7direct retainers ii

8/12/2019 7Direct Retainers II

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119

Fig. 7-31 Back-action circumferential clasp used on

premolar abutment anterior to edentulous space.

Chapter

7

Direct

retainers

Fig. 7-30 Ring clasp ma be used in re!erse onabutment located anterior to tooth-bound edentulousspace.

Fig.7-3" #mbrasure clasp used $here no edentulousspace e%ists. &lthough in this dra$ing both retenti!eclasp arms are located on buccal surface and nonretenti!earms on lingual surface' retention and reciprocation can

be re!ersed on both teeth or on either tooth' depending onrespecti!e contours of the teeth. (o$e!er' if second

molar is sound and suitable stabili)ing and retenti!e areascan be found' circumferential clasp originating on distal surface of abutment is

preferable.

ti!e areas are a!ailable or $hen multiple restorations are

*ustified' clasping can be accomplished b means of an

embrasure clasp +Figs. 7-3" and 7-33,.

ufficient space must be pro!ided bet$een the

abutment teeth in their occlusal third to make room for

the common bod of the embrasure clasp +Fig. 7-3,' et

the contact area should not be eliminated entirel.

(istoricall' this clasp assembl demonstrates a high per-

centage of fracture caused b inade/uate tooth

preparation in the contact area. Because !ulnerable areas

of the teeth are in!ol!ed' abutment protection $ith inlas

or cro$ns is recommended. he decision to use

unprotected abutments must be made at the time of oral

e%amination and should be based on the patients age'

caries inde%' and oral hgiene' as $ell as on $hether

e%isting tooth contours are fa!orable or can be made

fa!orable b tooth modification. 2reparation of ad*acent'

contacting' uncro$ned abutments to recei!e an tpe

connector and should ne!er be supported b a clasp arm

alone. f the occlusal rest is part of a fle%ible assembl'

it can4,.ot function ade/uatel as an occlusal rest.

Embrasure clasp

n the fabrication of an unmodified Class or Class

partial denture' there are no edentulous spaces on the

opposite side of the arch to aid in clasping.

5echanicall' this is a disad!antage. (o$e!er' $hen

the teeth are sound and reten


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5cCrackens remo!able partial prosthodontics

Fig. 7-33 5ultiple clasping in surgicall mutilatedmouth. 6n the right are embrasure clasp' bar clasp arm'and con!entional circumferential clasp engaging lingualundercuts on three abutment teeth. 6n the left is $ell-designed ring clasp engaging lingual undercut' $ithsupporting strut on buccal surface and au%iliar occlusalrest to pre!ent mesial tipping. ote rigid design of ma*orconnector.

1"0

&

B

Fig. 7-38 &' #%ample of use of embrasure clasp for aClass partiall edentulous arch. #mbrasure clasp ont$o left molar abutments $as used in the absence of

posterior modification space. B' 6cclusal and pro%imalsurfaces of ad*acent molar and premolar prepared forembrasure clasp. ote that rest seat preparations aree%tended both buccall and linguall to accommodateretenti!e and reciprocal clasp arms. Adequate

preparation confined to enamel can rarely beaccomplished for such a clasp, especially when claspedteeth are opposed by natural teeth.

pro%imal shoulders can be established +Fig. 7-38,. his is

done to a!oid interpro%imal $edging b the prosthesis'

$hich could cause separation of the abutment teeth and

result in food impaction and clasp displacement. n

addition to pro!iding support' occlusal rests also ser!e to

shunt food a$a from contact areas. For this reason'

occlusal rests should al$as be used $hene!er food

impaction is possible.

#mbrasure clasps should ha!e t$o retenti!e clasp

arms and t$o reciprocal clasp arms' either bilaterall or

diagonall opposed. &n au%iliar occlusal rest or a bar

clasp arm can be substituted for a circumferential

reciprocal arm as long as definite reciprocation and

stabili)ation result.

Fig. 7-3 Embrasure and hairpin circumferentialretenti!e clasp arms. he terminus of each engagessuitable retenti!e undercut. se of hairpin-tpe clasp onsecond molar is made necessar b the fact that the onla!ailable undercut li:s directl belo$ point of origin ofclasp arm.

of embrasure clasp of ade/uate interpro%imal bulk isdifficult' especiall $hen opposed b natural teeth.

he embrasure clasp al$as should be used $ithdouble occlusal rests' e!en $hen definite


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Fig. 7-37 5ultiple clasp is actuall t$o opposing

circumferential clasps *oined at terminal end of t$oreciprocal arms (mirror view).

