Symposium Oil Removable Partial Del/lures

RPI (Rest, Proxim~1 Plate, I Bar) Clasp Retainer and Its Modifications

Arthur J . Krol , D.D.S*

I n the last few yea rs various clasp designs fo r removable panial den-tures havc evolved fro m a cO lllinuing eff'on 1.0 prescrve the teeth and 1 heir su pportin g structu res, Emphasis o n prevcntivc de ntistry has added further in1petus to this eRo n . This anicle wi ll prese nt o ne of these newer clasp designs and the rationale f'or its usc.

h mi ght be well

632 AH:T H U R J. KROL

TOOTH BORNE AND EXTENSION BASE PARTIAL DENTURES

The esse ntia l di ssimila rity or the tooth borne an d th e ex tension base re movable pa rti al denture must. be recognized . In th e Loothborne rc- -movable par tial denture, the occlusal stresses a re lransmiucd to the bone by way o f the periodo ntal Iiga menL. Because a ll the eclenlUio lis spaces arc bounded by natural Leeth , this class of prosthesis functions like a jiXf'r/ partial denture. Since the abutment teeth provide re lat ively un yieldin g support, the choice of structural e le me nts is based o n co nvenience and the ir abilit.y LO provide rigidity, the re by using each re maining LOoth lO co ntr ibu te to th e stability of the olhe rs.

The extensio n base removable partial denture, the I. rue panial de n-ture, mliSI depe nd for its suppo rt upon IwO completel y diffe rent tiss ues. The teeth represent a relat ively immovable su ppon, while the soft tiss ues coverin g the ede ntulou s ridge have va l-ying deg rees o f displacea bility. The problem of designin g a removable pa rtial denture t.h a t. wi ll p rotect both from cvenlUa l destruction constantly plagues th e dentist. Any sli ght yie ld of t.h e so ft tissue structures, I.Ogether with the residual ridge resorption , will pe rmit venical displacement of the base a nd subsequen t tilting of the a butment toolh . Thi s potential hazard LO th e abutment tooth in the ex tension base pania l denture is quile uni ve rsall y recog-ni zed . Many ways lO protect an abutmen t tooth from destructio n , pe r-mitting it 1.0 funClion satisfactor il y fo r an extended pe riod o f time, have been proposed . These approaches a re a ll intended to in sure aga in st the tippin g actio n described above.

Most autho riti es agree Ih at in additio n to th e vario us types of stress directors, th e dentist can control the amount o f relat ive force de li ve red to the abutme nt by pro per selectio n of clasps. Th is is due in pan to the diffe re nt degrees o f Aex ibility possessed by the va rious types.

REQUIREMENTS OF A PROPERLY DESIGNED CLASP RETAINER

511./)/)ort: the resistance to the ve rtical components of masticatory force which preve nts th e panial fro m bein g displaced towa rd the so ft tis-sue. It is prov ided by the occlusa l rest.

Bracing (s tabili zation): the resistance to hori zonta l compo nents of force_ It is provided by the rigid co mponents of the cI ~l Sp incl udin g the occlusal rest and th e minor connecto r.

Retelltion: th e res istance to dislodgeme nt in an occlusa l direction. It is provided by the clasp tip engagin g the undercut when a dislod gin g fo rce is applied.

Adequate Encirclement: The clasp asse mbl y mu st engage more tha n

RPI C LASP R ET/\ I NER 633

1800 (more tha n ha lf the circumfere nce) LO prevent the LOoth fro m mov-in g o ut or the clasp.

R('(ijJ rocat iulI (reciproca l action): Each fo rce exerted on a LOoth by a clasp arm must be offset by an equal and o ppos ite (compensating) fo rce. T his is provided by th e reciprocal a rm lotat.ed o n t.he opposite side o f the LOo t.h . Normally. t.he reci proca l ar m does not e ngage an undercut.

