Clasp Retention: The Effects of Five Variables Arthur M. La Vere, DDS, FACD, FICD *

An attempt was made t o determine the relative retention of each of the four most commonly used clasp assemblies. They are the rest, proximal plate, I bar (RPI); rest, proximal plate, facial circumferential arm (RPA); modified T; and Akers [facial and lingual circumferential arms). A mandibular bilateral distal extension base removable partial denture design was selected because it is most commonly used. A test model was made by removing all mandibular molars from a Columbia Dentoform and replacing the ivorine second bicuspids with natural teeth. These, in turn, were later replaced with gold crowns that were identical in contour to the natural teeth. Eight removable partial dentures were constructed using preformed plastic lingual bars, minor connec- tors, denture base retention loops, and clasps. Artificial teeth and the denture bases were processed to each framework as for a clinical patient. Each removable partial denture was placed in the lnstron testing machine and pulled from the test model. The investigation examined the effectiveness of each of four types of clasps in resisting displacing forces, in both vertical (occlusal) and mesio-occlusal directions. Each clasp was tested using natural teeth and gold crowns, in dry and wet environments, both with and without indirect retainers. Results indicate that, (1) The RPA clasp was the most retentive of all on natural abutments, against both directions of pull, but only slightly SO over the RPI clasp with a vertical pull. (2) The Akers clasp was most retentive against a mesio-occlusal pull on the gold abutments. (3) Natural tooth structure gives slightly more frictional resistance than a gold surface. (4) The presence of artificial saliva caused l i t t le change in forces required for dislodgment. (5) The absence of indirect retainers greatly reduced the retentive properties of the RPD frameworks.

J Prosthod 2: 126-131. Copyright @ 1993 by the American College of Prosthodontists.

INDEX WORDS: clasp, removable partial denture, artificial teeth, chrome cobalt, crown, indirect retention

N 1930, Dr F.E. Roach wrote in the~~numal oJthe I A merican Dental Associatian that the clasp is the oldest and still is and probablywill continue to be, the most practical and popular means of anchoring partial denturcs. This statement is as true today as it was then, indicated by the fact that most distal extension removable partial dentures (RPD) con- structed use extracoronal direct retainers for reten- tion.?

A RPD must be sufficiently retentive3 to avoid being displaced in function. Retention4 may be dc- fined as thc resistance to dislodgment. It is provided primarily by the clasp,+ but other components of the RPD may also contribute to retention, eg, rests

*Pr$eswr, Department $Removable Prosthodonticr. Srhml $Lkntirtq: liniuwsig; @he Pncijc, San Francisco> C4.

This study w m supported by the Pa& A4edimi Center Grant and thc 7 %gm Fund.

Address repint requests to ilrthur M Lavere; DDS, FACD, FICD: Depadment ofReniouab1e Prosthodontics. School of Dentistqi, Unifieni!y of lhe Paijic, 2155 Webdm St. Snn Francisco, C A 94115.

Co,tyrigtit 0 I993 b ~ the Amencan Collqe ojProsihodontists 105~-!i4IYJY3/ 0202-0009$5.@@10

placed anterior to the fulcrum line act as indirect rctainers by preventing occlusal movement of the distal extension base.?

In 1936, Ur Eugene R. Stone5 showed experimen- tally that an infrabulge clasp could dclivcr 143% inore retention than a suprabulge clasp engaging the same undercut. Dr D.X. Firtell, in 1968, made an experimental model that simulated an abutment tooth. Castings ofvarious clasps were then Lested to de terrriine the amount of forcc required to dislodge them. The castings were not of an entire RPD, and there was no attempt to evaluate the cffect on retention of components other than clasps.

To evaluate the factors of retention, test RPDs in this studywere constructed to contain all the compo- nents normally present in a clinical prosthesis. The investigation cxarnined the efkctiveness of four types of clasps in resisting displacing forces in both vertical (occlusal) and mcsio-occlusal directions. Each clasp was tested using natural teeth and gold crowns, in both dry and wet states, with and without indirect retainers.