Lingual

Buccl

Fig. 7-3; mproper application of embrasure claspdesign (mirror view). Failure to locate retenti!e andreciprocating-stabili)ing arms in most ad!antageous

positions +proper third of cro$ns, is /uite e!ident.

f'

0 degrees.

sstem +the R2& clasp, should be considered +Fig. 7-8'

#). &pplication of each approach is predicated on the

distribution of load to be applied to the tooth and

edentulous ridge.

n most situations the bar clasp arm can be used $ith

tooth-supported partial dentures' $ith tooth-supported

modification areas' or $hen an undercut that can be

logicall approached $ith a bar clasp arm lies on the side

of an abutment tooth ad*acent to a distal e%tension base

+Figs. 7-; through 7-80,. f a tissue undercut pre!ents

the use of a bar clasp arm' a mesiall originating ring

clasp' a cast' or a $rought-$ire clasp or re!erse-action

clasp ma be used. 2reparation of ad*acent abutments

+natural teeth, to recei!e an tpe of interpro%imal direct

retainer' tra!ersing from lingual to buccal surfaces' is

most difficult to ade/uatel accomplish. ne!itabl the

relati!e sie of the occlusal table is increased'

contributing to undesirable and additional functional

loading.

he bar clasp arm is not a particularl fle%ible clasp

arm because of the effects of its half-round form and its

se!eral planes of origin. &lthough the cast

circumferential clasp arm can be made more fle%ible than

can the bar clasp arm' the combination clasp is preferred

for use on terminal abutments $hen tor/ue and tipping

are possible because of engaging an undercut


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Fig. 7-47 Bar clasp arm on distal abutment must bemade light enough to be fle%ible and ma be used onl$hen it can engage pro%imal undercut ad*acent toe%tension base. 5esial -portion of clasp arm had to

be placed to encompass abutment b more than 180degrees. t is placed on height of contour. otefinishing line (arrow) $here clasp and denture

base $ill *oin.

a$a from the distal e%tension base. ituations often

e%ist' ho$e!er' in $hich a bar clasp arm ma be used

to ad!antage $ithout *eopardi)ing a terminal

abutment. & bar clasp arm s$inging distall into the

undercut ma be a logical choice' since mo!ement of

the clasp on the abutment as the distal e%tension base

mo!es tissue$ard is minimi)ed b the distal location

of the clasp terminal.

he specific indications for using a bar clasp arm

are +1, $hen a small degree of undercut +0.01 inch,

e%ists in the cer!ical third of the abutment tooth'

$hich ma be approached from a gingi!al direction?

+", on abutment teeth for tooth-supported partial

dentures or toothsupported modification areas +Fig. 7-

81,? +3, in distal e%tension base situations? and +, in

situations in $hich esthetic considerations must be

accommodated and a cast clasp is indicated. hus use

of the bar clasp arm is contraindicated

$hen a deep cer!ical undercut e%ists or $hen a

se!ere tooth andGor tissue undercut e%ists' either of

$hich must be bridged b e%cessi!e blockout. Hhen

se!ere tooth and tissue undercuts e%ist' a bar clasp

arm usuall is an annoance to the tongue and cheek

and also traps food debris.

Chapter 7 =irect retainers 1"7

Fig. 7-> Bar clasp arm on ma%illar terminal abutment.ote uniform taper from point $here it $ill emerge fromresin base and that it engages tooth undercut on sidead*acent to distal e%tlmsion base. Butt-tpe *oint forfinishing line bet$een direct retainer and acrlic resin

base is pro!ided.

6ther limiting factors in the selection of a bar clasp

assembl include a shallo$ !estibule or an e%cessi!e

buccal or lingual tilt of the abutment tooth +Fig. 7-8",.

ome common errors in the design of bar-tpe clasps are

illustrated in Fig. 7-83.

here are se!eral other tpes of bar clasps' one of

$hich is the infrabulge clasp. t is designed so that the

bar arm arises from the border of the denture base' either

as an e%tension of a cast base or attached to the border of

a resin base +Fig. 7-8,. t is made more fle%ible than the

usual bar clasp arm in that the portion of the cast base

that gi!es rise to the clasp arm is separated from the clasp

arm itself' either b a sa$ cut or b being cast against a

separating shim of matri% metal' $hich is later remo!ed

$ith acid. t ma be made more fle%ible through

the use of $rought $ire' $hich is attached to a metal base bsoldering or is embedded in the border of a resin base.

ome of the ad!antages attributed to the infrabulge

clasp are +1, its interpro%imallocation' $hich ma be

used to esthetic ad!antage? +", increased retention

$ithout tipping action on the abutment? and +3, less

chance of accidental distortion resulting from its

pro%imit to the denture border. he $earer .should be

meticulous in the care of a denture so made' not onl


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1"> 5cCrackens remo!able partial prosthodontics

Fig. 7-9 Bar clasp arm on lo$er molar abutment engaging mesiobuccal undercut. ote proper useof parallel pro%imal guiding planes.