Passivity: ' '''hen th e clasp is in place o n t.he t.ooth , it. sho uld be aL res t. Its re tenti ve fun ct.ion is activa t.ed o nl y when a dislod gin g fo rce is applied . A fo rce in an occlusal directio n causes the re te nti ve arm to en gage th e undercut from a gingival direction and so re ta in the partial denture in place. The clasp should never "grip" the t.oo th.

PROPOSED CLASPS FOR EXTENSION BASE PARTIAL DENTURE

Agreement ge nerall y ex ists tha t t.he cas t. circumferential clas p, es pecia ll y the Akers, bein g rel at.i vel y ri gid , is considered best suited for st.ro ng abutment teeth becau se it transmits th e force direClly to the too th and provides the best braci ng effect. It. is. the re fo re , lI sed more oft.e n in all LOOlhbo rne removable partia l den tures and in those ex te nsio n base parti al dentures where th e abutment teeth a re unusuall y stro ng.

Bar or Roach type clasps, usua ll y bei ng mo re fl ex ible, are freque ntly used fo r somew hat wea ker abutment teeth . Applegate ,! however, ex-pressed fear th at the bar t.ype cl asp tran smit.t.ed too much tilt.in g st. ress to th e abutment. He based hi s sta tement. upon th e fact that it is half round and has there fore undergo ne a maximu m reduction of fle xibility. The use o f thi s type o f clasp in a d istal extensio n pa rtial denture, he felt . shou ld be combined with some form of stress brea ker. McCracken" a lso wa rned agai nst the lise o f a bar clasp in exte nsio n base pa rtia l denl.ll res. O n ly where it was possible to utili ze a dista l rest and engage distal under-cut. d id he emplo y this type o f clasp, because o f its rigidity (Fi gs. I and 2).

Swenson's so lution7 for the extension base partia l denture was the provision of two abut.ment.s for each base with res t. areas which were not adjacent to the edent.u lous ridge. For di st.a l ex t.ension base pa rtia l den-I ures , claspin g is accompli shed by a mesia ll y o rig inatin g back ac tion clasp using d istal re tention (Fi g. 3). However , such clasp design tends LO exert a buccal force on th e abutment tooth when the base and dista l portio n o f the clasp moves lissueward under fun ctio n (Fig. 4).

The cast combinatio n clasp (half cast and ha lf wrou ght wire), utili z-in g a di st.a l rest for di st.a l extension base partia l denture is report ed by man y authors to have stress breaking actio n (Fig . 5) . Appl egate] claimed that the combinat.io n clas p dissipated the stress o f torq ue and leverage besl because the ret.~nti ve arm is ro und and the refore fl ex ible in any di-rectio n. He claimed that sin ce it is of wro ug ht st.ructure, it is t.o ugher a nd

Text continued 011 jJage 636.

634 ARTH U R .J. K ROL

Figure I. i\fodified T bar buccal clasp arm of a combina tion Glsl clasp utili zing a di stal rest and distal undercut.

Figure 2. Ci rcu mferential C clasp wit h a di stal re st engagi llg a d istal unden:u l that 111

RP I CLAS P R ETA INER

Figure;l, Occl usa l vic\\' of a back actiOn clasp wit h a mesialrcsl.

Figure 'I. Oingrarmllacic illustration of a d istal view of a bac k act ion clasp orr a bicuspid with arrow (A) re presenting a buccal force ex-erted 011 the toot h \\,hcn all occ.:ius

636 AKTl I U R J. KRO L

Figure 5. Akers clasp with a wroug ht wi re buccal arm and a lingua l G is t arm. !\d\'o-Giles or this clasp for ex tension base partial dentures claim thal lhe flex ible wrouglll wi n: bucca l ar m minimizes IOHluing of the abu lmelll tooth under funct ion .

can be used in srna ller gauges, which furth e r increases flex ibility. Thi s in creased fl ex ibi lity o f th e retentive arlll permitted the clasp to fUIlCl ion as a stressbreaker. Because the reci procal arm of the combination clasp is cast and th ere fo re rigid , it is able to resist latera l forces effectively and also 1.0 recip rocate th e force exerted by th e rClcrllive wro uglu wire arlll as th e partial denture is placed o r removed. The supposed fl ex ibilit y of the rete nti ve wro ught wi re clasp arm has bee n questioned in a cl inica l stud y' and more recently by ClaYlOn and J aslow.2

