2. ENVOY
This handbook has been produced introdure some of the very important aspects of the
designand construction removablepartialdentures.
of
The student
who is newto this subjectwill, it is hoped,acquirethe theoretical
toolswith which
to beginthe processof design,constructionand discussionabout removablepartial dentures
with fellow studentsand the dental lecturersat Lambeth College,Tower Bridge Centre
We must remember,
however,
that the information
containedhereinwas not given to us on
tabletsof stone,but has been developedover many years by surgeonsand technicians
in
conjunction
with the, often forgotten,long suffering patient.
Designingpartial denturesis a process of applying theoreticalprinciples,backed up by
technical
expertise, meetthe requirements the individual
to
patient.
of
This moduleguide is intendedto be practicaland useful and will, it is hoped, increasethe
knowledgenecessaryto make successful partial dentures and at the same time, provide
muchfood for thought.
M. WelchCertEd.
Lecturer Dental
in
Technology
Lambeth
College
TowerBridge
Centre
TooleyStreet
London
SE12JR
3. t
I
I
I
I
ACKNOWLEDGEMENTS
I
i
r
-r I
i
-i
I
I
-a
i
I
I
I
J
I
.J
i{
J
J
J
f
I
t
I
-J
-J
I
I
#
I
I
i
I
I
I
I wishto thankthe following
lecturers Lambeth
at
College,
TowerBridge
Centre theirinput
for
and inspiration, Comwallfor notes,the discussions
Jeff
and consequently ideas that
the
grew.Rowland
Gardner his computerknowledge.
for
ChrisGrigson continued
for
support,
ideasand for editing
this latesteditionof the 'Handbook'.
KarenHorton providing
for
notes,
'Neddy'for more nbtes and
last, but not least, Robert Woods for ideas, handoirts
and
practical
suggestions timeswhentheyweresorelyneeded.
at
4. 1
CONTENTS
REMOVABLE
PARTIAL
DENTURE
PROSTHETICS
lndications contraindications the provision partial
and
for
of
dentures.
Page 1
Classification partial
of
dentures
based the nature partial
on
of
denture
support.
Page3
Classifi
cation partial
of
dentures.
Page 5
Castsurveying partial
and
denture
design.
Page 7
Principles partial
of
denture
design.
Page 14
Principles mucosa
of
bomepartialdenture
design.
Page 19
Principles toothbomepartial
of
denture
design.
Page 20
Theapplication the principles partial
of
of
denture
design the tooth/mucosa
to
bourne
case.
Page24
principles consider the tooth/mucosa
Design
to
for
bourne
case.
page26
Partialdenture
retention"
page31
Majorconnectors.
page3g
procedure designing
General
partial
for
dentures.
page41
Procedures the production acrylicresin/metallic
for
of
complete partial
and
dentures.
Page 44
Principles the lostwax process.
of
Page 46
Fumaces.
Page 47
Metalmelting
systems.
Page 48
Casting
forces.
Page 51
prcedures metallic
Finishing
partial
for
denture
bases.
Page 53
Bibliography.
Page 55
5. Removable
PartialDentures
Definition of a Removable Partial Denture
A partialdentureis an appliance,
removableby the patient,for the replacement one or more
of
naturalteeth in the mandibleor maxilla in which one or more natural teeth remain. In this
Handbook, term partialdenture imply one which is removable.
the
will
Reasons for the provision of a partiat denture
. To restoremasticatoryefficiency
. To restoreand improve aesthetics
. To improve impairedspeech
o To preventthe collapseof the dental arch and over-eruption teeth
of
r To maintain verticaldimension
the
These reasonswill be expandedduring discussionsessions.
Harmful effects of poor partial denture design
Poor partial denture design will cause damage to teeth, encouragecaries and damage the
tissuessupporting denture,includingthe gingival margins.This occurs becausethe partial
the
denture:6 fits too closelyinto the gingivalmargincausingphysicat
injury.
6 Allows food to pack down between the denture and the teeth. Food packed under pressure
againstgingivalmarginswill force them away from the teeth. lf this is allowedto remain it will
cause inflammation.
Such damage to the gingival margins, if left untreated progresses,
to
involvethe deepersupporting
tissuesof teeth leadingto periodontal
diseaseand endingwith the
loss of teeth.
@ lt can also causetraumatic
damageto the palateknownas hyperplasia.
Denturehyperplasia
is
an abnormalincreasein the cell arrangementof a tissue which can result in a flap or roll of
hyperplastic
tissue resultingfrom chronic initation by a denture.
Cases best left without dentures
The decision not to fit a prosthesiswhen natural teeth are missing is usually a difficult one to
make. lf such a course contemplated
the dental surgeonwill carry out an examinationof the
mouthto ensurethat no harmfuleffects result from the decisionnot to restorethe missingteeth.
As a generalguide the followingquestionsshould be answeredand if these are mainly in the
negative,
then the probability that the patientwould be betteroff withouta denture.
is
o ls mastication
inefficient
with the remainingnaturalteeth?
. ls appearance
and speechadverselyaffected?
I ls the occlusion of the teeth such that there is a possibility of over-eruption or tooth drift
occuning?
o Wll the benefitsof the prosthesis
outweighthe possibledamage it may cause to the tissues?
' Doesthe patientmaintain carefuloral hygieneor likelyto followhygieneinstruction?
a
' Are temporalmandibular
joint symptomslikelyto developas the resultof a disturbed
occlusion
followingthe loss of teeth?
. ls the patientsmental attitudeto denturessatisfactory?
6. ' Doesthe patientsmedical historysupportthe fitting of a partial prosthesis?
"The ansWer to this /asf guesfion may be the sole deciding factor regarding the fitting of a partiat
denture. A prosthesis may be contra indicated by a history of frequent and severe epiteptic
attacks".
FennLidlowand Gimson.
Partial dentures fall into four categories
El Fixedpartial
dentures
Theseare the besttype of denture
available
because
they only fill the denture
spaceie. the
edentulous
space.However
some anatomical
contouris lost due to tooth preparation
and
replacement retainers.
by
The basicpremise thatthe patient
is
with minimaltoothlossand goodoral hygiene
can be fitted
with bridgesthat restorelost anatomical
contours,ie. caseswhere the second premolaris
jaw.
missing either
in
upper lower
or
However, mainareas
the
where
fixedpartial
dentures
triumph
overany othercategory three
are
fold, the first being aesthetiqs
and the secondthe restoration occlusion as completea
of
to
functional
levelas possible. thirdis thatlargeareas mucosa notusedfor support
The
of
are
thus
reducing amount tissuecovered connectorsthe
of
by
[1 Acrylicremovable
partial
dentures
Acrylicpartialdentures, matterhow well they are constructed,
no
shouldalwaysbe viewedas
beingtemporary,
used mainlyfor transitional
periodsor when the long term prognosis
is
uncertain to poorpatient
due
response.
Patients supplied all acrylic
are
partial
with
dentures familiarise
to
themwithwearing eating
and
with dentures.
Partialdentures this type damagetissuestructures to pooiOesign
of
due
anO
construction
techniques,
however,
theyare stillthe mostcommon
produced
typeof denture
and
fitted
today,
simply
because can be easilymodified
they
whenexlractions the remaining
of
teeth
become
necessary.
tr Metallic
partial
removable
dentures
Metallicpartialdenturesare designedto replaceand preserveexistinganatomical
oral
structures.
Partialdenturesconstructed cobalt chromiumalloy can be designedand
in
construcled components suiteveryindividual
with
to
case.
Thistypeof partial
dentures idealfor all categories partial
are
of
dentitions.
Theyare the only
altemative fixedpartial
to
dentures
whichprolong
dental
health.
Patients arefitted
who
withthese
dentures usually
are
those
withgoodoralhygiene sound
and
remaining
natural
dentitions.
E Complex
partial
removable
dentures
By their complex
nature,thesepartialdentures outside remit of thismodule.
are
the
7. Classification PartialDentureSupport
of
Partialdenturesare designedand constructed restorelost anatomical
to
structures
and to
maintain
existing
ones,however,
damagemay be done by poorlysupported
dentures teeth
to
andtissues.
Partial
dentures classified theirmeansof support.
are
by
Dentures
whichderiveall theirsupport
from basaltissues classified mucosa borne.
are
as
Dentures
whichderivetheir supportfrom the remaining
naturalteeth are classified tooth
as
borne.
Dentures
which are supported a combination both teeth and mucosaare classified
by
of
as
tooth/mucosa
borne.
Fig- 1. Shols a case where a denturemay be entirelymucosa bome
Fig. 2. Shovtrs case where a denturemay be entirelytooth borne.
a
Fig. 3. Shorc a case where a denturemay be entirelytootVmucosabome
8. Transmission Masticatory
of
Stresseson PartialDentures
During masticationvarious stresses, or loads are transmittedto the underlyingmucosa (in the
case of the mucosa bome denture),to the teeth and periodontalmembrane (in the case of the
wholly tooth bome denture) and a combination of the aforementionedstructuresin the case of
the toothltissuebome denture.
Fig.4. Shons how the masticatory
stress is transmittedvia teeth to the underlying
bone.
The greatestload to which teeth are subjectedare applied along their long axes approximatelyat
rightanglesto the occlusal
plane.
These loads vary with the muscle activity, type of food and the occlusalsurface area of natural
and artificialteethand are divided into two functions:
' Maslicationis the stressor load appliedto a denture and the supportingtissuesduring chewing.
tissueswhen the teeth are held
" Clenchedloadingis appliedto a denture and its supporting
togetherunder pressureby the musclesof mastication.
9. Classification Paftial Dentures
of
quotea classification
Osbourneand Lammie(1974)
basedon support a denture a whole.
of
as
Class I Mucosa-borne
Thedenture entirely
is
supported the mucosa
by
andunderlying
bone.
Class ll Tooth-bome
All the support the partial
for
denture
comesfromthe teeth.
Class lll Combination mucosa-bome
of
and tooth-borne
Somepartsof the denture supported the teethandsomeby the mucosa.
are
by
Beckeftand Wilson'sclassification
Beckett
and Wilsonbasedtheir classification an earlierone by Bailynand felt that the
on
following
must be considered
when decidingthe proportionate
amountof supportprovidedby
teethandtissue(thewordtissuemeansmucosa
andunderlying
bone)
o Thequality abutment
of
support.
o Themagnitude the occlusal
of
support.
I The harmony the occlusion.
of
o Thequality the mucosa residual
of
and
ridge.
Theybelieved everyeffortshouldbe madeto avoidtissuesupport
that
alone.
Class I Boundedsaddle
Abutment
teethqualified support denture; mucosa usedfor support.
to
the
the
not
Class ll Free-endsaddle
(a)tooth-and-tissue
borne
(b)tissue-bome
Classlll Bounded
saddle
Abutment
teethnotso wellqualified support denture in Class
to
the
as
l.
Kennedy's Classifi cation
partialedentulous
Kennedy.classified
archesby relating edentulous
the
spacesto the remaining
standing
ieeth.
Fig. 5. CLASS | - Bilateral
free end saddlespcsteriorto the standingteeth
10. Fig. 6. CLASS ll - Unilateral
free end saddleposteriorto the standingteeth.
Fig. 7. CLASS lll - Boundedunilateral
saddlehavingstandingteeth at eitherend
Fig. 8. CLASS lV - Saddleanteriorto the standingteeth.
All classes except Class lV are subdivided into modifications,each modification denoting an
additional
bounded
saddlearea.
In additionthe followingpointswere noted:
. The most posterioredentulous
area determinesthe classification.
o The size (number)of modificationsis not important.
o lf a third molar is missing,and not to be replacedit is not consideredin the classification.
Many classifications partial dentures and their designs are available to clinicians and
of
technicians,however, even now there is no universally accepted classification.Classifications
describe either the various tooth and saddle positions, like those of Kennedy's,or else look
closelyat the amount of availablesupportfor partial dentures,such as Beckett and Wilson's.