Fig. 7-80 Bar clasps used for both retention andreciprocation. Bar-tpe retainer on right second pre-molar engages distobuccal undercut. Bar-tpe config-uration on lingual aspect of left molar is used forstabili)ation and reciprocation and does not engage

undercut.

for reasons of oral. hgiene but also to pre!ent

cariogenic debris from being held against tooth surfaces.

Combination clasp

he combination clasp consists of a $rought$ire

retenti!e clasp arm and a cast reciprocal clasp arm +Fig.

7-88,. &lthough the latter ma be

in the form of a.bardasp arm' it is llsualla

Fig. 7-81 Bar retainer is used on anterior abutment ofmodification space' and its terminus engages distobuccalundercut. =enture is designed to rotate around terminalabutments $hen force is directed to$ard basal seat onleft. uch.rotation !!ould impart force on right premolar

directed superiorl and anteriorl. (o$e!er' thisdirection of force is resisted in great part b mesialcontact $ith canine. =irect retainer on right premolarengaging mesiobuccal undercut $ould tend to force toothsuperiorl and posteriorl.

circumferential arm. he retenti!e arm is almost al$as

circumferential' but it also ma be used in the manner of

a bar' originating gingi!all from the denture base.

he ad!antages of the combination clasp lie . in. the

Ie%ibilit' the .ad*ustabilit' and the


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&

Chapter 7 =irect retainers 1"9

B

Fig. 7-8" Contraindications for selection of bar-tpe clasps. &' e!ere buccal or lingual tilts ofabutment teeth. H, e!ere tissue undercuts. C' hallo$ buccal or labial !estibules.

&' =albo e%tracoronal attachment $ith D-shaped male portion attached to abutment cro$n onma%illar right canine? B, female slee!e placed inartificial tooth ad*acent to abutment.


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&

Fig. 7-;9 &' terngold D internal attachments $ith femaleportions cast into distal pro%imal surfaces of splintedma%illar first premolars. B, 2rofile !ie$ of male portionattached to partial denture frame. C' Finished partial denture$ith male portion seated into female portions $ithin

primar abutments.

&

c

B

c

B

=

Fig. 7-70 Dong copings on prepared natural abutments canpro!ide support and retention in compromised dentition. &'2atient is adult $ith repaired Class @ cleft $ith resultantcross-bite' Class occlusion' and se!ere anterior occlusaldeficit. H, nternal surface of prosthesis fabricated to restorethe arch. C' @ie$ of the internal o!erdenture structure $ith

an C attachment (arrow) that engages a dimpled cro$npreparation. e!eral such copings are placed to enhanceretention. D, Dabial !ie$ of completed o!erdenture

prosthesis.


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Fig. 7-71 Do$-profile coping contributes someadditional retention but primaril pro!idesimpro!ed support and stabilit.

+E%-"++E++ME

"I+

1. he frame$ork of a remo!able partial denture

must furnish support' stabili)ation against

hori)ontal +off !ertical, mo!ement' and

mechanical retention. (o$ is mechanical

retention accomplishedN

". Hhat factor other than mechanical retention

contributes to resistance of the denture to

dislodging forcesN

3. Hhat is the function of a direct retainer

+clasp,N. here are basicall t$o tpes of direct retainers.

=ra$ and label their component parts in their

correct positions on an abutment tooth.

8. =escribe the principles b $hich the e%tra-

coronal direct retainer and the intracoronal

retainer pro!ide retention for the remo!able

partial denture.

;. Hhat is meant b the heiht of contour of an

abutment toothN

7. =ra$ a diagram of an abutment tooth and

illustrate the anle of cervical converence.

Chapter 7 =irect retainers 11

>. & direct retainer is an assembl of the follo$ing

three components that perform indi!idual function4

+1, support' b a rest? +", stabili)ation-reciprocation'

b a rigid clasp arm or other rigid component? and

+3, a retenti!e element. =o these elements

necessaril ha!e to arise from a common sourceN

9. Fle%ibilit is permitted for $hich component

of a clasp assemblN

10. he amount of retention that a directretainer is capable of generating depends on

three factors. Hhat are these factorsN

11. he retenti!e arm of a direct retainer must be fle%ible

to engage an undercut $ith its terminal portion.

Fle%ibilit of the arm is a

product of four phsical and composition

factors. Hhat are these important factorsN

1". Retention on all principal abutments should be as

nearl e/ual as possible. o obtain this' $hich is the

more important factor-the relation of the tip of the

retenti!e arm to the height of contour or its depth in

the angle of cer!ical con!ergenceN

13. =escribe the proportional tapers of a cast'

half-round retenti!e arm.1. =escribe the taper of a cast' half-round stabili)ing-

reciprocal arm of a direct retainer assembl. For

$hat reason must there be a difference in form

bet$een a retenti!e arm and a stabili)ing-reciprocal

armN

18. ame the t$o basic tpes of retenti!e

clasp arms.