126 Journal ofProJthodontici, L512, 2Vo 2 i June), 1993:Fp 126131

June 19.93, Volume 2. Number 2 127

Direct Retainers T h e following clasps were studied: rest, proximal plate, Ibar (RPI),8 rest, proximal platc, facial, circum- ferential a r m (MA), modified T,? and Akers (circum- ferential)? T h e RPI clasp consists of a mesio-occlusal rest, a proximal plate on the distal surface of the abutment tooth that contacts the tooth with its occlusal one-third resting against the bottom of the guiding plane. T h e direct retainer is an I bar located a t the center of the facial surface of the tooth a t the gingival one-third with .OlO-inch undercut. T h e RPA clasp is similar to the RPI clasp except that there is a facial circumferential clasp a r m o f f the proximal plate instead of a facial I bar. This arm is positioned on the facial surface so that only its superior border contacts thc tooth (positioned exactly on the height of contour survey line). T h e rest of the clasp arm stands slightly away from the tooth surface until its retentive tip reaches the point of entry into the .D 10-inch undercut on the rnesiofacial surface.

The modified T clasp is similar to the MI, except that the retentive tip is the T clasp with the mesial half of the T removed. The .OlO-inch undercut is located a t the distofacial surface of the tooth.

The Akers clasp has a disto-occlusal rest and two circumferential clasp arms, one that engages a n .OlO-inch undercut a t the mesiofacial line angle, and the other a reciprocal lionretentive a r m on thc lingual surface.

Materials and Methods A mandibular bilateral distal extension base RPD was selected because with this design, dislodging movements of the base generate torquing stresses on the abutment teeth.y A Columbia Dentoform (Columbia Dentoform Corp, New York, Nu) model was modified to simulate a mandibular arch requiring a bilateral distal extension RPD to replace all molars. The second bicuspids (no. 20 and no. 29) were selected as abutment teeth, and their sockets were shaped so that they would accept both natural extracted teeth and gold crowns that duplicated the natural teeth in form and position. The natural second bicuspids were positioned in the sockets and the model placed on the Wills Surveyor (Jelenko Company, New Rochell, NY) and positioned to provide .010-inch facial undercuts on no. 20 and no. 29 at the selected path of insertion and removal. The model was then placed on a tripod and the survey table securely locked so as to be maintained in the same position for all subsequent survey- ing. Using a parallelometer, rnesiolinLgual and distoproxi- ma1 guiding planes were prepared on natural teeth nos. 20

and 29, and mesiolinpal guiding planes were prepared on ivorine teeth nos. 21 and 28. Mesio-occlusal and disto- occlusal rest seats were prepared on natural teeth nos. 20 and 29. Mesio-occlusal rest seats were prepared on ivorine teeth nos. 21 and 28 for indirect retention. Gold crowns were constructed on teeth nos. 20 and 29, which duplicated the natural teeth in both form and position. One test model was used for all testing by interchanging the natural abutments and duplicate gold crowns for teeth nos. 20 and 29. ,4 threr-point vertical pull was used to determine

retention against a strictly vertical pull directed parallel to the path of insertion and removal selected upon the initial survey. A two-point vertical pull on the posterior bases was used to determine the amount of retention present during simulated function (ie, lifting forces applied to the distal extension bases).

Eight polysulfide rubber impressions were made with both natural and gold crown abutments in place. The impressions were poured in yellow stone. The eight master casts were surveyed and dcsigned for each type of clasp, and the necessary blockout was performed. The casts were sent to the dental laboratory (liernan-Leino Dental Lab, Oakland, CA) for fabrication of refractory casts and returned.

-The eight refractory casts were designed and waxed using preformed plastic patterns Uelenko, Armonx, hi) for lingual bars and clasps so that all similar components of each frame would have the same thickness, length, width, curvature, taper, cross-sectional form, and comparable flexibility. An attempt was made to control and standard- ize the size, shape, and form of the clasps. One withdrawal loop was placed in the center of the lingual bar and one at the distal of each abutment tooth (no. 20 and no. 29). Also, one loop was placed between the first and second artificial molar teeth on each side of the arch. (Fig 1).

Figure 1. A refractory cast is waxed for an RPD. Tiote the loops, ie, one at the midline, two behind the second bicuspids, arid two between the first and second molars.