Kennedy'sclassification usefulwhen describingthe basic outlinesof any particularcase and in
is
forming a common partial denture language amongst colleagues,however, it does not take into
accountthe available support upon which the success of any partial denture depends. For this
reason a support classifrcation preferable.
is
6
11. CastSurveying
and PartialDentureDesign
Cast surveying is an important sequence in the design and construction of partial dentures.
Before any decisions can be made about the component parts of the design, casts of the
patient's
mouth must be surveyedusing an instrument
which enablesthe technicianand dentist
to analyseany undercutareas of the soft and/or hard tissue in relationto the path of insertionand
removal of the denture.Survey lines drawn on the labial and buccal aspects of teeth may show
areaswhereclaspscan be positioned,
linesdrawn on the mesial,distalor lingualareas may also
show areas of undercut.
The Gast Surveyor
The cast surveyoris an instrumentthat holds a graphite rod above a horizontaltable upon which
the cast to be surveyedcan be mounted and locked in various positions.The table may be tilted
either antereo/posterior laterally. The marking rod can be moved up and down, but always
or
remainsverticalto ensurethat the angle it makes with the cast is constantfor any tilt.
The graphite rod marks survey lines at the greatest horizontalwidth of the tooth being surveyed
(area of maximum convexity).
Survey Lines
The survey line (fi9.9) is a line which is marked on a tooth by the graphite rod at the greatest
width of a tooth in relationship the common path of displacement, to the path of insertion
to
or
and removal.Survey lines divide teeth into two sections,the area below the line (towardsthe
gingival margin),possessundercutsthat lie in relationto the path in which the cast is being
surveyed,whilst areas above the survey line (towardsthe occlusal surface), do not lie in
undercuts relationship that path.
in
to
Fig, 9. A surveyline
12. Paths of Displacement
Paths of displacement
are all possible paths along which all or part of the denture may be
duringfunction.
displaced
Common path of disptacement
The most common path of displacement for partial dentures is at right angles to occlusal
surfacesduring function,ie sticky food tends to dislodge denturesalong this path when the jaws
are opened. Initiallythe cast is surveyedwith a marking rod parallelto this common path of
plane.
displacement, with the surveyofstable in a horizontal
eg
Path of insertion and removal
The path of insertion
and removalis the angulatedpath along which the dentureis insertedand
removedfrom the mouth. This path must be decided before the denture is designed.The cast is
analysed,
usingan analysing
rod, to decide the best path of insertion
and removal by tilting the
surveyortableto variouspositions.
Once decidedthe table is securedby the lockingdevice and
the path of insertion clearly marked on the sides of the cast. A second survey line is drawn
around the teeth and mucosal surfaces of the cast in relation to this path. The colour of the
marking rod should be differentfrom that used to mark the survey lines in relationto the common
path of displacement.
Undercut
The definition of an undercut is, according to the BSI Glossary of Terms Relating to Dentistry
(1983) "that area of a tooth, or soft fissue surface which is beyond the survey line when viewed
from a particulardirection"(fi9. 9). Undercutscan be eliminated,or createdwhilst surveying and
can be describedas true or false undercuts.True undercutsare those in relationto the common
path of displacement
and provide resistance removal along this path. False undercutsare
to
those undercutsproducedby tilting the cast. They provide resistanceto removal of the denture
alongthe path in whichthe cast has been surveyed,but not in the common path of displacement.
Once the path of insertion
and removalfor the denturehas been chosen,the retentiveportionof
the clasparms shouldlie in relation both the path of insertion
to
and removal of the dentureand
to the commonpath of displacement.
Fig. 10. Soft and toothtissue undercutsviewedfrom a horizontaldirection.
Guiding surfaces
Guidingsurfacesof.teethguide removable partial denturestowardstheir final and exacl positions
in the mouth. These surfacesideallyshould be parallelto each other, howeverthis is seldom
possiblebecausethe surfacesof teeth are curved. Guiding surfacesare ne@ssarybecausethey
decreasethe numberof pathsalong which a denture may be displaced.
The influencingfactor
when choosinga path of insertionand removal of the denture is the locationof these surfaces on
a cast of the mouth by the analysingrod of the surveyor.
13. Object of surveying
Objectivesfor surveyingare many and varied for whichever type of case the dental surgeon and
technicianmay be faced with, howeverthere exists a common theme for all situations.Surveying
ensures that no rigid portions of a partial denture lie in undercuts in relation to the path ol
insertion
and removalof the denture,
although
they may lie in undercuts
relativeto the numerous
possible
pathsof displacement.
The flexibleends of clasp arms must lie in undercuts relation
in
to all possiblepaths of insertion,
removal or displacement,
since they are the mechanismsby
which partial dentures are held secure in the mouth (ng.11). The most likely path oi
displacement,as mentioned before, is at right angles to the occlusal surfaces of the teeth,
thereforeit is imperativeto ensurethat the tips of the clasp arms and if possiblethe denture base
itself, resist displacementalong this path, but in so doing we must rememberthat patients need
to be able to insert and remove the denture easily so the flexibility of the clasp arms must
providethe only resistance insertionand removal.
to
Fig. 11. Shoainga claspand its relationship a surveyline_
to
To summarize, objectives surveyingmay be listedas follows
the
for
o To mark the bulbousparts of the teeth and soft tissues.
o To identifyareason teeth and alveolarridgesof a cast relativeto any given path of insertion,
removalor displacement partial
of
dentures.
I To locatetooth surfacesthat may help to guide partialdenturesto their exact positions the
in
mouth.
o To find undercuts
that can be used for retentionand to measurethe amount of horizontal
undercut the teethselected clasping.
on
for
. To aid the designand exact location the clasp arms.
of
o To block out unwantedundercuts(areasof interference)on the cast.
o To help decide where the extreme margins of the denture base should lie in relationto the
survey lines on the natural teeth, which helps to ensure that no unsightly gaps are
apparentbetweenthe teeth and the denture base to affect aesthetics.
o To assistthe planning fixed partialdentures(bridges)
of
beforethe preparation the teeth.
of
r To decidethe location
and parallelism precision
of
attachments.
. To prepareparallelsurfaces, surfacesat specifiedangles by millingon abutmentretainers
or
and bars,orto drill parallelholes.
14. A Surveying Procedure
! Securethe cast on the movabletable so that the occlusalplane is parallelto the base of the
surveyorand lock the table in position.Doing this ensuresthat the graphitemarker is at right
plane(fig. 12).
anglesto the oc,clusal
Fig. 12. Graphitemarkerheld at right anglesto the occlusalplane.
! Check the graphite marker to make sure it has a straight,vertical edge free from wear, loosen
the holderand insertintothe mobileverticaltoolholderthensecurethe graphitemarkerby lighily
tuming the screw thread of the holder. Be careful at this stage because it is very easy to Oieai<
surveying
leads.
I Bring the graphitemarker into contactwith all surfacesof the teeth by moving it around the
cast makingsurethat the tip of the markeris touchingthe cast. Linesare markedon the teeth by
the markerat theirwidestpartsbecausethis is the only point at which it touchesin relationto the
common path of displacement
with the tip of the marker outliningthe horizontal
exent of the
undercutrelativeto that path. These survey lines indicate areas of the teeth that can be used for
retention,
whichare those belowthe survey lines,or towardsthe gingivalmargins.
D Once the teeth and the soft tissue undercutshave been surveyedin relationto the common
pathof displacement,
replace graphitemarkerwith the analysing and beginthe procedure
the
rod
to establish path of insertion
the
and removalfor the denture.
I Beforedecidingthe most suitablepath of insertion
and removalfor the denturethe answersto
two questionsmust be found:
What are the positions
and anglesof the guidingsurfaces?
ls a labialflangerequired?
D To establish
the positions
and angles of the guiding surfacesloosenthe screwthat locksthe
tiltingtable of the surveyorso that it can be tilted easily but will stay in the position which it is
to
tilted. Movethe analysing to the saddleson the right side of the cast and positionit so that it
rod
is in line with the distal surfaceof the tooth anteriorto the saddle (ng. 6 ). Then move the rod
until it touchesthe mesial surface of the tooth distal to the saddle. lf lhere is a difference
betweenthe angles that the teeth present to the rod then tilt the cast anteroposlerior.ly
until the
anglesmesially
and distallyare equal.
Fig. 13. The analysingrod establishing
any available
guide surfaces.
10
15. tr Repeat the same to the saddle on the opposite side of the cast, then place the analysing rod
on the lingualsurface of each abutmenttooth in tum, tilting the cast until the angles presentedto
the rod are equal as before. Whilst carrying out this proceduresurfacesthat are parallel may be
foundmesially,
distally,buccally lingually
or
that will guidethe partialdentureinto place.
Continuecheckingthe parallelsurfacesuntil an angle of tilt for the table is found where the
maximumnumberof parallelsurfacesare in contactwith the analysingrod. Once this has been
achieved lock the table in positionas this is the most suitable path of insertionand removal for
the denture.lf teeth are tilted and ill positioned, minimum of two guidingsurfacesshould be
a
found.
n Mark the vertical walls of the cast by using the analysing rod as a ruler in conjunctionwith a
wax knife, by scribingat least three vertical lines in different areas,this enables relocationof the
cast in exactlythe same positionafter it has been removed from the surveyingplatform.
I Guidingsurfacescan usuallybe found for posteriorsaddlesof partialdentures,
but if a labial
flange is requiredthen it may be neressary to have an oblique path of insertionand removal to
restorelost alveolarand gingival tissues (fig. 14). However, oblique paths of insertioncan cause
problemsif posteriorsaddles are also present because areas of undercutare increasedcreating
zonesof displacement
(fig. 15) and lack of retention.
Fig. 14. Restoringanatomical
structuresin anteriorregionsusing an obliquepath of insertion
Fig. 15. Shov,rs
how zones of displacement createdin posteriorregionswhen posterior
are
titts are chosen.
The saddlearea createdis wedge shaped,which is easily displacedalong a greaternumber of
paths.Another problemariseswhen teeth are divergent (fig. 16) becausethis also creates a zone
of displacement
from which the denture can be displacedat any angle betweenthe diverging
lines.
11
16. I
lzoN€
I
I
of
DISPIACI^4ENIr,
F8. 16. Divergent
teeth createa zone of displacement.
! The final path of insertion
and removalwill be the anteroposterior
and lateralpositionof the
cast in relation to the vertical arm of the surveyor that meets the criteria of eslablishingguide
surfaces,areas of retention,freedom from interferenceand aesthetics.
! The next stage is to establishthe type and position of clasps (direct retainers)that are to be
used. Let us assumethat we have surveyed an upper cast that has two boundedposteriorsaddle
areas each with the second premolar and first molar missing. The clasps chosen will occupy a
position
alongand belowthe survey line,the part of the claspthat is belowthe survey line is the
part that supplies
the direct retention our partialdenture(ng. 11), to establish
for
the positionof
the retentive of the claspa surveyingtool called an undercut
tip
gauge is used.Undercut
gauges
measurethe amount of horizontal
undercutavailablebelowthe survey line for clasps,they are
gradedin three sizes,0.25mm,0.50mm and 0.75mm and are selectedaccording the flexibility
to
of the metal used to make the clasp. For instancea 0.25mm for cobalt-chromium,
0.50mm for
cast gold and 0.75 for wroughtgold, nickel chrome and stainlesssteel wire. These gauges are
fitted to the arm of the surveyor (fig 17) to ensure that both the side of the rod and the edge of
the disc touch the tooth simultaneously.
The edge of the disc then indicatesthe horizontal
undercut
and the position the clasptip.
of
Fig. 17. Showingthe use of undercutgauges.
The pointof contact thenmarked
is
usinga different
pointfrom
coloured
marker lhe furthest
at
the adjacent
tooth/saddle
undercut
area,this ensures
that the clasparm is madeas long and
flexible possible.
as
Having
established position the retentive
the
of
areas, decision
a
aboutthe
typeof clasp be usedcanbe made,ie occlusally
to
(single
approaching gingivaly
or
approaching,
armor roachclasps).