REST-PROXIMAL PLATE-I BAR CONCEPT*

This concept of clasping abutme nt teeth is a modification of the con-ce pt presented by Kratochvil.4 T he components o f the clasp a re a rest with its minor co nneclOr,:! proxima l p late , and an I bar clasp. (Fi g. 6) . The RPI clasp fu lfills the req uireme ills or proper clasp design , and minimizes stress o n the abutment tooth. The rest, located on th e mesi al occl usa l surface of th e abutment tooth, ac ts as the point of rOL

RP I eL/\ S!' R f:~TAI N E~ 637

Fib'l.lre 6. A. Huecal (I) and lingual (2 ) views of a n RPI clasp 1'01- the cXlellsion base panial denture . B, Occlusal view (J) of

638 A RTH U R J. K ROL MOUTH PREPARATION

Rest Seat Preparations

For di sta l exten sio n base pa rtia l de ntures where a bicuspid serves as the a butme nt LOoth , a mesia l res t pre para tio n is mad e. For poste r ior tee th , where resto rations a rc not placed . this rest sea l ca n be prepared in the a ppropriate tria ngula r fossa with a No. 6 carbide bur or dia mo nd SlOne. The marginal r idge must be lowe red abo ut 1.5 mm . and the resultin g pre pa ration must have Lhe d eepest po rtion in the cenle r o f th e trian gular rossa .

Sufficie nt bulk of me ta lmllst be provided to pe rmit the rest to fun c-tion witho llt fracturing or bendin g. Gold requires la rger and deepe r pre para tion s th an the non-precio us metals (chro me coba lt, nickel cobalt, etc.). This pre paration should be rounded and full y polished to pe rmit some ro ta tio n when depressio n o f the extension base occurs. If a cli sp id is to se rve as th e abutment, a mcsio lin gual rest preparation is made. The rest seat mll st be deep e no ugh to prevent the metal rest from slipping gingivally (Fi g. 8). As a gene ral rule, mandibular cuspids have a thin enamel coverin g and when preparin g an adequat.e rest seat, penetra tion into th e d e ntin is o f Len in evita ble. If d entin is ex posed , th e pre paratio n should be deepe ned and mod iried to accept a gold fo il , amal gam , o r othe r resLOraLioll which ca n be pro perly contoured (Fi g. 9). Guide Planes

A guide pla ne is pre pared o n the distal surface o f th e abutme nl looth at the occlusal one third as pro posed by Pouer6 and associates, a nd should exte nd lingually just f~lI' e no ugh so th at th e proxima l pla te togethe r with the mesial mino r connecto r will prevent ling ual migrat ion of the tooth (Fig. 68). The guide pla nes should be app rox ima tely 2 to 3 mm . in height occlusogingivally (Fig. 7). T his guide p la ne will o fte n pe r-mi t the proximal pla te a nd the mesia l mino r con necl.O r to contac t the

Figu,'c 8, Hall bur n is hCI' placcd ill rcst preparation to dete r-mine adeq ll :lc )' of sha pc and dept h, When fun:c is placed un 1 he ball bu rnisher in an axial direct io n Ull th e (;us pid , iT should not slip OUI 01 til e prc llaralioll ,

RPI CLAS P R ETA I NER

Figure 9. Amalga m rcsto ral iOll is placed in mandibular w spid to permit prope r prepara tion of a rounded occlusal Iy pe r~s t withou t exposure of de nt in.

639

lOoth simultaneollsly and provide proper reciprocatio n agai nst the fo rce exerted by the retentive buccal clasp arm durin g the seating and removal of the denlllre. I f th e mesia l minor connector and proximal plate ca nno t contact sinwlta neously, as may occur with cuspid abutme nts, then th e re-tenti ve I bar should engage the mesio buccal undercut and receive its re-ciprocation from the prox imal pla te alone (Fi g. 10).

