Intra & extra coronal restoration resistance form /certified fixed orthodontic courses by Indian dental academy

Resistance form for intracoronal
and extracoronal Dental
restorations

INDIAN DENTAL ACADEMY
Leader in Continuing Dental Education
www.indiandentalacademy.com


• Introduction.
building restoration is similar to building any
mechanical structure, in that the stress pattern
of the available foundation and contemplated
sytucture must be predetermined.
The fundamental concept form of resistance form
is based on reaction with the restoration and
remaining tooth structure to the occlusal loading.


• Def.
Resistance form may be defined as the shape and
placement of preparation wall that best enables
both the restoration and the tooth structure to
withstand, without fracture, masticatory forces
delivered principally in the ong axis of the tooth.


• Sress patterns of teeth.
Anterior teeth.
a.The incisal edges espicially if they are square are
subject to tensile and shear stress in normal
occlusion. Masive stresses are present in edge to
edge occlusion.
b.The slopes of the cuspids will bear concentrated
stresses (three types) espicially if the cuspid is
protector of occlusion.(canine protected occlusion).
c. The distal surface of the cuspid exhibits unique
stress pattern as a result of anterior component of
force concentrating compressive loading at the
junction of anterior and posterior of dental arch and
microlateral movement of cuspidsduring excrusive

Movements. These factors leads to tremendous
stress concentration with resultant abrasive
activity.
D. The lingual concavity in the upper anterior teeth
bears substatntial compressive stress during
centric and shear stress during protrusive
movements.
e. The incisal edges of the lower anterior teeth are
subjected to compressive stress in centric and
shear stresis in protrusive movements.
2. Srress patterns in posterior tooth.
a. Functional cusp tips bears compressive stresses.

b. Marginal and crossing ridges bears tremendous
compressive and shear stresses.

c. During function compressive stress is on the
occluding contacting side and tensionon non
contactiing side. The junction of crown and root bears
tremendous shear stress during lateral excursion.

Some Applied Mechanical Properties Of Teeth.
Compressive strength of enamel supported by vital
Dentin 36-42000 psi.
When enamel looses support of dentin it looses more
than 85% of its strength.
Compressive strength of dentin os 40-50,000 psi
Non-vital dentin looses 40-50% its strength.

• Resistance form is of two types.
• Primary resistance form.
Primary resistance form is established during the
initial tooth preparation stage.
Principles.
1. Use box shape with relatively flat floor.
2. Restrict extension of the external walls to allow
strong cusp and ridge areas with sufficient dentin
support.
3. Slight roundening of the ine angles
4. Cap weak cusps in extensive preparations.
5. To provide enough thickness of restorative
materials to prevent its fracture under load.

• Factors affecting the resistance resistance form.

• A. Remaining tooth structure.
The amount of remaining tooth structure also affect the
need and type of resistance form. Very large tooth
even though extensively involved with caries or
defects require less resistance form consideration,
espicially in regards to cusp capping because th
eremaining tooth structure is still bulky and strong
enough to resist fracture.
b. Type of restorative material.
Amalgam requires minimum of 1.5mm thickness for
sufficient strength, cast metal requires 1mm and
ceramics 2mm. www.indiandentalacademy.com

• Amalgam.
• Class I Preparation.
• 1.Should have a Mortoise shape.
• Features.
• Seat of the restoration is at right angle to the
direction of stresses.
• It prevents buccal and lingual splitting
• 2. If caries penetrates deeply removing the
undermining caries would lead to conical
preparation. (fig)
• 3. when th epreparation walls comes in cantact with
marginal ridge , the wall should be divergent pulpo-
occlusally. www.indiandentalacademy.com

• 4.when ever preparation wall comes in contact with a
crossing ridge make the wall perpendicular to the
pulpal floor.
• 5. The width of the preparation should be ¼ to 1/5 the
intercuspal distance(Not less than 1.5mm).
distance(Not less than 1.5mm).
• 6.The crossing ridges should be preseved ie. No
effort should be made to them in preparartion unless
dictated by caries. Loss or involvement of such
crossing ridges is often more detrimental to the tooth
than loss or involvement of marginal ridge.
• 7.whenever preparation comes very close to the cuspl
tips thinning them to the extent that they cannot be
self resistance, it is advisable to flatten these
cusps(Cusp www.indiandentalacademy.com
capping) creatng a table with a minimum

• 8. when it is required to cross marginal ridge,
faciolly or lingually, the following rules must be
observed
• i.preserve the width being crossed. It should
be 1.5mm or dictated by cariogenic factors.
• ii. If small cusp is being isolated from the rest
of the tooth (weakening it) elevate the pulpal
floor at the crossing isthmus is advisable..
• 9. all cavosurface angles should be right angles
to create butt joint with amalgam.
• 10. all line and point angles should be rounded.