U The undercut
areasare theneliminated
(blocked
out)withwax,whichis thentrimmed
using
thewaxtrimmer
attached the mobile
to
(fig
vertical holder 18).
tool
12
17. Fig. 18. The trimmer is used lo trim the wax to achieveparallelsurfaces
13
18. Principles PartialDentureDesign
of
Thereis little doubt a conectlydesignedpartialdenturepreserves
existingoral structures
by
spreading masticatory overas widean areaof the supporting
the
load
tissues possible.
as
obviously
this is easier
withan upper
thana lowersimplybecause
thereis a greater
surface
area
availablelncreasing
masticatory
particularly
efficiency,
when premolars molarsare replaced
or
to restore
occlusion
and maintain
workingand balancing
contacts
wherepossible, increase
will
the chances success anypartial
of
for
denture.
Comparatively patients
few
request partialdenture
a
due to the inability masticate
to
food,this is
because modemdietof softcooked
the
food meansthat people survivewithout needfor
can
the
teeth.Neverthe lessmostpeople
prefersometeethto masticate
normal
foods.
Aesthetics generally patients
are
a
main concemwhenteethare lost.The mostcommonreason
for the request a partial
of
denture for the restoration appearance,
is
patients quitehappily
of
will
tolerate lossof posterior
the
teethbut as soon as an anterior
toothis iost a changeof attitude
occurstowardsreplacement.
The loss of any tooth is liable to be followedby migrationof
sunounding
teeth resulting closureof the spaceand malposed
in
teeth. partialdentures
are
essential preserve maximum
to
the
width of theseedentulous
spacesso that a replacement
of
the conectsizecanbe inserted.
Speech
may be impaired the lossof upperor loweranterior
by
teeth,this is because
labiodental
sounds
suchas F and V require
contact
between lowerlip andthe incisal
the
edgesof the upper
anleriors.
Dentalsounds
suchas TH, CH, J, Z, require
intactupperand lowerarches.
Restoration
of anterior posterior
and
arches almostcertainly
will
phonetics.
resultin improved
Whenteethdrift it causes
themto losecontactwith the teethadjacent them.Foodcan pack
to
between
teethwithconsequent
damage gumsand an increase the actionof caries.
to
in
Lossof occlusion oftencauseover eruptionand if this is grossmay meanthe loss of that
will
tooth.Two or three teeth extracted
from differentparts of th-emouth,if not replaced
with a
prosthesis,
would in a few years lead to a completecollapseof the dental arches and a
derangement occlusion maybecnme
of
that
traumatic justior anyteethinvolved, alsofor
not
but
thetemporo
joint.
mandibular
Driftingteeth
With a partialdentureno driftingis possibleif the edentulous
spacesare adequately
filled by
contacling
artificial
teethor denture
surfaces.
OverEruption
When,
through
extraction toothonejaw is deprived an opponent occlusion,
a
of
in
changes
take
placein its supporting
tissues. boneand periodontal
The
membrane no longer
are
stimulated
by
the physiological
masticatory
stresses consequently atrophic
and
an
reaction
takls place.
Atrophy
A termin medicine
usedto describe slateof wasting
a
due to someinterference the function
with
of healthy
nutrition.
Partialdenturedesignattemptsto:
@ Preserve
existing structures prevent
oral
and
trauma.
O Restoremasticatory phonetic.
and
functionsby replacinglost occlusion
and conecting
the
sounds
madeduring
speech.
O Restore
aesthetics, support facialappearance.
lip
and
@ Preventarch collapse,
individual
tooth movement,
over-eruption opposing
of
teeth and
splinting teeth.
of
't4
19. @ Spread masticatorystressesover the supportingtissueswithin their physiological
limits.
g Be stablein the mouthduringnormalfunction.
Functions of the component parts of partial dentures
Saddles
That part of the denture which replaces lost alveolar tissue and normally contains the
replacementteeth, it transmits masticatory load and retains retention devices. These may be
describedas being, free end or bounded saddles or trssue borne or tooth borne.
Connectors
That part of a denturewhich combinescomponentsto form an integralunit.
o rigidlyconnectthe saddles.
o rigidlyconnectthe retainers
. ensureeven load distributionbetweenthe saddles.
r brace againstlateral movement.
e providei ndirectretention(anterio-posterior).
Occlusal supports
Means by which vertical occlousalload is transmittedto the alveolar bone. Usually through
metallicsupportspositioned the naturaldentitionwhich transmitmasticatory
on
load throughthe
abutmentteeth. (occlusalrests)
Retainers
Meansby which a partialdentureis retainedin the mouth. This is normallyachievedby flexible
metal extensions
encircling
the abutmentteeth to enter undercutareas,these extensions
being
calledclasps.
ldeal physical properties of partial denture components
Saddles
$ Cood stimulation natural
of
tissues.
$ Wett toleratedby the oral mucosa- non toxic, non initant.
$ Dimensionally
stable.
S ruotpermeableto oral fluids.
$ Maintains surfacepolish.
a
$ Goodthermalconductivity.
Gonnectors
Shouldhave the same properties the saddlematerials,
as
and in addition
o TheYmust be rigid.A connecting
bar that is flexible will allow saddlesto move under stress
resulting masticatory
in
forces not being transmittedverticallyto the supporting
tissue or being
distributed
evenlyover that supporting
tissue.
o TheYmust be strong. In practicea connectorwhich is rigid is also sufficientlystrong.
o TheYmust be unobtrusive.
Particularly the patientstongue.This meansthat they must not
to
be bulkyand caretaken in their positioning.
I TheYmust not be food traps or encroachon areas of tissue which are best left uncovered.
15
20. Peripheral Outlines and Basic Contours of partial Dentures
Extensionsof borderareas,or peripheries(fig, 19) for partial denturesmust take into accountthe
structureand function of tissues in the immediate vicinity. Muscles of masticationwhich lie in
sulcus areas must be avoided as must frenal attachments,if the denture is to stay in position
becauseduring masticationand speakingthese structuresare active and mobile, any extension
onto these structures
will resultin displacementof the denture and eventuallyulcers.
RAMUS F JAW
O
MED PT
MASSE
M
BUCCINATOR
MODIOLUS
L A B I A LF
Fig. 19. Shouring peripheral
the
extension a lower free end saddle.
of
As a generalrule denturebasesshould cover the maximum area possibte
and extend into the
labial and buccal sulcii. The contoursof the flanges should be slightly concave to allow the
cheeksand lips to rest againstthem insteadof applyingpressure,
which resultsin the dentures
being forced out of position.Buccal and lingual flanges provide some form of bracingagainst
lateralforces, claspsprovide bracing,against lateralforces and retentionby resistingvertical and
rotationalforces
tlcob
Po"c
Fig. 20. An exampleof how short extensions
offer litile resistanceto lateralforces.
16
21. /
The lengthof saddleareasdepends the amountof natural
on
teethlhat remainin the arch form.
Free end saddleareasshouldend at the hamularnotcharea for uppersand at the retromolar
pad
area for lowers.For bounded
saddlesthe most anteriorextension
shouldfit as closelyas possible
to the distal aspect of the abulment. The most posteriorextensionshould fit as closely as
possibleto the mesial aspect of the last standing abutment tooth so that retention ulilizing
undercuts gainedand rotation
is
reduced.
Where collettsare necessary
they should be on or above the survey line and be relievedin the
gingival margin area eilher prior to constructionof the denture or after to prevent physical
damageto the margins,knownas 'gum stripping'.
Collettsshouldjoin naturalteeth at an angle of
less than 90o so that they merge into the naturalcontourand deflect food away from the teeth
(fis.21).
Fig. 21 . Colletts should be constructed to deflect food awav from the tooth
Acrylic resin as a connector for paftial dentures
Acrylicresin connectors
have to be fairly large or they will break.Sometimesthey have to cover
all available
soft tissueto achievethe rigidityand strength
necessary a connector.
for
The saddlesof this tissueborne denture,$g 22) are joined by a large acrylicconnectorwhich
also providessupportand retenlion.
Fig. 2.
A large acrylic conneclor covering a large surface area of mucosa.
It is not always necessary cover as much tissue with an acrylic connector(fig. 23). This
lo
denture will be worn more for appearancethan for mastication,it needs less retention and
support beqausethe acrylic connector is subjectedlo less force, therefore it can be rnuch
smaller.
17
22. I
Fig. 23, Denturesfor aestheticpurposesonly can havesmallerconnectors.
The size and design of partial dentures is based on the assessment of the following
' The coveragenecessary give slrength,this dependson the number
to
and positionof artificial
teeth and whether or not occlusionis to be restored.
. Any extra coverageneededfor support and retention.
o The need to keepgingivalmarginsclearto avoid unnecessary
lissuedamage.
18
23. Principles mucosaborne partialdenturedesign
of
Replacement
teeth are set in relation the articulated
to
castsand the remaining
naturalteeth.
once centric jaw relationshiphas been establishedin the clinic with either wax squash bites or
occlusal record rims. The casts are articulatedso that the artificialteeth can be "set up,'to the
remaining
naturaldentition.
The relationship the upperand lower naturaldentitionmay differ from the classicskeletaljaw
of
relationships
that allows intercuspation
when setting complete dentures. lf this is the case then
certain modificationsto occlusal surfaces will be ne@ssary to provide the patient with an
occlusionthat is both functionaland stressfree.
To establishthe maximum amount of contact, the largest surface area, spot grinding of artificial
tooth surfaces is necessary.Before this procedure is commenced, any'opposing natural tooth
surfacesare coveredwith thin metal foil, which is burnished(adapted)ctoseiyto protectthe cast
from potentialdamage.
The teeth are then positioned
one by one to conformto the occlusalplane,lengthand arch form
of both upper and lower casts. The articulator is then closed down with a piece of articulating
paper betweenthe upper and lower casts, once contact has been made, open up the articulatoi
and look at the occlusal
surfacesof the artificialteeth. Marksleft by the articulating
paperwill be
apparenton the surface,colouredeither red or blue, these areas are then grounb with a small
metal bur, usually a rose head, and the articulatoris then closed again to check that the
occlusion established
is
and that the verticaldimensionis correct.This ii done by ensuring
that
the incisalpin is touchingthe incisaltable, however,if this is not the case then the procedure
is
repeated
untilit does beingcarefulnot to over grind the artificial
tooth.
This technique
establishes
centricocclusionin centricjaw relationship, the technician
now
has to
check both the lateraland protrusivejaw movements to ensurefreedom from interferenceand
where possibleworking contactby tooth surfaces.This is done by moving the articulatorfrom
side to side with articulating
paperin positionbetweenthe teethwith contaaingareas being
groundto establish
workingand balancing
contacts,
this may not howeverbe possible
with all
partialdenturecasesdue to the canineguidance,or lift.
19
24. Principlesof tooth borne partial denture design
partial
A typical
removable
denture havethe following
will
components
I Majorconnector
o Minor
connectors
. Rests
o Directretainers
o Indirect
retainers
o Reciprocal bracing
or
components
o Saddle
areas
Partialdentures
should be designedto resist displacing
forces in a vertical,horizontat
and
anteroposterior
direction.
This is achieved
throughthe use of components providesupport
that
andretention the partial
for
denture,
whichprevent
movement anyof these
in
diiections.
partsthat resistverticaldisplacement
The component
(movement
towards
and awayfrom the
mucosa), occlusal
are
restsandclasps.
Horizontal
displacing
forces are resistedby the bracing sectionsof retentiveclasp arms,
reciprocal
clasp
armsandmajorandminorconnectors.
Anterior-posterior
displacing
movements resistedby occlusalrests,claspsand major and
are
minor
connectors.
