Indirect retention /orthodontic nursing course

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IIndirect retentionndirect retention is required on allis required on all extension-extension-
base partial denturesbase partial dentures retained byretained by clasps.clasps.

It is required also when the tooth atIt is required also when the tooth at one endone end
of a denture base is clasped for retention butof a denture base is clasped for retention but
the tooth at the other end isthe tooth at the other end is not suitablenot suitable forfor
retention.retention. Extension-base partial dentures areExtension-base partial dentures are
needed more often than any other type ofneeded more often than any other type of
removable partial denture.removable partial denture. If theseIf these
statements are true, then we need to considerstatements are true, then we need to consider
indirect retention carefully.indirect retention carefully.

KennedyKennedy wrote thatwrote that CummerCummer advanced the ideaadvanced the idea
ofof indirect retentionindirect retention and had revolutionizedand had revolutionized
partial denture designingpartial denture designing.. CummerCummer reasonedreasoned
that the rationale of indirect retention was athat the rationale of indirect retention was a
modification of a C 1 levermodification of a C 1 lever. Indirect retention. Indirect retention
was a valuable contribution to partial denturewas a valuable contribution to partial denture
design. However, it is a much better example ofdesign. However, it is a much better example of
aa Class 2 leverClass 2 lever than, athan, a Class 1 lever.Class 1 lever.

Levers are classified according to theLevers are classified according to the
location of thelocation of the fulcrum,fulcrum, thethe powerpower,, and theand the
resistanceresistance in respect to each other. Therein respect to each other. There
areare threethree classes.classes.

In Class 1, the fulcrum is between the power and theIn Class 1, the fulcrum is between the power and the
resistance. The resulting mechanical advantageresistance. The resulting mechanical advantage cancan
be greater or less than onebe greater or less than one depending upon thedepending upon the
locationlocation of the fulcrum along the lever.of the fulcrum along the lever.

The Class 2 lever has theThe Class 2 lever has the resistanceresistance betweenbetween thethe
fulcrumfulcrum and theand the powerpower, and has a mechanical, and has a mechanical
advantage that isadvantage that is always greater than one.always greater than one.

In Class 3, theIn Class 3, the powerpower is appliedis applied betweenbetween thethe
fulcrumfulcrum and theand the resistanceresistance. Its mechanical. Its mechanical
advantage isadvantage is always less than one.always less than one.

TheThe mechanical advantagemechanical advantage of a lever is expressed asof a lever is expressed as
thethe quotient of the power arm length divided by thequotient of the power arm length divided by the
resistance arm length.resistance arm length.

Indirect retention is an application of a Class 2 leverIndirect retention is an application of a Class 2 lever

The tooth surface on which the indirectThe tooth surface on which the indirect
retention rests is theretention rests is the fulcrumfulcrum. The retentive. The retentive
part of the direct retainer clasp becomes thepart of the direct retainer clasp becomes the
resistanceresistance, and the, and the powerpower is represented byis represented by
any force that tends to move the dentureany force that tends to move the denture
base away from the underlying tissue.base away from the underlying tissue.

TheThe resistance arm lengthresistance arm length, then, is the, then, is the
distance between the retentive part of thedistance between the retentive part of the
direct retainer clasp and the pivotal edge ofdirect retainer clasp and the pivotal edge of
the indirect retention. The potential length ofthe indirect retention. The potential length of
thethe power armpower arm is the distance between theis the distance between the
free end of the denture base and the pivotalfree end of the denture base and the pivotal
edge of the indirect retention.edge of the indirect retention.

InIn all probabilityall probability, the power arm, the power arm never functionsnever functions at itsat its
full potential lengthfull potential length because forces that tend to unseatbecause forces that tend to unseat
the base act most often on thethe base act most often on the teeth on the baseteeth on the base..
InIn function,function, therefore, thetherefore, the length of the power armlength of the power arm isis
shortershorter than its potential length.than its potential length.

A list of factors thatA list of factors that influence the effectiveness ofinfluence the effectiveness of
indirect retentionindirect retention must include the following:must include the following:

1.1. The effectiveness of the direct retention (The effectiveness of the direct retention (thethe
resistanceresistance)-)-Indirect retention cannot be moreIndirect retention cannot be more
effective than the direct retention of the clasps.effective than the direct retention of the clasps.
Actually, indirect retention does not begin toActually, indirect retention does not begin to
function until the physiologic retentionfunction until the physiologic retention
contributed by the extension base is overcomecontributed by the extension base is overcome
and, simultaneously, the resistance offered byand, simultaneously, the resistance offered by
the direct retention is challenged. Therefore,the direct retention is challenged. Therefore, thethe
direct retention must be adequate.direct retention must be adequate.