FigU 1"(! 10. J ncisa J view of Rill cI

640 A~TH UR J. KROL DESIGN OF THE RPI CLASP

Mesial Rest

T he abu tment LOoth contains th e mesioccl usal rest with the mino r co nnector pl aced into the rnesiolin gual embrasure bUl no t. co ntactin g li1C adj ace nt tooth . Proximal Plate

""hen viewing a mandibul ar partial den tu re o n the maste r cast, the superior ed ge of the prox im al plme is located at th e bottom of the p repared guide plane which sho uld be allhejullctio n o f the occlusa l o ne th ird and midd le o ne thi rd o f the tooth (the remai nder o f the p rox ima l p late lies belo w th e guide plane) (Fig. II A, C and D). T he p roxi mal plate extends lin guall y just fa r enough so that the distance between the m ino r connector and prox imal pla te is less th an the mesiodista l width of the tooth (Figs. II Band 12A, B and C). It sho uld be I mm. t.hick and j o in the framework a t a righ t angle. At the j unctio n with th e framework , the prox imal plme is relieved so as not to con tact the gin giva and is h ighl y poli shed . A fi n is hin g line is placed at the base o f the proximal pla te LO pe rmit a butt joint with the resin base (Figs. 13 and 14). I bar

The approach a rm of the I bar extends from the fra mework so as LO re main at least 3 m m. from the gin gival margi n and then crosses the gin gival ma rgin at right angles (Fi g. IIA). Approximately 2 mm. o rth e I ba r contacts the tooth surface, usuall y at the gin gival one third o f th e LOOlh (Fig. I I A). The bo ttom ponio n o f the I bar co ntacting the tooth surface sho uld engage 0.0 I in . undercu t. The I ba r should ta per slightly fro m th e base LO the tip. It is usua ll y placed at the grea test mesiodista l promin ence o n the buccal surface or towa rds th e mes ia l, but no t toward the dista l. This is necessary to permit movement o f th e I ba r away fro m the entire buccal surface in fUIlClio n. Wh en the I bar is placed toward t.h e mesial , it has the advantage that when properly adjusted it brin gs th e prox imal plate into tig ht contact with th e dista l surface (Fig. 10).

ADVANTAGES OF THE RPI CLASP

Vertica l masticatory force o n the d istal exte nsion base causes the I bar to move mesiogin givall y away fro m the tooth and the prox imal p lat.e to move furth e r into th e undercut of th e tooth . T hu s, both th e I ba r and th e proximal plate disengage the abutmen t and the reby red uce to rq u-ing o f the tooth . T he mesial minor conn ector together with the p rox-im al plate provide the necessary reciprocmio n and e liminate the need

RP I eLf\S!' RET/\ I N ER 641

A

9 '-, ..... :; ~gp

c

Figure II. Diagranllnati(' illuslI'ation showing four views of an RPI clasp. A. Distal view. The guide plane (gjl) is approximatelr 2 to 3 mTl1. in height occ1usogin-

gi\'a1J)'. The buccolingual width of the guide plane and proximal plat.e (p) is de1.ermined by the contour of the tooth. The I bar (I) makes

642

........

ARTH U R J. KRO L

Figu re 12. ,-I, r' rox ima l pl att' (jJ) ,'clati vely narrow due to the w pc ring comou r of the first bi;. cusp id . The proximal plate shou ld be dcsigllcd 10 be as narrow as possi-ble but stilt be able to prevent li ngual mig ratio n of the tomh . A n

RP I e L/\ SP R ETt\lNER 643

Figure 14 . Unders ide of maxillary partia l denlure casting. Arrows POilll lO under-side of prox ima l pl ate: \ ... ith fmish line permiuing a butljoinl with the resin base.

for a lin gual arm . The mesial rest eliminates th e pot.entia l "pump handle" effect t.h at a force o n the base o ft.e n induces with a distal rest.

The RPI clasp contacts the LOoth minim all y and is advantageously lIsed o n cari es pro ne patient.s. and sin ce th e I bar itse lf makes very little co ntact with the LOath , it. is usually more esthe tic than most ot.her clasp arms.