Occlusal tooth bornesupportfor the partialdenturemust be provided some kind of rest
or
by
placedon exi9ing abutment
teeth.Theseshouldalwaysbe placedupontoothsurfaces
which
havebeenprepared receive
to
them.Restsmay be placeduponthe prepared
occlusal
surface
of
a premolar molartooth,usually the marginal
or
in
ridgearea gg. 24),or uponthe prepared
lingual palatal
or
surface an anterior
of
toothcapable withstanding forces
of
the
applied, upon
or
a prepared
incisal
surface.
ftct
Et
Fig. 24. Occlusalrest posilion.
Occlusalsupportis sometimesobtainedby using a tooth surfacewhich is inclinedocclusallyor
incisally,from its greatest convexity. Rest! tnat ire so placed upon an unpreparedsurface are
subjectto slippage
alongtooth inclinesproducing
orthodontic
forceswith the possibility moving
of
the abutment
teeth.
This is an infringement one of the basic rules providedfor rest design which is that a resf
of
should be so desrgned to transmit any occlusal force directly along the long axis of the supporting
tooth.
20
25. Fig. 25. Inclinedsurfaces,if unprepared subject to slippage.
are
A second
ruleis thata restmustbe placedso that it will prevent
movement the partial
of
denture
in a cervical
(downward)
direction.
Occlusal
restsmusttherefore rigid and receivepositive
be
support
from the abutment
tooth or
teethif the required
amount stabilityis to exist and be maintained.
of
Occlusal
rests,in addition
to distributing
occlusal
load,serveotherpurposes.
Restsact to maintain occlusal
the
relationship
withthe opposing
teethby preventing
settling the partial
of
denture. the sametime setilingof
At
the partialdentureagainst
gingivaltissuesis prevented
thus avoiding
any impingement
or
damage the gingival
to
tissues
adjacent the abutment
to
teeth.By preventing
movement a
in
cervicaldirection, positionof the retentiveportionof the clasp arm is maintained its
the
in
intended
relationship thetoothundercut.
with
portion a clasparm,whenin its terminal
The retentive
of
position
passive in
should
remain
and
contactwith the toothso that it is readyto resistany verticaldislodging
force. Thus when a
dislodging
forceis applied clasparm becomes
the
activeto resistverticaldisplacement.
When
partialdentures madewith claspassemblies do not contain
are
that
occlusal
restsas an integral
partsomesettling the denture
of
takes place,whichmeansthat the clasparm is standing
away
from the tooth, consequently
some verticaldisplacement possiblebeforethe retainercan
is
become
functional.
Rests,
whenincorporated retainers
prevent
into
suchsettling
thereby
helping
to maintain vertical
the
stability partialdentures.
of
Partial denture components used to provide occlusal support
Occlusal Rests
Theseare metallic
extensions
whichfit into prepared
areasof teethto provide
occlusal
support
by resisting
vertical
displacement the denture
of
towards softtissues,
the
theyresistlateral
forces
andactas indirect
retainers resisting
by
movement thedenture anteroposterior
of
in
directions.
Cingulumand Incisalrests
Cingulum
restsare smallmetallic
extensions
whichfit intoprepared
areasof anterior
teethon
either lingual palatal
the
or
aspects.
Incisalrestsare small metallic
extensions
whichfit into prepared
areason the incisaledgesof
anterior
teeth
A modified
formof incisal is the'embrasure
rest
hook',
whichis placed the embrasure
in
between
twoteethandcontinues the incisal
over
edgeontothe labial
surfaces.
In addition the above components, 'Kennedy
to
the
bar' is a continuous
lingualor palatal
cingulum
rest,whichshould
havea reit preparation eitherendto be effective.
at
Onlays
Thesecomponent oftenregarded enlarged
are
as
occlusal
rests,whichare usedto maintain,
or
re-establish vertical
the
dimension orderto restore original
in
the
function occlusion.
Onlays
of
apeoftennecessary
whenocclusal
surfaces groundawayover the yearsby mastication
are
or
bruxism,
theyalsoprotect occlusal
the
surface
from further
wear.
21
26. lntemalRests
Such restsare seatswhichare incorporated the surfaces cast retainers
into
of
formingparallel
slotsor dovetails utilizebothocclusal
to
support
and horizontal
stability.
c-)
t
rl
J
'2
m
-t
ffi
tYl
lA,
I
f
I
t
'- a.
Fig. 26. The varioustypesof rests.
clinical modificationsto tooth surfaces that provide occlusal support
Planning supportof tooth bornepartialdentures often necessitate grindingof tooth
the
will
the
surfaces the dentalsurgeon providespacefor occlusal
by
to
rests,the use of restswill prevent
complete
closure centric
in
relationship cuspscontact
if
them.
Fig. 27. Cusps of opposingteeth may haveto be modifiedas well.
Rests seats should not be cut very deep through enamel, if deeper rests seats are required
restorations
should be placed on abutment teeth into which rests seats may be cut. On.the
occlusal surfaces of posterior teeth they should be dish shaped with vertical walls and be
positioned eitherthe distal or mesial marginal ridges.The positionfor rest seats on anterior
at
teeth will be eitheron the cingulumor incisalareas and take the form of narrownotcheswhich
are flat and at right angles to the long axis of the teeth. The floor of occlusal rest seats on
posteriorteeth should be inclinedslightly towardsthe centre of the tooth. Rest seats on posterior
teeth should form an angle of less than 90o with the vertical minor connectorfrom which it
(ng. 28).
originates
It shouldbe notedthat anglesgreaterihan gOofail to transmitthe resulting
occlusalforcesatong
the long axis of abutment teeth because the force is not channelledtowards the centre of the
tooth.
22
27. ^A tr, C
N
Fig. 28. Occlusalrests shouldform an angleof less than 90' with the minor connectorto transmitocclusalforces alongthe long
axis of the supportingtooth.
Anterior
teethcan be usedto provideocclusal
supportif the lingual palatal
or
surface
slopesare
gradual
rather
thanperpendicular,
whichhelpsto prevent
slippage,
with canines
beingprefered
to incisors occlusal
for
support
because
they have a largerrootsurface
areaandgreater
alveolar
provide
bonesupport.Cingulum
greater
restson canines
support
than incisalrestsbecause
they
are placedcloserto the centreof rotationof abutments
therefore
reducing tendency tip
the
to
abutments.
Cingulum
restsare aesthetically
moreacceptable patients
to
thanincisal
rests.
Poorly
designed
indirect
retainers
resting toothsurfaces
on
may resultin
of
@ movement the toothawayfromthe component
loading a tooth resulting traumafollowedby inflammation the periodontal
of
in
@ excessive
of
membrane,
looseningof the tooth and loss of bone support resultingin the loss of the
tooth.
23
28. TheApplication the Principlesof PartiatDentureDesignto the
of
tooth/mucosaborne case
Tooth/mucosa
borne denture support is derived from the remainingabutmentteeth and
edentulous
areas.
Partial
dentures this kindof support haveocclusal
with
will
and incisalrestsincorporated the
into
designto allowthe transmission masticatory
of
forcesthroughthe abutmentteeth, however,
some of the mastication
force will be transmitted
throughthe mucosa.The most common
example wherethistypeof actionoccurs the Kennedy
of
are
Classes & ll.
I
Possiblemovements the partialdenture
of
Thereare at leastthree possible
movements a distalextension
partialdenture(ie bi-lateral
of
freeendsaddles).
Thefirstis rotation
aboutan axisformedby the two principal
occlusal
rests.
Thisaxis,whichis known the fulcrumline,actsas a centreof rotation the baseof the distal
as
as
extension
movestoward supporting
the
tissues
duringthe application an occlusal
of
load. When
the occlusal
load is released, fulcrumline then shiflsto any anteriorly
the
plecedre$s as the
basemovesawayfromthe supporting
tissues
whichallowsverticaldislodging
forcesto become
effective.
Thesedislodging
forcesare the verticalpull of food between
opposing
toothsurfaces, effect
the
of moving
border
tissues the effectof gravityagainst maxillary
and
a
denture.
Assuming
that direct retainers functional
are
and that occlusalrests remainseated,it will be
rotationof the prosthesis
that occursratherthan total displacement.
This dislodging
force is
resisted onedirection the tissues the residual
in
by
of
ridge,(theaccuracy the fit of the denture
of
base)
with resistance the opposite
in
direction
beingprovided the actionof the occlusal
by
rests
serving indirect
as
retainers.
The secondmovement rotation
is
abouta longitudinal
axis formedby the crestof the residual
ridge.Resistance provided
is
initially the rigidityof the majorconnector its ability resist
by
and
to
torquewith the occlusalrests contributing the stabilityof the partialdentureas indirect
to
retention
only. lf the major connector not rigid this rotationabout a longitudinal
is
axis either
appliesunduestressto the sides of the supporting
ridgeor causeshorizontal
shiftingof the
denture
base.
Thethirdmovement rotation
is
perpendicular located
aboutan imaginary
axis
nearthe centreof
the dentalarch. This movement
o@ursduringfunctional
occlusal
stresses
from a diagonal
and
horizontal
direction
and is bestresisted stabilizing bracing
by
or
components
suchas reciprocal
clasparmsand minorconnectors
whichmakecontact
withvertical
toothsurfaces.
Stabilizing
part of any partialdenture
components an essential
are
designregardless the
of
manner support
of
and the type of directretention
employed.
Thesebracing
components,
on
eachside of the arch,act to stabilize partialdentureagainstany horizontal
the
forcesapplied
fromopposite
sides. Rigidconnectors
mustbe employed achieve effect.
to
this
Movement tooth/mucosa
of
bomepartial
dentures result the breakdown supporting
will
in
of
bony
tissues abutment
of
teeth,if the majorpartof the masticatory fallson the saddle.
load
This is
because
compressibility tissueincreases
posteriorly
of
pad
towards regionof the retromolar
the
area.On application a vertical
of
forcethe abutment
toothmay be slightly
displaced rigid
if
(occlusally
approaching)
claspsare used.The alveolar
tissues displaced a greater
are
to
extent,
resulting rotation
in
arounda fulcrumaxis provided the occlusal
(rests).
by
support
The amount
of displacement compression
and
depends
uponthe condition the mucoperiosteum covers
of
that
the bone(thealveolar
ridge).
Increasing
verticalload will causebone resorption the breakdown cells underpressure
by
of
(fibroclasts).
The denturebase must cover the maximumarea possibleto give maximum
support,
thusspreading masticatory
the
load.
24
29. After resorptionhas taken place, if no attempt to reline or rebase is made, further resorptionis
likely to take place as the downwarddisplacementcontinues.
When rigid clasp arms embrace a tooth vertical loading will tend to rotate the tooth distally, the
centre of movement being locatedin the area betweenlhe apex and the gingival margin.Tensile
forces act on the bony socket in the distal area of the gingival margin and mesiallynear the apex.
Such forces acting upon abutment teeth result in a breakdown of the periodontaland bony
supportproducinga loosetooth.
Vertical loadingwill also result in some movement of the clasp arms over the enamel surface of
abutmentteeth, but this dependson
. The thicknessof the flexible portion of the clasp arm.
o The modulusof elasticityof the clasp arm material.
. The degree of retentiveundercutengaged.
Clasps constructedin alloys with a low modulus of elasticity will allow greater movement. lt
therefore follows that wrought clasps will allow greater movement, as they are usually round in
cross sectionallowinggreaterflexibility in horizontaland verticaldirections.
To summarisethe effects of clasping,the more rigid the clasp the greater is the torque ratio on
abutment teeth with less load applied to the mucrsa and alveolar ridge. Flexible clasp arms
effect less torque on abutmentteeth but allow greater mucosaland laterai loading.
25
30. bournecase
to
Designprinciples considerfor the tooth/mucosa
Reduction of load
The vertical load acting on the saddle area during masticationcan be lessenedby reducingthe
surfacearea of the oerlusaltable. This is achieved when caninesand premolarsare used instead
of molars, using teeth with a narrow bucco-lingualwidth and sometimes leaving a tooth off the
saddlearea (usuallythe last molar). This is most usual when the bite is heavy and possiblyclose
to the saddle area or the saddle is long or the bone supportis poor.