• Class II .
Occlusal loading and its effects.
a. A small cusp contact the fossa away from the
restored proximal surface in a proximoocclusal
restoration I centric closure.(Fig)


• 2. a large cusp contacts the fossa adjecent to the
restored proximal surface in a proximo-occlusal
relation at centric closure.


3. Occluding cuspal elements contacts facial or lingual
tooth structures surrounding a proximo-Mocclusal
during centric and excursion movements.


4. Ocludin cuspal elements contacts the facial or
lingual parts of restoration surrouded the tooth
structure during the centric.


5.Contacts at the facial or lingual parts of restoration
which completely replaces the facial or lingual parts.


• 6. contacts at the marginal ridges.


Design Features for the mechanical integrity of the
restoration.
a. Isthmus.
The junction between the occlusal part of the
restoration and proximal, facial or lingual parts,
potentially deleterious tensile stresses occur.
Studies reveals three things
I. Flucrum of bending ocurs at the axiopulpal line
angle
II. Stresses incresae closer to the restotation away
from fulcrum
III. Tensile stresses predominate at the marginal
ridge area.

Materials tend to fail therefore starting from the
surface near the marginal ridge and proceeding
internally.
These problems can be solved by applying three
engineering principles.


• The advantages of this design are
1. Increases accessibility to th eproximal surface
2. Sharp junctions are avoided ,which would otherwise
concentrate stresses in the restoration.
3. It increases the bulk of the amalgam
Other features for the isthmus are
a.The pulpal and gingival floor should be perfectly flat in
order to resist forces at the most advantageous
angulatrion.
b. Every prt of the preparation (Facial, occlusal, lingual,
proximal) should be self retenrive. If every part of
the restoration is locked independently there will be
minimum stresses at the junction of one part with
anotherie.www.indiandentalacademy.com
Isthmi.

Margins.
Four design features.
1.Butt joint
2.Leave no frial enamel at cavosurface margin
3. Remove flashes of amalgam on tooth surface
adjecent to the enamel margins.
4. As practically a s possible the interface between the
amalgam and tooth structure should not be at
occluding contact area.
Cusp and Axial angles.
1. Bulk of amalgam in all three dimensions should be
1.5mm.
2. Each portion should be independently immobilised
with retention modes.

3. Amalgam should be seated on a flat floor or table in
this area.
Design Features for protection of physiomechanical
integrity of tooth structure.
1..Isthmus.
The width of the preparation should not exceed more
than 1/4 to 1/5 the intercusapl distance. If it is
exceeded, it results in iuncresaed internal stresses
in the tooth, reduces tooth resistance to fracture
2.If the class II preparation includes the occlusal
surface all the basic features in the prepareation
should be included.
3.Cuspa and axial angles.
As in class I preparation the ideal length to width ratio

Should be less than 1:1 (Mesiodistally :buccolingually), if
the ratio is more than 2:1 then this part of the cuspal
wall should be shortened untill there is a maximum
ratio of 1:1 this is done in form of flat table to
accommodate 1.5mm thickness of amalgam.
Facial and lingual retentive grooves should not br deep
as to undermine enamel.
Margins.
Facial and lingual walls of the occlusal surface approach
the proximal surface, they should meet at right angle.


Class III
1. Anterior component of forces concentrates at the
junction between the anterior segment of the arh
and the premolar region.the cuspids stand at this
corner where these forces will first be receieved
and distributed
2. The incisal slopes of cuspids in normal excursion
mechanism will be the last to disocclude during
lateral excursion so the canines carries maximum
occlusal load for the longest time.
3. If the incisal margin is located far enough onto the
distal slope, restoration will be directly loaded
vertically and horizontally a situation that
necessiates maximum strength.