2.2. The effectiveness of the tooth ant its rest seatThe effectiveness of the tooth ant its rest seat
((the fulcrumthe fulcrum) on which the indirect retention) on which the indirect retention
rests or pivotsrests or pivots.- A fulcrum must offer positive.- A fulcrum must offer positive
support for a lever to function most effectively.support for a lever to function most effectively.

Its efficiency may be reduced if indirect retention isIts efficiency may be reduced if indirect retention is
placed on inclined surfaces of teeth. It should beplaced on inclined surfaces of teeth. It should be
placed inplaced in rest seatsrest seats on teeth capable of supporting itson teeth capable of supporting its
function. The capability of any tooth to support thefunction. The capability of any tooth to support the
function of a rest can be enhanced by preparing thefunction of a rest can be enhanced by preparing the
rest seat so thatrest seat so that forcesforces are directedare directed parallelparallel to theto the
long axislong axis of the tooth.of the tooth.

3.3. The rigidity of the major and the minorThe rigidity of the major and the minor
connectors (connectors (the leverthe lever).-).-A lever must be rigid toA lever must be rigid to
effect a positive action. This means that theeffect a positive action. This means that the
connectorsconnectors must bemust be rigidrigid..

4.4. The mechanical advantage of the lever-The mechanical advantage of the lever- TheThe
lower the mechanical advantage, the more liftinglower the mechanical advantage, the more lifting
force is necessary on the extension base toforce is necessary on the extension base to
overcome the resistance of the direct retentionovercome the resistance of the direct retention
and move the base off its seat - in other words,and move the base off its seat - in other words,
decreasedecrease the mechanical advantage of the leverthe mechanical advantage of the lever
andand increaseincrease the effectiveness of the indirectthe effectiveness of the indirect
retentionretention. This is the. This is the key factorkey factor in indirectin indirect
retention.retention.

Basically, there areBasically, there are two waystwo ways to reduce theto reduce the
mechanical advantage of a Class 2 lever.mechanical advantage of a Class 2 lever.
OneOne is tois to shorten the power arm at the power endshorten the power arm at the power end
of the leverof the lever. If the power arm is shortened at the. If the power arm is shortened at the
fulcrum end, the mechanical advantage will befulcrum end, the mechanical advantage will be
increased because the resistance arm is shortenedincreased because the resistance arm is shortened
the same amount but proportionately more.the same amount but proportionately more.

Since forces that tend to unseat extension bases actSince forces that tend to unseat extension bases act
most often on the teeth on the bases, themost often on the teeth on the bases, the
mesiodistal span of these teeth should be kept to amesiodistal span of these teeth should be kept to a
minimum to keep theminimum to keep the effective length of the powereffective length of the power
arm to a minimumarm to a minimum. Where possible,. Where possible, smaller teethsmaller teeth
should be used to reduce the span rather thanshould be used to reduce the span rather than
reduce the number of larger teeth. The smallerreduce the number of larger teeth. The smaller
teeth not onlyteeth not only reduce the mesiodistal spanreduce the mesiodistal span but alsobut also
thethe buccolingual widthbuccolingual width, thus presenting a, thus presenting a smallersmaller
area for application of force.area for application of force.

The denture bases should not be shortened toThe denture bases should not be shortened to
decrease the potential length of the power armdecrease the potential length of the power arm
because thebecause the base itself contributes physiologicbase itself contributes physiologic
retentionretention through its intimate relationship with thethrough its intimate relationship with the
underlying tissue. Moreover, the retention isunderlying tissue. Moreover, the retention is
proportionateproportionate to the area covered by the dentureto the area covered by the denture
base.base.

TheThe other wayother way to reduce the mechanical advantageto reduce the mechanical advantage
of a Class 2 lever is to lengthen the resistance armof a Class 2 lever is to lengthen the resistance arm
at either of its ends. The optimum length of theat either of its ends. The optimum length of the
resistance arm can be attained on extension-baseresistance arm can be attained on extension-base
partial dentures by placing the indirect retention aspartial dentures by placing the indirect retention as
far from the denture base as feasible and byfar from the denture base as feasible and by
placing the retentive clasp tips of the directplacing the retentive clasp tips of the direct
retainer as near to the denture base as possible.retainer as near to the denture base as possible.