Reducingmasticatoryload will also reduce the amount of lateral loadingthat acts on the saddle,
so helping to preservethe ridge form and the supportingtissues of the abutment teeth, this is
achievedfurther by the use of flat cuspedor shallow cuspedteeth.
of
Distribution the masticatoryload over a wide area can be achieved by the coned peripheral
extensionof the saddle areas of partial dentures.Under extendedsaddleareas have the effect of
concentratingthis load on a very small area, particularly if a well defined residual ridge is
present.
of
Anteriorplacement occlusalsupports
and
surfaces premolars
of
Although prefened
the
sitesfor occlusalrests may be the occlusal
the
simplybecause premolar
molars may be neecssary include reston a canineor incisor
it
to
a
or molar missing.
is
is
rest.
retainer an auxiliary A canine much
or
Anterior
teethmaybe utilized support indirect
to
an
prefened
restseatscan be
are
multiple
over an incisor this purpose.
for
When canines missing,
incisor.
incisors
rather
thana single
spread
overthe remaining
crownto the
inclination the tooth and ratioof the lengthof the clinical
Rootform,rootlength,
of
before decidingthe site and form of rests placed on
alveolarsupportmust be considered
to
they are placednearer the centre
restsare prefened incisalrestsbecause
to
incisors.
Lingual
are
Lingual
rests
or
to
of rotation abutments havelesstendency tip teethdistally mesialy.
of
and
moreacceptable the patient.
to
alsoaesthetically
of
on
are
the
restsshouldbe positioned the mesialaspect the
Whenpremolars present, occlusal
of
ridge.The distribution forces
in
rest seat cut into the marginal
occlusal
surfac,e, a prepared
as
support
occlusal
by
abutment
teethandthe ridgecan be alteredsignificantly placing
between
possible
in
an increase the load on the ridge,but proportionately
far forwardas
This resultsin
of
less on the abutments, giving a more even distribution stress in an anteroposterior
so
direction.
to
of
Combined
actionof restsand claspsreversethe direction rotation a forwardmovement
whichis resisted contac{ anterior
by
with
teeth.
of
Distribution load betweenteeth and ridges
and
clasps saddles.
between
of
Thiscanbe achieved varying nature the connection
by
the
Stressbreaking
Insleadof having a rigid connection
betweenclasp and free end saddlesome degree of
is
the
movement allowedbetween two. The stressesacting upon the dentureare distributed
differently.
as
unit
unit
whichallows
movement
between saddle andthe retaining is known a
the
Anydevice
breaker.
stress
the
covering
into
downwards the softtissues
Whenvertical
loadis applied saddleis displaced
the
the
greater
existsbetween
a rigidconnection
edentulous
ridgeto a
extentthan is the casewhen
retainer,
occlusal
rest and the saddle.This meansthe mucosaand alveolarbone has to
overthe whole
increased
vertical
withstand
load,but alsothatthe loadis muchmorewidespread
thanconcentrated the freeend.
at
ridgerather
26
31. ln generalstressbreakerscan be divided into two groups
e Those having a movablejoint betweenthe direct retainerand the saddle ie hinges,sleeves.
cylindersand ball and socketdevices. These are also termed precisionattachments.
. Those having a flexible connectionbetween the direct retainerand the saddle, ie wrought wire
connectoror a divided (cast)major connector.
The resultof stressbreakingaction is
Vertical load - a greater proportionof load is assignedto the edentulousridge and less to the
abutments.
Lateral load - greater lateral stress is placed on the alveolar bone and less on the abutment
teeth.
Stress Breaker Flexibility
The flexibilityof a stress breakergovems the amount and distributionof load betweenthe ridge
and the standing
teeth but will dependupon
o The metallurgical
conditionof the connector(cast or wrought).
o The lengthand positionof the connector.
. The crosssectional
dimensions
and shape of the connector.
Advantages of the stress breaking principle
O Horizontal
forcesactingon abutmentteeth are minimized,the alveolarsupportof these teeth
is preserved.
O Stress-breakers
distributethe load between the teeth and the mucosa so that the mucosa
takesa greaterproportion the load when the periodontal
of
cnndition poor.
is
O With carefulselection the type of flexibleconnectorit may be possible obtaina balance
for
to
of stress betweenridge and the abutment.
@ Sptintingof weak teeth by the denture is made possible,despitethe movementof a distal
extensionbase.
pressure the denturebases massagethe mucosa,thus providingphysiological
O Intermittent
of
stimulation,
which prevents
bone resorption
and eliminates needfor relining.
the
O Stressbrokendesignsare best when the remainingteeth are of poor prognosis
and cannot
providesupportfor conventional
partialdentures.
Disadvantages of the stress breaking principle
@ me dentureis more expensiveto construct.
@ A concentration forces on the edentulousridge results in its speedierresorption.
of
@ tne more complicatedappliance lessens the tolerance of the patient. Spaces between
components sometimes
are
openduringfunctionleadingto food trapsand tonguenipping.
6 tt a lightand flexibleconnector used it can be easily bent and distortduringcleaning.
is
@ lf rebasingis not undertaken
when necessarythen marked resorptionwill occur.
@ Repairand maintenance any stressbreakeris difficult,costly,and frequently
of
required.
.
27
32. Fig. 29. A flexibleconnectorfor a free end saddle
Advantages and disadvantages of rigid designs
Advantages a rigiddesign
of
g Technically framework easierand lesscosflyto make.
the
is
O Equitable
distribution stressbetween
of
abutments the residual
and
ridge(s) possible
is
with a
rigid
design.
O tne needfor relining rigid prosthesis less frequent
the
is
sincethe residual
ridgedoes not
haveto carry functional unaided.
the
load
O Indirect
retainers otherrigidcomponents
and
may act to preventrotational
movement the
of
dentureand will providehorizontal
stabilization
that is not possible
when stressbreakers
are
used.
O By reducing number flexibleor movingpartsthere is less dangerof distortion
the
of
by
careless
handling the partof the patient.
on
g Moving
parts
being
absent appliance moreeasily
the
is
keptclean.
Disadvantages a rigiddesign
of
@ Objectionable
torquemay be applied the abutment
to
teeth if abutment
retainers not
are
passive"
6 nigiO
continuous
clasping
maybe hazardous
whenstress
breakers notused.
are
@ Intracoronal
retainers
may not be usedat all without
stress-breakers,
because
theyare locked
within abutment tipping
the
and
forces
wouldbe transmitted
directly the abutment
to
tooth.The
onlyexception whenmultiple
is
splinting abutments,
of
coupled
witha minimum occlusion
of
on
thedistal
extension
base,
actsto reduce
abutment
torque.
@ tne useof wrought-wire
retentive
particularly
clasparms presents
sometechnical
difficulties
when high-fusing
chromealloys are used. Wroughtwire may be crystallized improper
by
application heatduring
of
casting soldering
or
operations,
resulting earlyfracture. may also
in
lt
be easily
distorted careless
by
handling,
leading excessive insufficient
to
or
retention, ultimate
or
fracture to repeated
due
adjustment.
@ lf reliningis not done when needed,
the abutment
tooth may be loosened
and suffer
permanent
periodontal
damagebecause the repeated
of
application torqueand tipping
of
sil.resses.
28
33. Type of clasps used in conjunction with rigid connectors
The type of clasps used in conjunctionwith rigid connectorshas markedly different effects on
load distributionlf gingivallyapproachingclasps are chosen,a conditionexistswhich is similar in
principleto stressbreakingaction ie Roach back action (half r or L).
The effectivenessof the.stress breaking action of bar clasps is increasedsimply by increasing
the length of the bar further into the sulcus, or attachingthe clasp to the minor connectoras far
from the abutmentas possibleand by the type of material used, ie wroughtor cast alloy.
With gingivally approachingclasps a phenomena known as 'trip action' exists. This increases
retention of free end saddle designs and may be defined as 'fhe ftictional resrsfance fo
movement, as opposed to deformation resr'sfance'.On attempted removal the angle between the
tooth and clasp increasesand the clasp beds into the tooth surface,thus resistingdisplacement.
A combinationof rigid connectionand occlusaly approachingclasps is the opposite extreme to
stress breaking. In this situation the maximum load is placed on the abutment tooth and the
minimum on the ridge. However,if wrought gold occlusalyapproachingclaspsare provided with
rigid connectionthen the added flexibility means that a reductionof stress occurson the tooth but
increaseson the ridge,the oppositebeing so for cobalVchrome.
Altematives to cast components for metallic denture base connectors
Connectorsfor the partial denture may also be formed by using wrought stainlesssteel, these
connectors
beingthe bar type ie labial,lingualor palatalbars. Usingwroughtconnectors
has the
followingadvantagesover cast connectors;More flexibility exists Oetween clasp unit and the
tfre
saddle,the cross sectionalshape of the material allows flexibilityin universaldirectionsand
Iastly,the structureof the wrought alloy used makes it less likely to fracture.However,there are
also disadvantages wrought connectorsin comparisonwith cast conneclors.Cold working of
to
alloys can produce work hardeningof connectors,which leads to fatigue and eventual fracture
due to the compression of the structure, soldering can anneal wiought components, thus
reducingflexibility,elasticityand the ability to retain shape or resistdeformltion.
Physical properties required for connectors
The physical propertiesrequired for connectors used in stress broken and non-stress broken
designsare similarto that of cast or wrought clasp units and cast major connectors.
The following applies to stress broken connectors
They shouldhave a high proportional
limit to resistpermanent
deformation,
they shouldpossess
a low modulusof elasticity allowfor flexiblemovementduringfunction.
to
The following applies to non stress broken connectors
The major connector non stressbrokendesignsshould have a high modulusof elasticity
for
for
rigidityin thin sections
and a high proportional
limit to avoid permanent
deformation.
29
34. lmpression Techniquesfor Removable partiat Dentures
Functional
lmpression
techniques
ln the mid 1950's
McClean
recognised denture
that
bearing
areashadtwoforms(anatomical
and
functional).
Anatomical
formbeing
denture
bearing
areasfree fromocclusal masticatory
or
loading.
Functionalform beingonewhichis underpressure, loadduringfunctions mastication
or
of
and
occlusion
McClean's
methodof taking impressions
involvedthe use of occlusalrecordrims lined with
paste.
impression
The patient
bitingtogether providethe functional
to
loading the mucosa
on
of
the edentulous
areas. overallimpression the recordrims in situwasthentakenand sent
An
with
to the laboratory.
Mucco-displacement
impression
techniq
ue
A mucco-displacement
impression
technique
seeksto produce impression the denture
an
of
bearing
areaswhilstundersimulated
functional
loading.
The type of materialused for this
technique zinc oxide/eugenol
is
paste,which displacesand compresses soft tissue in a
the
similarmanner that of a denturebase,thus givinga more accurate
to
cast upon whichto
construct denture
any
basefor tooth/mucosa
bornepartial
dentures. aim of thistechnique
The
is
thesameas McClean's.
Technique
1. Special
traysaremadefor secondary
impressions
using studycastspreviously
the
taken.
The
typeof traythat is constructed a dualpurpose.
has
The tray will haveboth alginateand zinc oxideeugenolpaste used in it to producea secondary
cast.
2. Firstthe saddleareasare relievedby using0.5 mm wax spacer.
3. A 2 mm wax spaceris laiddown over the remainingnatural
teeth on the cast.
4. Locatingstops cut into two different areas of the occlusalsurfaces.
5. The tray is constructed the usualmannerand sent to the surgery,
in
wherethe dentisttakes an
impression the saddle areas using zinc oxide eugenol paste,this materialis viscous under
of
pressureand will flow to all sectionsof the saddle area. Once set any excess is removed in the
abutment
areasand the next stageusingalginateis begun.