To overcome these forces, the tooth preparation on the
distal of the cuspid should have
1. Bulkiest walls possible
2. Minimal incisal extension.
Tooth coloured materials have no chance of
withstanding these types of forces. If esthetics is of
great concern here due to extensive labial extension,
an amalgam restoration can be built with a windiw
like cavity on the labial side which can be later
restored with tooth colouredmaterial.


• Class V .
• 1. the margins should not be placed on the cementum
as it lacks self resistance.
• 2.in cases the lesion is close to the bifurcation and
trifurcation areas, special attention should be given to
the gingival margins and walls. Gingival floor should
be flat, one planed, and with no retentive grooves ,
this allows more bulk to the tooth structure.
• 3. to protect the critical axial angles of the tooth the
mesial and distal walls should diverge axiofacially or
lingually.


• Pin retained amalgam restorations.
• Mechanical aspects of pin retaine
dfrestorations.
• Stressing capabilities of pins.
• Stresses are always inudced in dentin as a result os
pin insertion.if the stresses exceeds th elastic limit of
the dentin permanent deformation occurs.
• Types of pins.
• 1.Smaller the diameter of the pin compared to pin hole
lesser willl be the stress.
• 2. Diameter of pins.
• Greater the diameter of pin greaterr will be the stress.

3. pin depth and dentinal engagement.
greater the depth of pin channel greater will
be the stresses
4. Bulk of dentin.
the greater the bulk of dentin pulpally or
towards the surface of pin is, the less will be
the stresses per unit volume.
5. Type of dentin
6. Inter pin distance.
7. Loose pins.
8. Ratio of the depth of the pin into the dentin to
that pritruding into the preparation.(2:1)

• Retentive Features in the remaining portion of the
tooth.

• Pins and restorative material..
• 1. Pin will not increase the compressive strength of
restorative material. They will only help in retaining
it.Infact there will be a drop in compressive strength
and tensile strength.


Cast metal restorations.
cast metal restorations are alternative to amalgam
when higher strength is needed. Their main
advantages are high conpressive and tensile
strengths.
They are basically indicated to support the
weakened tooth like large restorations,
endodontically treated teeth, teeth at risk of
fracture.


• Resistance features.
• Flat pulpal floor
• Incluson of weakened tooth structure
• Preservation of cusps and marginal ridges
• Reduction of cuspa when indicated.
• Minimm thickness of the metal can be 1.mm.
• 30 degree marginal bevel provides burnishable metal
which protects the enamel margins.
• Resistance form can be incresaed by conservig the
tooth structure.
• a. enameloplasty
• b. marginal bevel can be used to include (eliminate)
terminal end of the fissures in outline form.

if majhor facial and lingual extension is required to
remove undermined enamel capping the weakened
cuspal structure is indicated.


• Tooth coloured restorations.
• Ideally restorations made of ooth coloured materials
should not be loaded directly ,ie., there should be
intervening tooth structure between occluding tooth
and restorations.
• Composite resins are only non cast materials that can
combine undermine enamel and cuspal elements to
sound tooth structure there by improving resistance
form.
• Consrevative composite restorations.
this typr of design is used for smallpits and fissures.
It allows for restoration of lesion or defect with minimal
removal; removal of tooth structure. It is termed as
conseravtivewww.indiandentalacademy.com
composite restoration(CCR).

Resistance features.
The design is primarily conventional design.
The thickness of th erestorative material should be
minimum1.75mm.
The roundness of the lineangles should be more
exagerrated trhen amalgam because of brittle nature
of materials.
Walls directly loaded i.e., in contact with opposing teeth
in centric and non centric occlosion should be formed
by enamel supported by dentin.
In areas where extensive caries undermines the cusps it
is advisable to skirt these areas. It is applicable for
functional cusps rather than non functional cusps.
Knife edge enamel should be trimmed and flattened.

In high stressed areas in centric an eccentric contacts it
is essential to create sufficient eeduction of tooth
structure for bulk of restoartion.
Of all the surrounding walls in class II, III andiV
preparation, the gingival wall is most important in
shaping the resistance form. It should be as
pronounced as anatomically possible
The margins should not be placed close tro the
important anatomicx struictures like incisal angle,
marginal ridge and axialangle, this will leave the tooth
bulk espicially dentinal support to these structures.


• The modofied preparation for composites conserve
more tooth structure because retention is obtained
primarily by micromechanical adhesion to the
surrounding enamel and underlying dentin, rather than
by preparation of retention grooves or coves in dentin.