TheThe objectiveobjective is tois to increase the distance between theincrease the distance between the
direct and indirect retention as much as feasible.direct and indirect retention as much as feasible.

MMajor connectorajor connector stabilizers transmit forces fromstabilizers transmit forces from
the major connector to the teeth and other tissues,the major connector to the teeth and other tissues,
and vice versa. They serve many functions. Itand vice versa. They serve many functions. It helpshelps
toto stabilizestabilize the major connector against vertical andthe major connector against vertical and
lateral forces, andlateral forces, and decreasesdecreases stresses on thestresses on the
principal abutment teeth by transmitting forces toprincipal abutment teeth by transmitting forces to
other teeth in the arch.other teeth in the arch.

Thus, the so-called indirect retainerThus, the so-called indirect retainer servesserves
many functionsmany functions and is really a majorand is really a major
connector stabilizer that offers indirectconnector stabilizer that offers indirect
retention as its primary function.retention as its primary function.

Any minor connector plus its rest (s) -incisal,Any minor connector plus its rest (s) -incisal,
cingulum, ledge, or occlusal-on tooth surface (s) is acingulum, ledge, or occlusal-on tooth surface (s) is a
major connector stabilizermajor connector stabilizer. Clasps for stabilization. Clasps for stabilization
and retention may be included. At least three types ofand retention may be included. At least three types of
major connector stabilizers employing the minormajor connector stabilizers employing the minor
connector are possible.connector are possible.

OneOne is the minor connector plus its rest (s) withoutis the minor connector plus its rest (s) without
clasps.clasps. AnotherAnother is the minor connector plus its rest (s)is the minor connector plus its rest (s)
with clasps for stabilization only. Awith clasps for stabilization only. A thirdthird is the minoris the minor
connector plus its rest (s) with clasps for bothconnector plus its rest (s) with clasps for both
stabilization and retention.stabilization and retention.
Any of the 3 may serve as indirect retention, but theAny of the 3 may serve as indirect retention, but the
first is used most oftenfirst is used most often for that purpose.for that purpose.

Other partial denture units that serve to someOther partial denture units that serve to some
extent as major connector stabilizers are theextent as major connector stabilizers are the
secondary lingual barsecondary lingual bar, the, the lingual platelingual plate andand
thethe extension baseextension base. Any of these can. Any of these can
function as indirect retention.function as indirect retention.

Their efficiency however may be reducedTheir efficiency however may be reduced
since the secondary lingual bar and the lingualsince the secondary lingual bar and the lingual
plateplate rest on inclined tooth surfacesrest on inclined tooth surfaces, and the, and the
denture base rests on soft tissue. Thedenture base rests on soft tissue. The rugaerugae
areaarea of the palate is covered sometimes forof the palate is covered sometimes for
indirect retentionindirect retention, but rests on the teeth are, but rests on the teeth are
much more effective.much more effective.

A secondary lingual bar and a lingual plateA secondary lingual bar and a lingual plate
alsoalso tend to stabilizetend to stabilize the teeth that theythe teeth that they
contact. That is, theycontact. That is, they preventprevent lingual tippinglingual tipping
of the lower anterior teeth by theof the lower anterior teeth by the occludingoccluding
forcesforces of the upper anterior teeth and theof the upper anterior teeth and the
muscle actionmuscle action of the lower lip.of the lower lip.

On extension-base partial dentures with basesOn extension-base partial dentures with bases
extending distally, the fulcrum line is usuallyextending distally, the fulcrum line is usually
defined as andefined as an imaginary line passing through theimaginary line passing through the
rest seat in the most posterior abutment tooth onrest seat in the most posterior abutment tooth on
each side of the archeach side of the arch. Then the fulcrum line passes. Then the fulcrum line passes
through the rest area of the most anterior abutmentthrough the rest area of the most anterior abutment
tooth on each side of the arch when the denturetooth on each side of the arch when the denture
base extends mesially.base extends mesially.

For these definitions to be valid, theFor these definitions to be valid, the
principal abutment teeth must have rests onprincipal abutment teeth must have rests on
themthem. This is not always true, since some. This is not always true, since some
partial dentures do not have a rest on thepartial dentures do not have a rest on the
principal abutment tooth but on the nextprincipal abutment tooth but on the next
tooth in the arch.tooth in the arch.