128 Cia$ Retention a ArthurM. LaVere

The eight waxed refractoiy casts were returned to the dental laboratory for fabrication of chrome cobalt frame- work castings (Vitallium Howmedica, Chicago, IL). A total of eight framew70rks were made: four with the natural abutments and four with the gold crowns. The frameworks were rough finished and deplated only, and returned. This eliminated the possibility of any component being overpol- ished or made too thin. Artificial teeth nos. 18, 19,30, and 3 I were processed to the eight denture bases using auto- polymerizing orthodontic acrylic resin (Fig 2). Pressure relief cream was applied to the inner side of the minor connectors, rests, clasps, and guide The frame was seated and removed. Slight pressure areas were removed from the minor connectors by the use of a handpiece and a pointed stone.

The Instron testing machine (Instron Corporation, Canton, MA) was used to test the eight RPDs for reten- tion. The test model was rigidly fixed in the center of the Instron machine to a table with a universal ball joint. Ihis permitted orientation of the test model in line with the origirial surveyed path of insertion and removal. A level was used toverify this orientation, and the model positioned so that dislodging forces were alwa).s directed vertically (Fig

Three 2-inch chains were joined together by the end link of a single 3-inch chain, which was attached to the load cell. Chain tension was set at zero so that no premature pull was introduced before starting the experiment. The main chain link connection to the Instron had three connecting links entering a single loop. There exists the likelihood that exact positioning on each test recording will result in subtle length changes in the chain and thus yield fluctuations in dislodgment forces at the three (or two) connections to the test specimen.

3).

Test One: Retention of Four Clasp Designs on Cold Crowns The WI clasp RPD constructed for the gold abutment crowns and the premeasured chains was connected to the

Figure 2. An example of a completed mandibular E D .

Figure 3. An RPI test model is rigidly fixed in the center of thc Tnstron machine, the RPI removable partial denture is s(:iltt:d, and one premeasured chain is c:onnc:ctcd to thc antrrior l ~ p and tm) prt:measm-t:d chains to the right and Icft postc:rior loops to mcasurr thr three-pint dislodging vertical pull.

anterior loop and to thr right i d left posterior loops locatcd distal to the two abutments. 4 three-point vertical dislodging pull, which matcht:d the path of insertion, was applied to the Rll). Aftcr each testing, the ND was reseated, and a total of seven replications were accom- plished in the dry state. The chain was then distnnrirctrd, and two legs of the chain were attached to the two loops located between the first arid second artificial molars on each side. Seven pull tests were then repratcd using thc posterior or functional two-point dislodging pull, resulting in a mesial rotational lifting force k i n g applicd to the RPD (Fig .I). The RPA, rntxlified T, and Akers clasped R P D s construc:ted for crowns wm: similarly tested in the dry state.

Test Two: Retention of Four Clasp Designs on Natural Teeth Thr natural abutment teeth were substituted for the Sold abutrimits and thr RII, KPA, modifird T, and Akers clasped NDs constructed for the natiiral abutment teeth

June 1993, IJolume 2. Number 2 129

Figure 4. A posterior or functional two-point dislodging pull is applied to the RPD.

were individually tested in the dry state using both the vertical (three-point) and posterior (two-point) dislodging pull tests.

Tests Three and Four: Eflects of Salivuty Substitute on Retention Artificial saliva (Sal-eze, North Pacific Dental Inc, Kirkland, WA) was applied to the gold and natural abutments before each testing to maintain a u'et condition, and the proce- dures describcd in tests one and two were repeated.

Test Five: Efects of Loss of Indirect Retainers on Ouerall Retention Ivorine teeth nos. 2 1,22,27, and 28were removed from the test model to eliminate any interrerences and indirect retention from the RPD. The teSt model was reoriented t o the Instron machine to the original path of insertion and removal. The eight RPDs were then individually retested for resistance to the posterior two-point dislodging puI1 without indirect retention, under both wct an dry condi- tions, and with both gold and natural teeth as abutments for the four clasps.

Results On natural teeth, thc RF'A clasp was the most retentive under a vertical pull. The RPI, modified T, and Akers clasps showed nearly cqual retention when removed in a vertical direction, regardless ofwhether the abutment tooth was wet or dry (Fig 5). Katurd tooth structure gave slightly more frictional resis- tance than the gold surfaces (3.8 Ib versus 3.7 lb).