• Effect of polymerization shrinkage on resistance form.
Moder n composites undergo polymerisation shrinkage
from 2.6% to 7.1%.
During polymerization resin composite may pull away
from least retentive margin resulting in gap formation.
Tensile forces developed in enamel margin can result in
margination degration mastication.
Contraction forces on cusps can result in cuspal
deformation, enamel cracks and crazes and ultimately
decreased fracture resistance of cusps.
Prevention.
Incremental palcement.
Beta quartz inserts
Auto cured resin has less polymerisation shrinkage.

• Two step or soft start polymerisation.
Initial irradiation150 mW/cm2 followed by high level
irradiation 650 mW/cm2.


Extracoronal resistance form.
Cusp capping.
It is a method of encompassing a cusp with a restorative
material to increase the resistance form of the tooth.
Indications.
If the preparatiion outline extends 2/3 the distance from the
central groove to the cusp tip.
Cusp cappng provides
1. Protection of the weakened tooth structure.
2. Remove occlusal margin from a region subjected to heavy
stress and wear.
Materials used
Amalgam
Cast gold.


• Cuspal reduction for capping for amalgam is doneby
reducing the cusp 2mm and making a flat table.
• For cast metal it is done by placing occlusal bevel ,
the Table and the counter becvel.


• Capping of margianl ridge.
• It is done when the marginal rige is
weakened and the embrasure is pronounced
• It is similar to capping a cusp but on a klesser
scale.
• C apping the ocluso proximo facial or lingual
corners of preparation.
• It is done to protect the thinned corners due
to over preparation or wide preparation in
ovoid tooth.
• Over the facial or lingual extension a bevel
extension is added ending in a knife edge or
chamfer finishing line.

• Peripheral sleeve.
• This feature add resistance for amalgam
preparation design.
• It is 2mm axial and 11mm cervicall deep
prepared feature.


Onlay.
It is partly intracoronal and pertly extracoronal
restoration which has cuspal proection as main
feature.
Mainly indicated for cuspal protection,when the width
of the lesion has exceeded ½ the intercuspal
distance. The cusps are protected by dong cusp
capping on the functional cusp and <Shoeing< on
the non functional cusps.
Resistance features.
Secondary Flare.


It is indicated when the surface extension are
required to include facial lingual defects beyond the
line angles of the tooth.
Surface extension is necessary to encompass an
axial angle for reinforcement and support.
Secondry bevel is is in the form of partial bevel. It
involves enamel only, with its maximum depth at
the main cavity preparation. It ends at the facial and
lingual surface with knife edge finishing line.


• Skirt.
• For cast metal.
• It is more extensive surface extension than
secondary flare. It is imparts retention and retention
to a missing or shortened facial or lingual wall.
• It is used
• Features.
• It includes a part of the facial and lingual surfaces
near the axial angle to a depth of 0.5 to 2mm.it ends
with a chamfer or a hollow ground bevel.
• It should be terminated in the facial or lingual groove.


• For composites.
• In areas where extensive caries undermines the
cusps it is advisable to skirt these areas. It is
applicable for functional cusps rather than non
functional cusps


Collar.
This type of surface extension is most involving surface
wise and depth wise.i could be
a. cuspal collar. (ivolving one cusp )
b. Tooth collat. (Involving the entire facial or
lingual surface)
The help in retention and resistance when the entire cusp is lost.
A collar can accommodate both metal and porcelin b.ulk in PFM
restoration.
Features.
It has axial depth of 1.5 to 2mm.
It ends gingivally in bevelled shoulder finishing line.
There should be a definite line angle at the shoulder and axial
collar. www.indiandentalacademy.com

• STEP 3 PRIMARY RETENTION
• Definition:
• Is that shape (or) form of the prepared
cavity that resists displacement (or)
removal of the restoration from tipping (or)
lifting forces.
• According to their locations, retention
means can be classified as
• 1. Intracoronal (With in the tooth)
• 2.Extracoronal (On preparation surface
• replacing external surface )


• PRINCIPLES MEANS OF RETENTION :
• 1.Frictional retention, depends on 4 factors:
• a)Surface area of contact between tooth structure and
restorative material. Greater surface area produces a
greater frictional component of retention.
• b)More opposing walls / surfaces involved greater
retention and a more stable restoration within the
preparation.
• c)Parallelism & non– parallelism, a higher degree of
parallelism between opposing walls. Higher convergence
of walls – in intracoronal preparation. Higher divergence
of walls – in extra coronal preparation, provides a greater
locking ability of the tooth to the restorative material.
• d)Proximity : bringing the material closer to tooth
structure during insertion will increase the frictional
retention.