6. Firstthe locating
stops are removedand the alginate(ineversible
hydrocolloid) loaded into
is
the tray and the impression
completed.
Other impression
materialsthat can be used are silicone rubber base and tissue conditioners
such as ViscoGel for the saddleareas.
Applegate
technique
Anotheralternative
methodis the alteredcast techniquefirstdescribed Applegate.
by
30
35. Partialdentureretention
Retentionof removablepartial dentures is achieved by means of direct retainersof one type or
another. A direct retaineris any unit of a removable prosthesisthat engagesan abutmenttooth
so as to resistdisplacement that prosthesisaway from the basal tissues.
of
This can be accomplishedby frictional means, by engaging a depressionin the abutment tooth,
or by engaginga horizontal
tooth undercutlying below the survey line.
Direct retainer types
Basically
therearetwotypesof directretainers,
theseare
Precisionattachments
Theseare eitherprefabricated, made by the technician provideretention
or
to
and/orsupport.
They can be elilra coronal, that is lying outsidethe naturalcontours the teeth, or intra
of
coronal,that is lyinginside natural
the
contours the teeth.
of
Extracoronal
retainers
haveedensions
withhinges
and railswhichengage
slotsandslides.
lntracoronal
retainers
engage
verticalwallsbuilt intothe crownof the restored
abutment
toothto
create
frictional
resistance movement removal.
to
and
Clasps
Othertypesof extracoronalretainers
engageextemalsurfaces abutment
of
teeth in an area
cervical the greatest
to
convexity in a depression
or
created that purpose.The mostcommon
for
extracoronalretainer the flexible
is
retentiveclasparm- Ratherthan creatingfrictionalresistance
to removal, flexiblearm is forcedto deformtherebygenerating
a
resistance removal.
to
Clasp
retention baseduponthe resistance metalto deformation.A clasp,if it is to be retentive
is
of
must be placedin an undercut
area of the tooth whereit is forcedto deformwhen a vertical
dislodging
forec,suchas occlusally
directed
force is applied
and resisted.lt is this resistance
to
deformation
whichgenerates necessary
the
retention, is only proportional the amountof
but
to
flexibility the metalfrom
of
whichthe clasparm is constructed.
Clasps
should
beara passive
relationship the teethexcept
to
whena dislodging
forceis applied,
any activerelationship claspto tooth,that is to say a claspwithoutocclusal
of
supportor
reciprocal
action resultin orthodontic
will
movement or damage the periodontal
and
to
tissues.
Claspunitsresistocclusally
directed
forcesby the inclusion an occlusal
of
rest,whichprevents
movement a downwards
in
direction
towards mucosa,
the
resistance lateral
to
forcesis provided
bythe broaderpartof
theclasparm,
orreciprocal
clasparms, is known bracing.
and
as
Teeth,if
purposes
they are to be usedfor retentive
must have a heightof contour
cervical whichthe
to
surface
converges, the gingival
eg
margin.
All teeth,when surveyed, possessa heightof contour,or an area of greatestconvexity
will
(width), areasof cervicalconvergence
but
may not exist when a tooth or teeth are viewedin
relation a givenpathof placement
to
because,
areasof mnvergence
change
withthe tilt of the
platform.
cast on the surveyor
Sometimes
areasof convergence too closeto the gingival
are
margins
and are therefore available the placement retentive
not
for
clasps.
areas
of
Conversely
of occlusal
convergence
may be utilisedfor the placement non-retentive,.reciprocating,
or
of
slabilisi components.
ng
Flexible
components the onlycomponents
are
whichmay be placedbelowsurveylinesbecause
if rigidcomponents
wereso placed, undercut
the
areaswouldbecnmeareasof interference
to
placement removal
and
rather
thanareasof retention. degree toothundercut location
The
of
and
available retention the partialprosthesis therefore
for
of
is
relative the pathof placement
and
to
removal.
31
36. .!:'1r
: ,llt
..
': a.::. .
.
Factors determining the amount of retention generated by a clasp. Size and positionof undercutareas.
. How far into the angle of cervicalcrnvergenc€ (undercut)the clasp terminal is placed.
r Flexibilityof the clasp arm, which is the producl of
. lts length, measuredfrom its origin to its terminal end.
- lts relativediameter,regardless its cross sectionalformof
. lts cross sectionalshape,that is whetherit is round, half round or some other shape.
. The materialfrom which the dasp is made, ie cast gold alloy, cast chrome alloy,wrought
gold alloy,orwroughtnickelchromealloy.
Uniformity of retention
:
The placement of clasp arms into areas of convergencewill be determined by the size of the
angle of convergence,uniformityof retentiondepends upon locating the clasp terminal not in
relation to the height contour but in relationshipto the angle of cervical convergence.lt follows
then that all retentiveclasp arms should be located in the same degree of undercuton each of
the abutment teeth, howeverclasp arm positionswill be different for each tooth due to dissimilar
contoursThe degree of undercutused for retentionof partial dentureswill vary with the choice of alloy for
the retentivecomponent,ie 0.25mm for cobalt chrome alloys, 0-50mm & 0.75mm for gold alloys
and wrought alloys.
Factors affecting the flexibility of the clasp arm
Length
The longer the clail arm the more flexible it will be, the length of a circumferentialclasp is
measuredfrom the point at which a uniformtaper begins.Retentivecircumferentialclaspsshould
have an arm that is tapered uniformly from ils point of origin continuous along its full length to
the terminalend.
Clasp arrn cross section
The greater the cross sedion of a clasp arm the less flexible it will Oe, if the taper is completely
uniform (fig. 30), then the average cross seclion should be at a point midway between its origin
and its terminal end. howeverif its taper is not uniform a point of flexure and thereforea point of
weakness will exist that will then be the determining factor in its flexibilily regardlessof the
average cross seciion of its entirelength.
l
'J
l*€tat
rt,n! o2 tctt * -*
t*"*
Fig. 30. The uniform taper for clasp arrm.
32
37. Cross sectionalform.
Flexibilitycan exist in any form of materials used to make clasps whether they be half round or
round. The only universallyflexible form is the round form, which is practically impossible to
obtain by casting and polishing.This flexibility is limited to only one direction in the case of the
half round form. Cast claspsare essentiallyhalf round in form and flex away from abutments in
horizontalplanes only, this ability of the alloys used to cast clasps allows for adjustmentof the
arm, howeveredgewiseadjustmentand flexing is limited (verticaldirection). lt is for this reason
that retentivecast clasp arms, when used for tooth borne partial dentures are more acceptable
becausethey will be called upon to flex only during placementand removal of the prosthesis.
Tooth tissue bome cases ideallyshould have clasps which have a round cross sectionalform.
This is best illustratedby the example of a distal e)ilensionbase which has a retentiveclasp arm
on an abutment adjacent to the free end saddle. A clasp in this situation must be capable of
flexing not just during placementand removal but also during functional movement of the dislal
extensionbase, in short it must possess either universal flexibility to avoid the transmissionof
tipping forces to the abutment tooth or be capable of disengagingthe undercut when vertical
forces directed against the denture are towards the residual ridge. Round clasp forms are the
only circumferential
clasp form that may safely be used to engagea tooth undercuton the side of
an abutmenttooth adjacentto a distal extensionbase.
Clasparm materials, and wrought
cast
physical
properties
Theideal
required analloyused construct
of
to
clasps
should
be
o A highproportional to resistpermanent
limit
deformation.
o A low modulusof elasticity that the clasp is flexibleenoughto allow withdrawal
so
over
undercuts
without toothor claspbeingoverstressed.
the
Clasparm assemblies be madefrom cast alloyssuchas gold and cobaltchromium well
can
as
as preformed
wire of nickelchrome,stainless
steel and gold (wrought
clasps). previously
As
stated action a claspdepends
the
of
uponthe resistance the alloyto deformation. alloywith
of
An
a lowmodulus elasticity
of
flexeseasily,
whilstonewith a highmodulus stiff.
is
The elasticlimit,whichis the pointat whichpermanent
deformation
takesplace,needsto be as
highas possible prevent
to
suchpermanent
deformation the clasparm during
gold
of
function,
alloy,nickel
chrome
andstainless
steelclaspsare placed 0.50mm 0.75mm
at
&
because
their
limitsare simplylowerand greater
flexibility
can be achieved,
theseclaspsare usually
made
frompreformed
wire.
The amountof horizontal
undercut
usedfor retentive
clasparms has to be proportionate the
to
modulous elasticity, cobaltchromealloyclasparm tips are placedat 0.25mmbecause
of
ie
this
is theamount deflection canbe achieved
of
that
physical
within alloys
the
limits.
Type lV gold alloysshouldbe heat treatedin orderto provideadequate
stiffness.
Cast cobalt
chromium
alloys
cannot
usually hardened heattreatment
be
by
unless
theycontain
MolyMenum.
A disadvantage cobalt
of
chromium
alloysis thatthe grainsizeis large,
whichmeans
thatthere
may onlybe two or threegrains
across wholethickness an alloyclasp,and
the
of
therefore is
it
y
easif distorted broken. presence 0.25- O.5o/o molybdenum the alloyreduces
or
The
of
of
in
the
grainsizeandincreases strength thinsections, alsoproduces solidsolution
the
of
it
a
hardening
effect.Nickelincreases ductility cobaltchromium
the
of
alloys,but decreases
theirstrength.
The
methodof coolingafter castingalso influences strength
the
and structure cobaltchromium
of
alloys.
33
38. Clasp design and influencingfactors
Clasps are extra coronal retainersand fall into two main groups
Occlusally-approaching
clasps,
(supra
bulge)The clasparm approaches undercut the
the
of
toothfromits occlusalsurface.
Theseclasps alsotermedcircumferential.
are
Gingivally-approaching
clasps(infrabulge)The clasparm approaches undercut the
the
of
toothfromthe gingival
surface.
Theseclaspsmay alsobe termedbarclasps.
The following general principles of clasp design should be observed
S RigiO
portions the clasparm shouldlie on or abovethe survey line.
of
S Retentiveflexible tips of clasp arms should lie below the survey line to provide retention.
$ Cast clasparms shouldbe made as long as possible.
$ Claspsor collettsshould be on oppositesides of the teeth to provide reciprocalaction.
$ Where ever possible
claspassemblies
should be bilaterally
opposed,
that is, retentionon one
side of the arch shouldbe complimented retentionon the opposite
by
side.
Clasp assemblies should consist of
. One or more minorconnectors
from whichthe clasp arms originate.
o A principlerest.
. A retentiveclasp arm engaginga measuredtooth undercutwith a retentiveterminal.
o A non-retentive
clasp arm on the oppositeside of the tooth for reciprocation
and stabilization
againsthorizontal
movementof the denture.Rigidityof this clasparm is essential its purpose.
to
TENTION
BRACING
parts of clasps.
Fig. 31. Component
Poordesign
andchoice direct
of
retainers
may result
in
S partlat
dentures
rotating
around toothusedfor retention
the
causing gradual
a
loosening the
of
tooth
dueto boneloss.
I Placing
claspsin undercuts are incorrectly
that
measured
may causetraumato the toothand
fracture clasp.
the
34
39. It rl
i
I
I
I*
I
I
Blatterfein's classification of buccal and lingual survey lines
Blatterfein
derived a classification survey lines accordingto their positionon the tooth, the idea
of
being that this classification
would prove helpful and practicalwhen deciding on clasp design.
What Blatterfeindid initiallywas to divide teeth in half vertically by using the long axis and once
divided then gave names to the divisions, he called such divisions on teeth nearest to a saddle
the Near Zone and the divisionfurthest away from the saddlethe Far Zone.
Classification surveylines
of
Medium
Survey
Line
Situated approximatelyacross the centre of a tooth it will curve in a slightly occluso-gingival
direction from the near to the far zone. The type of clasp most used in this situation is the
circumferential occlusaly approaching clasp arm but this does not mean that gingivally
approachingclasp arms cannot be used. Clasp arms of either type can be made of cast cobalt
chromeor gold alloysas well as preformedwire.