CONTRAINDICATIONS FOR RPI CLASP

Any o f the following conditio ns would precl ude the use of the RP[ clasp: ( I) insuffi cie nt de pth of vestibule to permit t.he approach arm o f the I ba r to be located at least 3 mm . from gingiva l margin (Fi g. 15) ; (2) a tooth which has a severe lingual tilt and no labial o r bucca l under-cut (Fig. 16A); (3) tissue undercut so seve re tha t the approach arm of the I bar would be too far away from the tissue and act as a food trap or irri tate th e mucosa of the lip or cheek (Fi g. 16B) ; (4) teeth which are severel y fl ared labia ll y or bucally (Fig. 16B) ; (5) a tooth which has o nl y a distobuccal re ten tio n undercut and does not require a restoratio n; and (6) a mOllth with a hi gh Aoor in which a lin gual plate is used on the lin gual surface of th e abutmen t LOath .

RPA CLASP

T he RPA clasp is similar to the RPI clasp except that in place of the I bar a circurnferential Akers clasp arm arises fro m the proximal plate.

644 A RT I-I U R J. KROL

Figure 15. 11, Soft tissue ir riw l iO l1 unde r base or I bar due 10 insufficient depth of ves-tibul e. T he depth of the vestibu le should be care rully exa mi ned before decid ing to utilize an I ba r clasp arm since the approach arm of (he I b,lr Illll S! be localed at least 3 mm. from the gingival margin . B, Ulcera tio n caused by the approach arln of lhe I ba r is apparent. Also nOte innammation of g ingiva l margin .

RPI CLAS I' R t:TAI N ER 645

,

B Figure I (1. II, Diagrammalic i liuSIJ":lIio n of sc\'creiy lihed bicuspids whcre no under-

CUI is prcsCIlI as indicated by (I ). B. Diagrammalic illustl"a tio n of sC\'crely tilled bicuspid whe re an cxccss i\'c tissue undercut is indicatcd b)' (2).

The RPA clasp may be used in place of the RPI clasp when the re is insu'" fi cie nt d e pth in the buccal ves tibule or wh e n th e buccal tissue undercut is LOa great (Fig. 17). The shou lde r of the clasp arm Illust be constructed so th at o nl y its occlusal border contacts the LOmh aL th e hei ght of conLOur. The undercut area in that portion of th e clasp arlll must be bloc ked OUl.

MODIFICATIONS IN THE CONSTRUCTION OF THE RPI CLASP

Wh e n two teeth are splinted the mesial rest is placed on th e anterio r LOath and th e proximal plate on the posterior LOoth with the I ba r o n the mesiobucca l surhlce of the posterior LOath (Fig. 18).

I r a three unit fix ed prosthesis is constructed , the mesial rest is placed in the anterior abutment and the prox im a l plate on the poste rior abutme nt with the I bar placed o n the mesiobu cca l su rface of the poste-rior abUl.menl.

Wh e n designin g an extension base partial d enture. where an iso-lated LOath is in vol ved , the RPI clasp may be placed on th e LOolh an terior LO th e isolated tooth and prox imal plates on th e mesial and distal sur-faces o f th e isolated tOoth wilh no resl. Th is d esign permits re mo va l of stress on the isola ted LOoth (Fig. IS). It is imponant that the superior borde r o f the mesial proximal plate on the isolated tooth be located a t o r gin giva l to I.he level of th e occlusal rest of the RPI clasp.

The RPI clasp may be designed not to release in function by ex te nd in g the proxima l plate 1.0 contac t the entire guide plane. This lype o f' clasp may be used ", here it is d esirable to load th e tooth more than the ed en lUlous ridge.

T('xt continued 011 page 648.

646 A RTH U R J. KROL

Figure 17. A, RP[ clasp pennitlcd on right second bicuspid c\'cn though a sli ght li s sue un derCLIt ex isIS. 8, RPA clasp . Akers arm on' proximal plate. Due 10 seve re tissue un-d ercli t the I bar buccal arm cou ld n OI be used.