• 2.Elastic deformation of dentin :
• Changing position of dentinal walls & floors microscopically
by using condensation energy within the dentin’s
proportional limit, can add more gripping action by the tooth
on the restorative material. This occurs when the dentin
regains its original position while the restorative material
remains rigid thereby completely obliterating any remaining
space in the cavity preparation.
• 3.Inverted truncated cones (or) undercuts.
• 4.Dovetail : This is a purposeful modification in outline form
in some cases but usually extension for prevention will
create a dovetail outline.
• 5.Mechanical interlocking with cements counteracts pull
due to sticky foods.


Means of retention for materials.
Amalgam.
Primary Retention features.
1. Mechanical locking into the surface irregularities
2. Preparation of vertical walls which converge
occlusally
3. Special retentive features such as locks, grooves
pins steps and amalgapins
4. Bonding of the amalgam (Optional)
Retention locks.
They are placed in the axiogifacial and axiolingual line
angle(S p 706)

• Groove extensions.

• Occlusal dove tail.
• Secondry retention form.
• Slot.
• A slot is a retention groove in dentin whose length is in
a horizontal plone
• Slot retention may be used in conjunction with pin
retention or as an alternative to it .
• Outwaire et el reported pin retained amalgam has
more tendency to slip whereas slippage does not
occur in slots.
• Preparation.
• Using a No. 33 ½ bur a slot is placed in the gingival
floor 0.5mm www.indiandentalacademy.com
axial of the DEJ.

• Slots in the gingival floor may be used to
provide additional retention in an extensive
proximal box. Slot dimension will depend on
the size of the proximal box.
• Normal Dimensions of the slot
• 0.5mm to 1mm- Deep gingivally
• 2 mm to 3 mm - Length
faciolingually
• 0.2mm to 0.3mm - Inside DE
Junction.


• Groove.
• In classII, III and Class V
• Features.
• Prepared b y ¼(0.5mmdiameter) round bur.
• Groove is prepared by placing the bur at the line
angle cutting half the depth of bur i.e.,o.25mm..

• Cove.25 mm into dentin.
• They are`palced inj cl III amalgam
• They are palced at axiogingivofacial and
axiogingivolingual point angles. .


• Retention of amalgam in special situations.
• In Box type preparation.
When restoring a small, cavitated, proximal lesion in
tooth, a proximal box preparation is recommended.
To maximize retention, preparations with facial and
lingual walls that almost oppose each other are
advised.
• To compensate for the lack of an occlusal
preparation, the proximal retentive locks should have
a 0.5mm depth at the gingival point angle, tapering to
a depth of 0.3mm at occlusal surface
• In slot type preparationsentive grooves are given at
the occlusoaxial and gingivoaxial line angles.
•

• Cusp capping.
• Cusp reduction significantly decreases
retention form caused by loss of height of
vertical walls. When additional retention is
indicated, slots and pot holes can be
prepared along the gingival floor, 0.2mm
pulpally from the ‘DE’ junction.


• Pin retained amalgam restorations.
• DEFINITION
• A Pin retained restoration may be defined
as any restoration requiring the placement of
one or more pins in the dentin to provide
adequate retention form.


• INDICATIONS
• For restoration of mutilated and badly broken down teeth
especially in young patients where the gingival lines are still
high, where massive tooth preparations necessitated by cast
restorations are contraindicated, pins are required for
auxiliary retention.
• In badly broken down teeth, prior to endodontic or
orthodontic treatment, pin retained restorations are placed as
a transitional restoration to act as a build up for rubber dam
application or band attachment.
• As a foundation for partial or full veneer cast restoration or
metal ceramic restorations, thereby saving considerable
amount of tooth structure by eliminating the need to remove
undercuts.
• As a provisional restoration in teeth with questionable
prognosis endodontically or periodontally, until a definitive
prognosis is established.
• In preparations where adequate retention form cannot be
established with slots, locks or undercuts, pins are used as a
means of providing auxiliary retention.