DiagonalSurvey
Line
Situated
approximately the occlusal
near
surface linetravelsdiagonally
the
across toothfrom
the
the nearzonetowards far zonefinishing or very closeto the gingival
the
at
margin.
Suchsurvey
lines found upper
are
on
canines premolars either
and
in
maxilla mandible.
or
Bothtypesof retentive
clasparmscan be utilisedon teethwhichhavethe diagonal
surveyline,
listedbelow someof the specialist
are
designs
employed.
o Thereverse
actionor hairpin
claspandthe ringclaspwhichare bothcircumferential
clasps.
. The bar clasp,
whichis a gingivally
approaching
claspand mostcommonly
called.aRoach
clasp.
I
Allthese
designs
haveadvantages disadvantages, example, reverse
and
for
the
action hairpin
or
clasphasthe disadvantage havingno application
of
wherea shortclinical
crownexistsbecause
insufficient
space available accommodate double
is
to
the
clasparm,alsoas the diagonal is
line
foundmainly teeththatareconspicuous
on
(whilst palient smiling) double
the
is
the
clasparm is
easily
seen.
Ringclasps
encircle toothon threesides,firstlyit follows lingual
the
the
surface
abovethe survey
line,thenflowsinto the embrasure
between toothto be clasped
the
neighbour
and its adjacent
andthencrosses buccal
the
surface
fromthe far zoneto the nearzone. Clasps this type may
of
require
extensive
modification toothsurfaces.
to
Gingival
approaching
clasps
over
may be prefened
because the obvious
of
aesthetic
advantages
portionof the claspcan be L or T shaped.
the occlusally
approaching. retentive
The
The L type
part
hasa portion
whichliesin the undercut
bracing
areabelow survey andan additional
the
line
whichfiesabovethe surveyline whilstthe T type has a retentive
element
whichlies entirely
withinthe undercut
area.
The High SurveyLine
Found
muchcloser theocclusal
to
surface
on
thanthe gingival
margin bothnearandfar zones
in
teeth
thathavea small
degree convexity because toothis lingually
of
or
the
inclined.
Occlusally
approaching
clasps
shouldencompass
if
threesurfaces theyarethe casttypeand one
if a wrought
clasp used.Sucha wrought
is
the
clasparm is positioned
immediately
below survey
gold wire is
line havinga greaterlengthof the arm engaging
the undercut
area (wrought
prefened stainless
to
steelbecauseit is more flexible).Bracingeffectsof this clasp are poor
becauseinsufficient
space existd betweenthe survey line and the occlusal surface to
accommodate rigidportion a castclaspsinceit couldinterfere the occlusion.
the
of
with
surface
Wherehighsurveylinesresultfrom an inclined
toothit will be foundthaton the opposite
area,the
thereis littleor no undercut, thesecasesa clasparm that encircles non-undercut
in
the
nearproximal
surface the surface
and
with the highsurveyline is oftenthe type used. The first
portion the arm lieson the non-undercut
of
area or surfaceabovethe surveyline and is thicker
35
40. thanthe rest of the arm.to ensure rigidity,the portion contactingthe proximalsurface is variable
positionwise lying in or out of the undercut,the portion which lies below the survey line is
resilient
alongits lengthgradually
tapering the terminalend.
to
This clasp supplies unilateralretentionwith a considerablebracing effect on the non-retentive
side, and will be attached to the denture by a thick strut joining together with the thicker
extremityof the clasp arm.
When this sfruf rs buccally placed,as is frequentty the case with tower dentures fhe c/asp r's
designated a REVERSE BACK ACTION CLASP.
Whenthe sfruf is palatallyorlinguallyplaceditisdesrpnafedaBACKACflON CLASP.
Ring clasps possesssimilar action becausethey encircle the tooth with the terminal point being
in the near zone of the undercutarea, this clasp is used on single slandingmolar teeth which
have a severe tilt and consequentlya high survey line. This clasp is attached in a more direct
fashion to the denture but should always have a buccal or palatal reinforcingarm lying against
the mucosato lend extra strengthand rigidityfor the bracingseclion of the claip arm.
The Low Survey Line
Low survey lines are found on the buccalor lingualaspectsnear to the gingivalmargins,they
occur in this positionwhen a tooth is conical in form or when there is a marked ihclinationin one
directionor another(a high survey line appearson the oppositesurfaceif inclination a tooth
of
has occurred).
Claspinga tooth with this type of survey line has obvious disadvantagesbecausethere is very
little undercut available for a retentive clasp arm to be effective in retaining a denture, also a
clasp in this positionis almostcertainlygoingto be near the gingivalmarginwhich could lead to
traumaof the mucosaand oral hygieneproblems.
Tooth surfaceswith these low survey lines can always bear a bracing arm as with the ring and
reverse back action clasps, however if additionalretentionis requiredthree alternativemethods
are available obtainit.
to
The first is the use of the extendedarm claspwhich is similar to the circumferential
clasp arm but
covers two teeth, it remains above the survey line on the first tooth and then crosses over into
the undercutarea of the adjacenttooth. This increases
the splintingaction and dislributionof
lateralloadingovertwo teeth.
Secondlyeven if no suitableundercuts
exist buccallyor linguallycertainlyone will be found on
the near zone aproximalsurfaceallowingthe use of a DeVan clasp.The DeVan clasp is a bar
clasp with a very small retentivehead that lies below this approximalsurvey line arising from
very low down near the peripheryof the saddle area. This clasp has reciprocalaction achieved
by a lingual or palatal strut which is in contact with the lingual or palatal far proximal surfaces
ending in an occlusalrest,the primary occlusalrest being positionedon the near proximal part of
the ocrlusal surface,however,little or no bracingeffect is achievedby this clasp.
Thirdly an abutment may be crowned so that suitable contours can be incorporatedinto the
restoration clasp positioning.
for
36
41. Efficiency of occlusally and gingivally approaching clasp arms
Retention
Gingivally
approaching
claspsprovidebetterretention
due to the phenomena
knownas ,trip
action',
this actionoccursas the retentive of the clasparm wedgesagainst tooth as it is
tip
the
displaced to the flexible
due
nature the extended
of
clasparm.
Bracing
Occlusally
approaching
clasparms have a rigid portion
which lies in contaclwith the tooth
surface a non undercut
in
areawhichprovides greaterbracing
a
effectagainst
horizontal
forces
thanthegingivally
approaching arm.
clasp
Tolerance
Gingivallyapproaching
clasps are not toleratedso well as occtusally
approaching
clasps
particularly a largetissueundercut present
if
is
because means
it
that the flexiblebar clasparm
hasto be positioned a largegap between andthe mucosa.
with
it
Aesthetics
Gingivally
approaching
clasps
maybe lessconspicuous
portion the mouth
towards anterior
the
of
particularly a canine to be usedfor clasping.
if
has
Indirect retention
Assuming
claspunits
havebeenusedfor retention,
displacing
forces occur
will
around clasps
the
during
function. drawing linethrough teethcarrying claspunitson a horizontal
By
a
the
plane
the
(ng.32),the axisor fulcrum
lineaboutwhichthe denture movecan be seen.lt is possible
will
thattheremay be morethanone fulcrum
linefor the samepartial
denture.
Indirect
retention
is
achieved placing
by
components, as occlusal
such
rests, the partial
of
denture rightangles
at
to
thefulcrum
axis.
The efficiency indirect
of
retention increased moreanterior
is
the
thesecomponents fromthe
lie
fulcrumaxis,because
they resistrotation the saddleareasawayfromthe mucosa
of
around
the
directretainers.
When components placedcloseto free end saddleareasindirectrelention achievedby
are
is
resisting
rotation the saddle
of
towards mucosa
the
round directretainers.
the
Thequality indirect
of
retention
depends the position the fulcrum
on,
of
line,the distance the
of
indirect
retainer
fromthefulcrum
axisandthe overall
surface
areacovered thedenture
by
base.
37
42. t'
-_-/
)t
,
)f
I
ll
Fig. 32. lllustrations
shoring the fulcrumaxes.
Components
which give indirectretentionare
Occlusal
rests,
onlays,
cingulum
and incisal
rests,embrasure
hooks,
Cummer
arms,Kennedy
barsandmajorandminorconnectors.
Poorlydesigned
indirect
retainers
resting toothsurfaces
on
may resultin movement the tooth
of
away from the component.
Excessiveloading of a tooth results in trauma followed by
inflammation the periodontal
of
membrane
and loosening the toothcausedby a reduction
of
in
bonesupport
resulting
eventually the lossof thetooth.
in
38
43. MajorConnectors
Types and Functions
"A maior c:annector the unit of a partial denture that connects the partsof the prosthesis/ocated
is
on one side of the arch with those on the opposite. W.L. McCracken.
"
The functionof a major connectoris to rigidlyjoin all the other components
together.lt is that
partof the dentureto which all othercomponent
partsare joined.
A major connectormust be rigid so that any load applied to it, is distributedover the whole of the
denture bearing area. Connectorsthat are rigid will resist torque (a tuming force) which may
otherwiseact as leverageon abutmentteeth.
Connectors
shouldbe placedawayfrom movingtissueand not put pressure
upon gingivaltissue.
Bony or soft tissue prominences,should be avoided, however,this may not always be possible
and it will be necessaryto relieve these areas before the denture is constructed.The connector
shouldalso be positioned as not to interferewith the tongue.
so
Maxillary connectors fall into the following categories
o Palatalplate
r Palatalbar.
e Combination Anteriorand Posterior
of
pattem).
bars(skeleton
o Palatalstrap.
Mandibular
connectors
o Lingualbar
Best constructed a half pear profile
in
I Lingualplate
Covering
the lingualalveolarwall form sulcusto the tooth cingulumof anteriorteeth of finishing
abovethe surveyline on otherteeth.Somewriterssuggest
that lingualplatedesignsare in effect
an extension the lingualbar.
of
o Buccalor labialbar
The exception usinga lingualbar or plate is that of a buccalor labialbar where the teeth are
to
so lingually
inclinedas to preventthe use of a lingualconnector.
However,
thesedesignsshould
only be used as a last resort.
Minorconnectors
Arising
fromthe majorconnector, minor
joinsthe majorconnector otherparts
the
connector
to
of
thepartial
denture.
joins
Where minorconnector the claspunitto the majorconneclor saddle, alsohasthe
a
and
its
function contacting guiding
of
the
surface so will position clasp
and
the
unitaccurately.
Wherea minorconnector required join an indirect
is
to
retainer, an auxiliary to a major
ie
rest
connector,
suchas a lingual
bar,the minorconnector
should
followthe interdental
embrasure
ratherthan coverthe convexsurfaceof the tooth.This is to reduceany wedging
effecton the
part
teeththe deepest of the embrasure
mustbe relieved
whensurveying cast.
the
On a cast metallic
partialdenlurebasethat part of the framework the saddlethat formsthe
of
retention the acrylic
to
resin(saddle
baseand teeth)is alsoa minorconnector.
Thesemay be
described eitheropenlattice
as
workor mesh,whichis spaced
the
fromthe master
castallowing
denture be more easilyrelinedand is ideal for free end saddles
to
is
wheretissueresorption
likely be greater
to
thanthatof bounded
saddles.
The othertypeis described platewith nail
as
39
44. head retentionand is used more often on short boundedsaddleswhich are unlikelyto need
relining.
Minorconnectors
shouldhave sufficient
bulk to be rigid withoutinterfering
with the placementof
artificial
teeth.
Connectors
As previously
mentioned, connector the componentof a partialdenturewhich connectsthe
a
is
saddles allthe remaining
to
parts.To fulfil this functiona connector
component
must be rigid and
capable of transmittingor equalisingstress over the whole supportingtissues (teeih and
mucosa).
Functions of major connectors
$ Rigiotyconnecls
saddles,
distributing
load over as wide an area as poasible.