On gold crowns, the retention of the RPI, RPA, and Akers clasps were nearly the same under a

wet

0 Dry

RPI R PA Mod T Akers

Average Value ( 7 RECORDINGS I

Figure 5. Average value in pounds of seven recordings of a vertical (three-point) pull on the natural abutment.

vertical pull, except that retention of the Akers clasp was markedly reduced by wetting the surface of the crown. The rndfied T showed the least retention on both wet or dry surfaces (Fig 6).

The RPA clasp was also the most retentive against a posterior (two-point) rotational pull on the RPD on natural teeth. The other three clasps showcd nearly equal retention against a posterior pull, and there was little difference between the wet and dry record- ings for any clasp (Fig 7).

On the crowned abutment teeth, the Akers clasp was the most retentive below a posterior (two-point) rotational pull on the RPD, both in the wet and the dry state. The WI and WA clasps were less reten- tive, and the modified Twas significantly less (Fig 8).

There was a significant loss of retention by the

-0

-I 8 3

2

1

0 RPI

F3 we1 Dry

Average Value ( 7 RECORDINGS 1

Figure 6. Avrrage value in pounds of seven recordings of a vertical (three-point) pull on the crowned abutment.

130 Clasfi Retention 0 Arthur M. LaVere

5 l -

1

0 RPI R PA

wet

0 Dry

Mod 'T' Akers

Average Value ( 7 RECORDINGS 1

Figure 7. Average value in pounds of seven recordings of a posterior (two-point) pul1 on thc natural abutment.

RPDs (about 82%) with all four clasp designs during the rotational pull when indirect retention was re- moved. This was true under wet or dry conditions and for natural as well as gold abutments (Figs 9 and

The analysis showed that there was no marked statistical difference at the 95% level of confidence between the wet and dry testing conditions (Table 1). The results in Table I represent the average of seven replication tests for each of the four clasp designs.

10).

Discussion To test direct and indirect retention accurately on a laboratory model, a RPD should be designed and constructed in the same manner as it would be for insertion in the mouth. For consistency, the blockout, waxing of the RPD frames, positioning of the artifi-

7t -

Dry

RPI R PA m d : r ' Akers

Average Value ( 7 RECORDINGS1

Figure 8. Average value in pounds of seven recordings of a posterior (two-point) pu11 on the crowned abutment.

7 t

R3 Wet Dry

RPI R PA Y d T ' Ahers

Average Value ( 7 RECORMNGS 1

Figure 9 . Average value in pounds of seven recordings of a postrrior (two-point) pull without indirect retention on the natural abutmmt.

cia1 teeth, and processing of the dentures bases should all be accomplished under carefully controlled conditions. The W D s in this study showed the most retention when a three-point pull in a vertical direc- tion was applied. This is probably due to the com- bined effect of the clasp engaging the undercut, the proximal plate contacting the guiding planes, and the lever action of indirect retainers. The natural tooth structure gave slightly more frictional resis- tance than the gold surfaces. Adding artificial saliva to the clasped tooth surfaces caused very little change in the forces necessary to dislodge the RPDs, and showed no consistent pattern of improving or reduc- ing retention. However, indirect retainers were ex- tremely effective in increasing the retention of all the RPDs in this study. This was contrary to the findings of some prior inve~tigations.'~,'~

7t

1

n

RPI R PA MGd.'T' A h -

Average Value ( 7 RECORDINGS )

Figure 10. Average value in pounds of seven recordings of a posterior (two-point) pull without indirect rctention on the crowned abutment.

June 199.7, Volume 2, Number 2 131

Table 1. Experimental Results With a Mean of Seven Replications and One Standard Dekiation, in Pounds Load Trpe RPI RPA Mod@ T A k e r S

Natural wet 1.760 -t .2915 5.172 & .2881 3.073 t .I493 3.730 ? 2413 3.109? .9048 6.860 t ,7701 2.333 f .1581 2.161 ? .5036 Vertical dry

Posterior wet 1.333 2 ,0761 3.814 i .1342 1.699 ? .O 157 2.003 f ,291 1 4.429 ? .0910 1.320 2 ,0780 1.32U ? .0882