CEMENTED PINS FRICTIONAL GRIP THREADED PIN

• .
Pin channel is larger in diameter than 1. Pin channel is slightly narrower 1. Pin diameter is narrower than
pin. in diameter than pin. that of the pin.
Pin Channel Diameter 2. Pin Channel Diameter 2. Pin channel diameter
- 0.020 0.021 0.027
- 0.021 3. Pin diameter 0.021
Pin Diameter 0.022 0.018
- 0.025 It is 2-3 times more retentive than 0.013
- 0.020 cemented pin. 3. Pin diameter
0.031
0.023
0.020
0.015
It is 3-6 times more retentive than
cemented pin.


• FACTORS AFFECTING THE RETENTION OF THE PIN IN
DENTIN AND AMALGAM
• Type of pin: In order of retentiveness, self-threading pin is
most retentive, friction lock pin is intermediate and cemented
pin is the least retentive.
• Surface characteristics: Retention of the pin in amalgam is
influenced by the number and depth of elevations in the pin.
Therefore, self-threading pins are most retentive.
• Orientation of the pins: Retention of the pins is increased by
placing them in a non-parallel manner.
• Number of pins: Within limits, increasing the number of pins
increases the retention in dentin and amalgam.
• Pins placed closer than 2 mm in dentin to each other in one
tooth will result in
• Crazing of dentin and increased potential for fracture.
• Decrease in the amount of available dentin between the pins.
• Strength of amalgam restoration decreases.


• Diameter of the pins: Within limits, as the
diameter of the pin increases the retention in
dentin and amalgam increases.
• Extension into dentin and amalgam: Pin
extension into dentin and amalgam greater
than 2 mm is unnecessary for pin retention
and contraindicated to preserve the strength
of the dentin and amalgam.


• Amalgam Foundations.
• It is an initial restoartion of broken down tooth . The
tooth is restored so that it will provide resistance and
retention fprms needed for indirect cast restoration.
• It also provide resistance form against forces that
otherwise might fracture tooth structure.
• It does not depend primarily on coronal; tooth
structure instead it rely mainly on secondary
preparation retention features(Pins, slot,coves
and`proximal retention locks.)


• CHAMBER RETENTION :-
• A Nagyar et al has described technique for
developing foundations in multi rooted endodontically
treated teeth. It is recommended only
• Dimension to the pulp chamber is adequate to
provide retention and bulk of amalgam.
• Dentin thickness in the region of pulp chamber is
adequate to provide rigidity and strength to the tooth.
• Extension into the root canal space 2 to 4mm in
recommend when pulp chamber height its 2mm or
less
• When pulp chamber height is more than 6mm no
adavantage is gained from extending into root canal.


• Amalaga pin
• These are circular`chambers that are cut in the dentin
to provide rewsistance and retention of the restoration.
These are also called Amalgam inserts.
• They provide retention similar to self thresded pins.
• It is 1.5 to 2mm in depth and 0.8mm in diameter.


• Cast metal restoration.
• – Principle retentive feature is close parallelism
of longitudinal walls. Small range for
divergence (2-5 degrees / wall) from the line of
draw will enhance retention form (concept of
taper)
• Since exact parallelism create technical
problems in processingl,a slight taper is
desired.
• The average taper is 2-5 degrees.
• The taper can be altered by following afctore
• 1.length of the preparation.

• 3. need for retention.

• Bevels.

• Grooves.
• These are indicated when th eprepaartion walls are
short.

• Groove extensions.
 Well defined line angles are also important in
obtaining resistance and retention form.


Secondary modes of retention :

1) Luting cement : fills gap between inlay
and tooth giving a physio chemical
bonding. Physical – Zinc phosphate,
chemical : glass inomer +
polycorboxylate. The exposed cement
dissolves in the oral cavity and so
should not be considered as main
retentive factor.


2) Grooves : Place two
grooves, one on bucco
axial and other on lingual
axial line angle with #165
bur. It is totally placed on
dentine. Depth of groove
should be 0.3mm at the
expense of buccal and
lingual walls and never at
expense of axial walls.


3)Reverse Bevel :
It is given on
gingival seat. This bevel has
generally three planes, i)
reverse bevel plane where
inclination is on gingivoaxial
plane which prevents
proximal displacement.
ii) secondly, flat plane made
of dentine.
iii) thirdly ,plane which is
sloping away from the axial
wall made of enamel and
dentine, www.indiandentalacademy.com
this helps in

4) Internal box:
made on the
pulpal floor, which
improves the retention by
4-5 times. this is on the
uninvolved side. it should
not have sharp line and
point angles and definite
walls. This prevents
micromovement of the
inlay. Internal box should
always be reciprocated
with a reverse bevel or
groove to avoid
micromovement.