Moreover, aMoreover, a second fulcrum linesecond fulcrum line may bemay be
established when indirect retention is present.established when indirect retention is present.

Therefore, “Therefore, “the fulcrum linethe fulcrum line” should be redefined” should be redefined
and referred to as the “and referred to as the “primary fulcrum lineprimary fulcrum line” to” to
distinguish it from the secondary fulcrum line. Thedistinguish it from the secondary fulcrum line. The
fulcrum line on distal extension partial dentures,fulcrum line on distal extension partial dentures,
then, isthen, is an imaginary line passing through the mostan imaginary line passing through the most
distal rest seat (of a tooth) on each side of the arch.distal rest seat (of a tooth) on each side of the arch.

If the denture base extends mesially, theIf the denture base extends mesially, the
primary fulcrum line passes through theprimary fulcrum line passes through the
most mesial rest seat on each side of themost mesial rest seat on each side of the
arch.arch.

When a second fulcrum line is produced by indirect retention,

the prosthesis rotates about the primary fulcrum line if
the denture base is forced tissueward,

and about the secondary fulcrum line if the dentureand about the secondary fulcrum line if the denture
base is lifted off the tissue.base is lifted off the tissue.

However, if a second fulcrum line is notHowever, if a second fulcrum line is not
produced by indirect retention,produced by indirect retention,

the prosthesis rotates about the primary fulcrum linethe prosthesis rotates about the primary fulcrum line
whether forces move the denture base toward orwhether forces move the denture base toward or
away from the tissue. Theseaway from the tissue. These fulcrum linesfulcrum lines are notare not
only the fulcra produced by vertical stops on theonly the fulcra produced by vertical stops on the
teeth, but the others are irrelevant to this discussionteeth, but the others are irrelevant to this discussion
since they are not concerned with indirect retention.since they are not concerned with indirect retention.

These designs emphasize the fact that indirect
retention is located in relation to the retentive part
of the direct retainer clasps, not to the primary
fulcrum line.

As a matter of fact,As a matter of fact, indirect retention does oneindirect retention does one
thingthing: it prevents the retentive part of the direct: it prevents the retentive part of the direct
retainer clasps from becoming a fulcrum or point ofretainer clasps from becoming a fulcrum or point of
rotation about which the prosthesis could rotaterotation about which the prosthesis could rotate
when forces move the denture base(s) away fromwhen forces move the denture base(s) away from
the underlying tissue.the underlying tissue.

When rotation about the clasp tips is prevented byWhen rotation about the clasp tips is prevented by
any unit(s) of the partial denture, the dentureany unit(s) of the partial denture, the denture
base(s) cannot move away from the tissue until thebase(s) cannot move away from the tissue until the
resistance to deformation of the retentive clasps isresistance to deformation of the retentive clasps is
overcome.overcome. IndirectlyIndirectly then, partial denture unitsthen, partial denture units
preventing this rotationpreventing this rotation retainretain the denture bases inthe denture bases in
contact with the underlying tissue, hence the namecontact with the underlying tissue, hence the name
indirect retentionindirect retention..

The principle of indirect retention is shown to beThe principle of indirect retention is shown to be
an application of aan application of a Class 2 leverClass 2 lever. The so-called. The so-called
indirect retainer is aindirect retainer is a major connector stabilizermajor connector stabilizer. A. A
secondary fulcrum linesecondary fulcrum line is usually established whenis usually established when
partial design includes indirect retention. Indirectpartial design includes indirect retention. Indirect
retentionretention preventsprevents the retentive clasp tips fromthe retentive clasp tips from
becoming a fulcrum about which the prosthesisbecoming a fulcrum about which the prosthesis
could rotate when forces move the denture basecould rotate when forces move the denture base
away from the tissue.away from the tissue.

1.1. Kennedy E.Kennedy E. : Partial denture construction.: Partial denture construction.
2.2. Cummer WECummer WE. : Partial denture service.. : Partial denture service.
3.3. McCrackenMcCracken WEWE. : Partial denture construction. : Partial denture construction
4.4. Steffel V LSteffel V L. Planning removable partial dentures.. Planning removable partial dentures.
J Prosthet DentJ Prosthet Dent 19621962;; 22:: 3535..

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Indirect retention /orthodontic nursing course

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