RPI CLAS P RET/\INER 647

Figure 18. Panial delllure showing modified RPI clasp used on spli nted ma xillary left bicuspids. Resl is located 0 11 lIlesial of first bi cuspid wit h proxi mal plate onlhc distal of the second bicli spid. The I ba r (nol visi ble) is loca led 011 the Illesiobucc;ll o r I he lefl second bicus pid . :\ole I he usc of t\\'o proxi mal plates on I he 1l1;lxillary riglll SCCOlld bicllspid with-OU I Ihe li se or rests to reduce possible 100'quing of the loot h, An RPI clasp is localed on the maxillar), righl cuspid .

648 ARTHUR J. KRO L

Figure 19. Clasps on biclispid and mol"r fulfilling the requirements of proper clasp des ign. :Htlxilllll)' jirsl biruspirl: Suppon is provided by occlusal rest: braci ng is provided by minor con nector. occlusal reSI, and proximal plate: retention is provided by I bar on Ihe buccal; reCiprOGllioll is provided by Illcsial minor connector and proxinlai plate ; cng"gc-ment of more tha n 180" uf the circulnl"cl'cncc o f the (oolh is pro\'idcd by proper placement of mesial rest with its minor con neCLO r . proximal plate. a nd I bar.

M axilhll)' :)/?/"Qlltllllular: Suppon is provi ded by mesial rest; bracing is provided b}' oc-clusa l rest, lIlesial minor conneClo r. and rig id lingual I bar: rCICllliOIl is provided b)' bucca l I bar: reciprocation is provided by mesialminorconneclor and rigid lingual I bar: e ngage-ment of lllore than 180 0 of Ihe circumference of the tOOl h is prov ided by propel' p lace-lllent of Illesial rest with its minor COI1llC(;lOI' and buccal and lingua l I bars.

Although the RP.I clasp has been presented for use in extension base removable partial dentures, a number of its characteristics make th is clasp suitable for use in LOolhborne removable partia l dentures as well. Such an RPI clasp and its modificatio ns are shown in Figure 19.

SUMMARY

A clasp design that ca n be employed for abutment teeth on exten-sion base removable partial dentures has been presented. The clasp referred lO as an RPI clasp retaine r minimizes lOoth coverage and reduces stress o n the abutment LOath. T his sa me design may a lso be used LO adva ntage on a ll toothborne removable partial dentures.

REFERENCES

I. Applegate. O. c.: Essentials of KelllO\ahlc Pania l De nture Prosthesis, 3rd cd. Philadel-phia. W. B. Saunders Co., 1965.

2. CIa)'lOll ,J . A., and .las low. c.: A nlcaSllrc mCl'l1 of clasp fOI'ces on teet II. J. Pros. Dent. 25: 21-43. 1971.

RP I CLASP R ETAINER 649 3. Fen ller. W .. Ged>el'. A .. ;l llci Muhle lllann . 1-1 . R.: Tooth mo bil ity changcs d uring treat-

ment \\'ith pan ial dc ntlWc prosthesis. J. Pros. Dent. 6:520-5:l5. 1956. 4. Kratochvil. F. J. .: Influc nce or occlusa l res t posilion and clasp on move ment o f abU! -

Hl cm tceth . j. Pros. Dml. lJ: 11 4- 124. 19t):-J. 5. Mt:Crack('ll. W. L. : I'anial Dcmure Construction. 51. Louis, C. v. ~ I ()sb)' Co. , 1964. 6. POllel'. R. B" Appleby. R. C .. and Adams, C. D.: -Rcmovable panial de nture d esign : a

rcvic w and a challc n){c . .J. I'ros . De nt. 17: 63-68. [96i. i. Swel1son . M. C .. and T erkl a. l.. G.: Partial DClllllres. 51.. Loui s. C. V. MoshyCn .. 1955.

Uni \'c l~')il y or the Pacific School of Dc nti stry 2 155 Webslc r Street S;l ll Francisco. c... lifol'llia 941 13