5) External box : these are box
shaped preparations opening
to axial tooth surface .they can
be proximal, facial or lingual.
They can be either stepped
occlusally or gingivally.


6) Roughening of pulpal floor :
at specific areas of tooth
preparation, esp. in pulpal
floor, is done for more
retentive and laterally locking.
irregularities should have no
frail or undermined enamel.
Creating different levels out of
flat, dished up, gingival or
pulpal floors could change a
mechanically negative
situation into a positive one.


• 7) Precementation grooves : after casting
make grooves on the walls of the inlay
and/or grooves on the cavity wall exactly
opposite. This will house with solid mass of
cement which helps in good retention


8)Electrolytic etching of inlay :

This gives hinge like projections.

Procedure : Protect proximal and occlusal
surface with sticky wax and keep it in the
electrolyte solution of 0.5 normal nitric acid.
Inlay is kept in anode and metal
with increased EMF as cathode.(316
stainless steel )


current causes microporosities on fitting
surface which enables the luting cement to
flow into it for better retention. This
process takes place for around 10 to 15
mins.


It is usually advantageous to use a diamond stone for
preparing the tooth for a composite restoration. This
results in a roughened prepared surface, which
increases the surface for bonding.
• The modified preparation for composites conserve
more tooth structure because retention is obtained
primarily by micromechanical adhesion to the
surrounding enamel and underlying dentin, rather than
by preparation of retention grooves or coves in dentin.


ADVANTAGES OF ENAMEL
BEVEL
 The ends of the rods are more
effectively etched than
otherwise only the sides of the
enamel rods are exposed.
 Increase in etched surface
area results in a stronger
enamel to resin bond, which
increases retention of the
restoration and reduces
marginal leakage and marginal
discoloration.
 Incorporation of cavosurface
bevel may enable the
restoration to blend more
esthetically with the coloration
of the surrounding tooth
structure.


Even recognizing these
advantages, bevels are not
usually placed on the
occlusal surfaces of
posterior teeth or other areas
of potential heavy contact
because a conventional
preparation design already
produces end on etching of
the enamel rods by virtue of
the enamel rod direction on
occlusal surfaces
• Bevels are not placed on
proximal margins if such
beveling results in excessive
extension of the cavosurface
margins.
• Therefore this design is rarely
used for posterior composite
restorations.


• For restoration of large carious lesions, wider bevels
or flares and retention grooves, coves or locks may be
indicated (similar to amalgam) in addition to the
retention afforded by the adhesive procedures.
• Class III
• The boxlike design is considered a part of retention
form.the external walls are retentive because of
opposing wall parallelism or slight undercuts.
• Retention groove may help in minimizing the potential
negative effects of polymerization shrinkage and
increae retention.. It is palced .25 bb bisecting external
wall and axial wall. It is not indicated in bevelled
conventional preparation.


• Coves are prepared with no.1/4 bur at the axioincisal
point angle with the bur oriented in a similar angle,0.2
mm inside the DEJ and 0.25mm deep.


• Cleass IV.
• Gingival and incisal
retentive undercuts may
be indicated in large class
IV preparations in which
rounded undercuts are
placed in the dentin along
line angles and into point
angles wherever
possible.


• Retention form is provided primarily
by the micromechanical bonding of
the composite to the enamel and
dentin.
• Additional retention may be
obtained by increasing the width of
the enamel bevels or placing
retention undercuts.
• If retention undercuts are deemed
necessary prepare a gingival
retention groove using a no ¼
round bur. It is prepared 0.2mm
inside the DEJ at a depth of 0.25
mm and at angle bisecting the
junction of the axial wall and
gingival wall.


• Class V.
If retention grooves are necessary they are
prepared with a no.1/4 bur along the full length
of the gingivoaxial and incisoaxial line angles.
• For bevelled conventionalGroove retention
usually is not indicated when the periphery of
the tooth preparation is located in enamel.
Many of these larger preparations will be a
combination of beveled enamel margins and
90-degree root-surface (non enamel) margins,
with the root surface areas having groove
retention .
•

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