S RigiOty
connectsthe retainerspreventing
any torque actionwhich could act as leverageon
individual
abutment
teeth.
$ Directsocclusal
stressto all the abutment
teeth.
S Stabilizes dentureby providing bracingactionagainstlateralmovement.
the
a
s Indirect
retention preventing
by
tippingin an antenor-posterior
plane.
Criteria construction connectors
for
of
S Coveras wide an area as possible distribute-load.
to
S Compromiseof bulk and thicknessto strength of material used. Thin sections must be
sufficiently
rigidto lransferload and resistfracture(more of a problemwhen constructing
acrylic
partialdenturebases).
S A bar or skeletontype of connectorshould ideally be a minimum of 6mm away from the
gingival tissues.This preventsfood from collectingbetweenthe teeth and the denture base
whichcausesinitationand possible
periodontal
problems.By leavingthe gingivaeexposedthere
is an unrestricted
bloodsupplyand stimulation tissuethat maintains healthymucosa
to
a
$ Platetype connectors,
whichcovergingivaltissuemust avoid pressure
whichis liableto cause
traumaor,damage. relieving gingivalmarginspriorto construction problemof damage
By
the
the
will be reduced.In the case of a conectly designedand supportedpartialdenture,where the
patienthas moderate
oral hygiene, problemshouldoccur.
no
The greatest
damageoccurswhen the gingivalmarginsare not relievedand the partialdenlure
is of the tissue bome type. Often these are constructedin acrytic resin, (some incorporate
stainless
steel lingualbars)and are knownas a "gum strippers".
However,
this is not the limit to
potential
damageto oral tissue.Lack of verticalsupportmay also lead to accelerated
bone loss
(atrophy)
and tissueresorption the localised
in
tissueareas.
NB. See handoulfor information
regarding
various advantages
and disadvantages individual
of
connector
types.
40
45. General
Procedure designingpartialdentures
for
Patientswho are about to be fitted with a partial denture should be free from caries, have good
oral hygieneand have no uncontrolled
periodontal
disease.There are three groups ot partialty
edentulouspatientswho will have dentures fitted they are either, tooth/bome, or tooth/mucosi
bome, or totallymucosaborne.
These patients,are
sub divided into two further groups
' Patientswho have sufficientteeth to indicatethe intercuspalpositionof occlusion.
e Those with so many teeth missingthat the intercuspalpositionis not indicated.
Initially surgeontakes impressions,
the
usuallyin alginate, the upperand lowerjaws to provide
of
study casts. When a sufficient number of teeth are present a wax record is taken to record the
intercuspalposition.Where insufficientteeth are present it may be necessaryto delay the taking
of this recorduntil wax occlusalrecord rims can be made.
The impressions pouredin ClassI stone,tdmmed,then mountedon an adjustable
are
articulator.
(Recordrims are made at this stage if there are not enoughteeth presentto assume the correct
occlusalrelationship).
Special impressiontrays can now be made for the secondaryimpressionsto provide the master
cast, (the cast on which the metallic denture is made). Survey the study casts in relation to the
commonpath of displacement
(blackcarbon)and in relation the path of insertion
to
and removal
(red marker). Once these stagesare completethe dentalsurgeonexaminesthe occlusionin the
intercuspalposition and decides what modifications,occlusal or otherwiseare ne@ssary before
proceeding
with the designsequence
and takingthe secondary
impressions.
Design sequence
The following
designsequence but one of the varioussequences
is
that are available, has been
it
chosenas a generalstartingpointfor thosewho are new to designing
partialdentures.
Plan the support of the denture
Decidethe positionand outlineof occlusaland incisalrestsand outlinethe saddles.ln the first
instanceyou are decidingwhich of the oral structures,both hard and soft tissues,are capable of
supporting dentureagainstthe forcesof mastication.Later,whenthe retention plannedthe
the
is
occlusal rest positions can be checked to ensure they provide indirect retention, that is they
should resist rotationof the saddles,away from the mucosa, round the fulcrum axes through the
clasp arm tips.
Plan the saddle connectors
In general bilaterally
similar saddles (tooth borne) should be connectedrigidly,and bilaterally
dissimilar
saddles,
which are a combination tooth borne and tooth/mucosa
of
borne.in lhe same
arch form, can be connectednon rigidly.Some rigid and flexibleconnectorsare shown, along
with various rigid connectorsfor maxillary and mandibulardentureswith bi-lateralfree end
saddles(fig. 33).
41
46. Designs of removable panial upp€r dentures ro itlusrrare
various types of connecrors and to show the
placement of clasps and rests' I' II. III. Rnteiior, middle
and posterior rigid saddle connecrors. Iv. v. vI, srress-broken
designs-
Sectionsof various types of rigid connectorsfor mandibular
dentures with bilareral free-end saddles.A,
ungual bar' B, Sublingualb.ar- goliiouous clasp and
-f"?r.
.lingual bar. D. L6;i ' plare covering gingival margins. E,
plate on teeth onlv' 9' l-abial bar. c, l-abial -and
F
i.ng*r
H, Linguat plate and swing-lock
:1H*
Fig. 33- Variousdesignfor connectors.
42
47. Plan the retention
The type and positionof retainersmust be decided. When precisionattachmentsare uSedthey
shouldprovidestress-breaking placed at the ends of mucosabome saddles,but may be rigiOit
if
used for retainingtooth borne dentures. Wherever possible,retention by extra coronal ctasps
should be assistedby rigid parts of the denture base lying in undercutsrelative to the common
path of displacement.
Where the black and red survey lines are at different positions,flexible tips
of clasp arms should be in appropriateundercutsin relationto both lines.
Where designsincorporate
flexiblesaddle connectors,three or even four retainersare necessary
to hold the tooth bome partsof the denture in positionwhen the mucosa bome saddles are under
load and the connectorsbend.
The effectivenessof the indirect retentiondepends chiefly on the direct retentionof the clasps,
but also on the distance of the occlusal rests from the axis of rotation. Thus, although the
occlusal rest positionsare decided early in the designing procedure,these positionsshould be
reviewedlaterwhen indirectretentionis being looked at in the overall assessmentof the design.
When the planning is completed the exact design is drawn on the laboratory card and the
materialtobe used is recorded
iegold,cobaltchromealloys,wroughtnickelchromeor stainless
steel, together with the dimensions and types of various components (clasps, rests and
connectors). casesin whichthe occlusionis not indicated, designshouldonly be tentative
In
the
untilthe occlusal
relationship been verified.
has
The next stage is canied out in the clinic, occlusal conections,any restorations,guide surface
preparation
and rest seat preparations
must be completedbefore the secondaryimpressionsare
taken. These impressions
are usuallytaken in alginate,or other elastic materialusing special
trays. Next the intercuspal
positionis recordedwith a wax wafer, or when this is not indicatedthe
use of wax occlusal record rims constructedon study casts prior to the secondary impression
stage is recommended. A facebow record is then taken. Next, the shade and any unusual
characteristics the standingteeth are noted on the laboratoryworksheet. In cases where the
of
designhas only beententative,mountthe primarycasts on the articulator
beforeconfirming
the
design.
Once the secondary impressionshave been received in the laboratory,they are cast using a
Class ll stone. The best methodto use is the two stage pouringtechnique.This is where the
anatomical
sectionis pouredfirst (withoutinverting)followedby basingthe anatomicalsection
with plasterof Paris. Casts producedfrom these impressionsneed to be treated with the utmost
care,they shouldnot be rubbedor damagedin any way.
When the definitive cast is produced,it is surveyed and any undercutsthat are not requiredfor
retention relation the path of insertion
in
to
and removalare blockedout usingwax. Shelvesare
waxed in position indicate
to
wherethe clasp arms shouldlie. Free end saddleareasthat needto
be relievedhave wax saddles laid down on them, (this is necessaryfor saddle area relief).
Borderseal linesare then scribedon the cast.
The mastercast is now duplicated,
using reversiblehydrocolloid, the wax pattem is 'waxed
and
patternis completeit is sprued and investedin a refractorymaterialand cast.
up'. Once the wax
43
48. :.:
Procedures the productionof acrylic resin/metallic
for
complete
and partialdentures
Modificationsto flasking techniques for acrylic/metallicdenture bases
The open method is recommendedfor free end distal extensiondentures, but where there are
individual bounded saddle areas a method known as the hooded technique can be used. This
technique requires the complete coverage of teeth, flanges if present and any metal work to
leaveonly the palatalor lingualaspects wax showing.
of
Once boilingout has been completedall
that can be seen is the taggingand the tooth'slingualsurface.This techniquehas the advantage
of securingsingle teeth in the conect position,however it does pose other problems,eg it may 6e
difficult to pack the acrylic resin through to any labial flange for single ieeth due to eithei the
presen@of retentiontaggingor the lack of space. Continuedpacking may fill the mould but may
lead to excessive pressurebuild up in the mould, resulting in breakage and movement of the
tooth beforethe mould can be completelyclosed.
The hoodedtechniquecan be combinedwith the open technique,this has the advantageof
allowingfree end saddle areaswith limited space to be open whilst isolatedanteriorteeth can be
hooded. This makes the flasking process much simpler because attempting to use the open
methodon several isolatedteeth is besetwith all the problemsconcemingfracture of the mould,
creatingundercutsand movementof teeth.
Siliconeor plastercasts may be used where small saddle areas are involved may be processed
usingauto-polymerising
resin.
Flaskingedentulous
cobaltchrome basesemploysthe use of 'capping',
which means covering
the metal palateslightlyabove the metal acrylic finishingline with the investingplasterto prevent
movement of the metal base. Trial packing presents a problem due to acrylic joining with the
metal retentiontagging.The Stellon flaskingtechnique,which employs wlx as a separating
medium prior to topping, allows a thickness of flash without a considerable increase in the
verticaldimension,
thus removing
the needto trial pack (although
this is advisable).
This problem
also appliesto distal extensionsaddleareas for partialdentures.
procedures
Other processing
includeinjectionmouldingand modernday pour techniquesusing
specialistacrylic resins.
Types of faults that can occur
The faultsthat occurduringthe conversion the wax saddleareasto acrylicresinare not much
of
different from those that occur when processingall acrylic resin complete or partial dentures,
however
there are significant
differences the flaskingand deflasking
in
techniques.
Flaskingprocedures
should leave land areas free from undercuts,the presenceof any undercuts
or lack of separating
mediumwill make openingof the flasksto boil out difficultand is tikelyto
resultin the breaking or fractureof the mould.
up
When boilingout wax, insufficient
scaldingmay leave tracesof wax which resultsin incomplete
acrylicsaddlesand chemicalunionwith acrylicteeth.
Teeth may become loose after boiling out due to excess wax on their surfaces and may move
out of the correct position.Failureto replace them properlywill result in teeth embedded in the
saddleareas and acrylic resin replacingthe void left to form a pink tooth that has to be replaced
with a new tooth using self cure.
Fine edgesof investing
plasterat the junctionbetweenacrylicand metal finishinglines are likely
to breakoff duringthe packingprocedure,
thus, all featheredges must be trimmedto avoid the
inclusion plasterin the acrylicresin.
of
44
49. I
I
i
I
I
I
"J
Heat soaking of flasks prior to boiling out should only soften the wax enough to open the flask
withotd damage, usually 5-7 minutes in boilirg water. Loqger periods result in the impregnation of
wax inlo the cast, which inhibits the separating medium and alfiowsthe acrylic resin to adhere to
the cast surfaces. Once processed the acrylic will be dirty' and need to have a lot of time spent
cleaning and trimming to remove the investing plaster.
Excessive use of cold mould seal results in areas called 'ptddles'which can be incorporated into
the acrylic resin, which have to be removed after processingby'bodging'with cold cure.
I
I
I
I
J
I
I
i
I
I
I
-J
I
I
I
J
I
I
-J
I
I
I
I
I
-J
i
.J
I
I
_i
I
I
45
-J
I
I