1;. & circumferential clasp arm approaches the retenti!e

undercut from an occlusal direction. From $hich

direction does a bar clasp arm approach the

undercutN

17. & clasp assembl ma be a combination of cast

circumferential and bar clasp arms andJ or $rought-

$ire retenti!e arms in one of se!eral combinations.rue or falseN

1>. & bar clasp arm is tapered in e%actl the

same $a that a cast' half-round circumferential

retenti!e clasp arm is tapered' differing onl in

configuration. Hhich arm is the more fle%ible if the

t$o different arms are the same lengthN HhN

1" 5cCrackens remo!able partial prosthodontics

19. Permissible fle%ibilities of retenti!e cast

circumferential and bar clasp arms based on length

ha!e been gi!en in ables 7-" and 7-3. Can a 6.7-

inch bar-tpe arm be safel placed in the same

depth of undercut that a 0.7-inch circumferential

arm canN Based on the information contained in

ables 7-" and 7-3'

e%plain the differences bet$een permissible

fle%ibilities of duplicate retenti!e clasp arms madefrom a pe @ gold allo and a chromium-cobalt

allo.

"0. Cast clasp arms are essentiall half-round in form'

permitting fle%ing in onl one direction. Hhich

direction is thisN

"1. Hrought $ire' >-gauge round' is often used as a

circumferential clasp arm. ts round form $ill permit

fle%ing in $hich directionsN

"". He speak of a reciprocal clasp arm. #%plain $hat is

meant b reciprocation and describe the condition

that must be met for true reciprocation to occur.

"3. & basic principle of direct retainer +clasp, design is

that the retenti!e and reciprocal arms must

encompass more than 1>0 degrees of the greatestcircumference of the tooth' passing from di!erging

to con!erging a%ial surfaces. Hhat $ould probabl

happen if a clasp failed to meet this criterionN

". imple mechanical la$s +of le!ers, demonstrate that

the closer a direct retainer assembl is located to the

tipping a%is of the tooth' the less likel that the

periodontal ligament $ill be ta%ed from rotation

tendencies of the denture. =ra$ the coronal portion

of an abutment? di!ide the enamel cro$n into thirds?

and locate support' retenti!e' stabili)ing' and

reciprocal components optimall.

"8. Clasp retainers on abutment teeth ad*acent to distal

e%tension bases should be designed so that the $ill

minimi)e direct transmission of tipping androtational forces to the abutment. rue or falseN

";. he location of a usable undercut is perhaps the

most important single factor in selecting a clasp for

use $ith distal e%tension partial dentures. rue or

falseN

"7. here are man tpes and configurations of clasps?

Hhat factors are important to determine clasp

retention and designN

">. nder $hat circumstances ma circumferential

embrasure clasps be usedN Hhat are some real

disad!antages of this tpe of retainerN

"9. i!e the indications for the use of a cast

circumferential direct retainer.

30. Hhat obser!ations $ould lead to the selec

tion of a bar-tpe claspN

31. Hhat is a combination clasp and $hat are

the indications for its useN3". tate three ad!antages of the combination

clasp.

33. ame the essential parts of a dental sur

!eor.

3. here are si% factors that determine the amount of

retention a clasp is capable of generating. 6ne of

these is the tpe of metal from $hich it is made.

ame the other fi!e.

38. (o$ does tilting the cast affect the selected

areas a!ailable for clasp retentionN

3;. he pro!isions for support and retention are t$o of

the si% basic principles of design of an e%tracoronal

retainer. Hhat are the other fourN

37. =ra$ four common errors in the design of acircumferential retainer. & bar-tpe retainer.

3>. Hould ou agree that the single most important

factor in selecting a tpe of direct retainer for a

distal e%tension partial denture is the location of the

undercutN HhN +#%plain our ans$er.,

39. He kno$ that guiding planes control the

. . path of placement and remo!al of a remo!

able partial denture. Can the also contrib

ute to additional retentionN f 4O0' ho$N

0. #%plain $h it i4O neces4Oar for retenti!e

clasp arms to be bilaterall opposed in Class

partial dentures.

1. n Class partial dentures should the

retention be bilaterall or diametricall opposedN #%plain.

". =ifferentiate bet$een three basic approaches

to the application of the R2 retainer sstemN3.

(o$ does the amount of contact of the minor connector

pro%imal plate $ith the corresponding guiding plane in

the R2 sstem influence the $a stress is transferred to

the abutment tooth and the residual ridgeN

7direct retainers ii

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