.631 f .2196 .lo0 2 ,0612 ,965 * .0288 .271 2 ,0540 .940 2 .0871

3.351 k ,2521 3.550 4 .1614 .m t .oav 1.608 2 .6873 Vertical dry 4.440 t 3587 5.013 f ,3707 1.105 ? ,1842 4.295 f ,2841 Posterior wet 1.833 & 2350 2.082 f .lo81 .576 2 ,0459 5.277 f .1646

dry 1 . a 4 2 .0674 2.235 f ,3512 .715 4.0386 2.827 2 .4722 Posterior without \vet ,215 f .0088 ,286 ? .0931 .358 f ,0071 1.085 ? ,2375

indirect retention dv ,236 ,0093 .505 f ,0593 ,361 ? ,3505 1.113 ? .0683

dry 1.212 5 ,1632 Posterior without wet ,196 4 .0377

indirect retention dry ,208 2 .Of505 ,710 ? .0969 Gold w-et

Overall, the RPA-type clasps shouTed more reten- tive influence than the other three clasp types tested. Apparently, the combination of the proximal platc/ guiding plane of the abutment teeth and the rigidity of the shorter buccal retentive arm makes the RPA clasp more retentive. Because of a lack of reciprocal arm and the manner of the construction of the buccal arm! the RPA is not considered to be a circumferential clasp but a modified RPI clasp.

Summary and Conclusion An investigation was conducted to cvaluatc four clasp designs for retention against vertical and poste- rior/rotational forces, using natural and gold crown abutments, both wet and dry states, with and without indirect retention. There is no attempt to indicate desirability of one type of clasp over another, but only to compare the retentive properties of differently clasped RPDs when they are pulled off the abutment teeth.

The following conclusions may be drawn from this study: The RPA clasp was the most retentive on natural abutments against both vertical and poste- rior pulls. The RPA clasp was only slightly more retentive than the RPI clasp on the gold abutments against a vertical pull. The Akers clasp was most retentive on the gold crowned abutment against a posterior pull. Natural tooth structure gives a slightly more frictional resistance than a gold surface (8 Ib versus 7 Ib average). Artificial saliva caused little change in dislodgment forces (Average 3.7 Ib wet versus 3.8 Ib dry). The most striking finding for all the variable test conditions was the large reduction in retention of the RPDs when indirect retainers were removed. This study showcd that indirect retainers are extremely effective in increasing the retention of a distal extension RPD.

Acknowledgments I wish to express my thanks to Dr Eric Skinner and Dr Keith Marc:rtift for their help on this project, Dr Joseph Moffa for use of the Public Health Facility, and Dr Armand Lugassy and COL Lewis Lor tion for their help with the statistical analysis.

References 1.

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Roach FE: Principles and essentials of bar claspipartial dentures. JAm DentAssoc 1930;17:124-138 Boucher L, Renner R: Treatment of Partially Edentulous Patients. St Louis, MO, CV Mosby, 1982, p 29 Kabcenell JI,: Efkctive clasping of removable partial den- tures.JProsthet Dent 1962;12:104-110 Academy of Denture Prosthctics: Glossary of Prosthodontic Terms (ed 4). St Louis, MO, CV Ivlosby, 1977 Stone E R Tripping action of bar clasps. J Am Dent Assoc 1936;23:596-617 Firtell DN: EIIect ofclasp design upon retention of removable partial dentures. J Prosthet Dent 1968;20:43-52 Zach G A .4dvantage of mesial rests for removable partial dentures. J Prosthet Dent 1975;33:32-35 Krol.45: Rernovablt: partial denture design-utline syllabus (ed 3) Sari Francisco, (2.4, University of the Pacific School of Dentistry, Bookstore, 1981 McCartnryJ\IV: Motion vector analysis of an abutment for a distal cxtcmion removable partial denture: A pilot study. J Prosthct Dent 1980;43:15-21 Frank RP: Evaluating refractory cast wax-ups for removable partial dcnturcs. J Prosthet Dent 1976;35:388-397 Fkatochvd FJ, CaputoiW: Photoelastic analysis of pressure on teeth and bone supporting removable partial dcnturcs. J Proqthrt Tlent 1974;32:52-61 Thompson WI), Kratochvil FJ, Caputo P A Evaluation of photoelastic stress pattcrns produced by various designs of bilateral distal extension remov-able partial dentures. J Pros- thet Dent 1977;38:261-273 Frank RP, Nicholls J I An investigation ol the eflctiveness of indirect retainers.,J Prosthet Dent 1977;38:494-506 Fisher lU,,Jaslow C : The efficiency of an indircct retainer. J Pro.;thet Dent 1975;34:24-30