SECTION EDITOR

RESEARCH AND EDUCATION

JOHN J. SHARRY

Removable partial denture research and its clinical significance Bert T. Cecconi, D.D.S., M.S.* Wilford Hall USAF Medical Center, San Antonio, Texas

The purpose of this article is to review research reported in the dental literature pertaining to removable partial denture prosthodontics and to discuss briefly some of the implications of that research. PARTIAL D E N T U R E D E S I G N The first survey of removable partial denture design was reported by McCracken 1 almost a decade and a half ago. Identical mandibular casts were sent to a number of commercial dental laboratories (selected at random from all areas of the United States) to compare designs made by them. An evaluation of tlhe various castings returned (28) demonstrated great variations in design. Results of this survey led to the conclusion that the practice of delegating the design of partial denture frameworks to dental laboratories is to be condemned. A survey of maxillary removable partial denture designs made by commercial laboratories was reported by Sykora and Calikkocaogle 2 in 1970. Twenty-five laboratories participated, 14 in North America and 11 in Europe. Again a wide variation of designs was observed, and the practice of delegating the designing of partial dentures to the laboratory technician was once more condemned. In 1973 a survey of removable partial denture designs by dentists was compiled by Frantz. 3 A surveyed maxillary cast was duplicated and given to 97 dentists who had varying degrees of experience in removable prosthodontics. T h e y were asked to design the partial denture that they considered ideal for that particular cast. All necessary information pertaining to the test case was given to the dentists. Ninety-six variations of design for the partial The views expressed herein are those of the author and do not necessarily reflect those of the United States Air Force or the Department of Defense. *Colonel, USAF (DC).

0022-3913/78/0239-0203500.80/0 9 1978 The G. V. Mosby Co.

denture on that cast were submitted by the 97 dentists. At a later date Franz 4 conducted another survey of dentists' designs for a removable partial denture. This was undertaken to determine if more uniform designs would be obtained when the dentists had the opportunity to examine the patient, view the radiographs, survey the diagnostic casts, and modify the existing abutment teeth. A maxillary partial denture was again selected as the test case. Fifty-seven dentists participated in this survey, and 57 different designs were submitted. A 1969 article by Atkinson and Elliott 5 was concerned with the ability of 10 recent dental school graduates to provide the clear, concise, and meaningful instructions and designs that a dental laboratory technician needs to fabricate a removable partial denture. An average score of 47% demonstrated to the authors that those recent graduate did not, Under the guidelines of the study, give adequate instructions to the dental technicians. In 1972 Trainor and associates 6 expanded on the survey of Atkinson and Elliott. The performances o f 19 dental officers (representing 15 dental schools ) were evaluated before and after they had received a graduate-level course in removable partial denture design and fabrication. T h e data indicated that, following the course, the group improved in every category under evaluation. After reviewing these surveys two p h e n o m e n a in the field of removable p a r t i a l denture design are identified. T h e first is that regardless of who designs the partial denture (dentist or technician), a variety of designs can be expected. Second, given a controlled educational environment a group of dentists will be more uniform in their design concepts. One is now confronted with the need of explaining

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the existence of these at least somewhat disturbing phenomena. The source of the first dilemma very likely is that the relatively few available research facts from which one could justify design concepts are not identified or applied. As to the latter factor we have to date directed our efforts in partial denture design education toward reaching each individual student and practitioner. This is done in the hope that they m a y understand and perform this professional duty. T h a t this hope has not materialized is self-evident. Perhaps the time has come to reexamine and reorganize this approach to partial denture design. FUNCTION AND AWARENESS Yurkstas 7 undertook a study to approximate, in a general way, the effect of missing teeth on masticatory performance and efficiency. He reported that the loss of one or two molars decreased masticatory efficiency from 33% to 66%. The average efficiency of complete denture wearers (191 were tested) was found to be about 24%. T h e food platform area of occluding teeth was identified as an important factor in determining masticatory efficiency. Forces exerted by removable partial and complete dentures were measured by strain gauges in a 1953 study by Yurkstas and Curby. 8 T h e y observed that patients with complete and removable partial dentures utilized similar forces during mastication. Relatively constant forces were used with each stroke, with masticatory difficulties overcome by an increase in the n u m b e r of strokes. Some interesting information was collected when Abel and Manly 9 measured masticatory efficiency of removable partial denture patients before and after completion of prosthetic restorations. With removable partial dentures opposing maxillary dentures mastieatory efficiency was approximately 20% to 25%. Efficiency with the partial dentures occluding against a natural dentition was 30%. There was no indication of superiority of any type of partial denture (tooth-supported or free-end distal-extension). Functional evaluations of fixed and removable partial dentures were reported by Yurkstas and associates 1~ and Nagasawa and Hiromichi. 'I Both found masticatory efficiency to be the same for both removable and fixed replacements for a missing mandibular first molar. Nagasawa and Hiromichi also electromyographically recorded the activity of the anterior belly of the temporal muscles and found

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that the rhythm of the masticatory movement became more regular and movement bursts shorter with the fixed partial dentures. An excellent review of the literature dealing with oral perception and proprioception and their significance to prosthodontics was compiled by C r u m and Loiselle. '2 They reported K a w a m u r a ' s work on sensitivity in the anterior part of the mouth, particularly the tongue, periodontal ligament, and mucosa. Sensitivity becomes progressively less acute from the anterior part of the mouth t o the middle portion. Oral stereognosis (the capacity to identify forms without the aid of vision through oral manipulation) decreases toward the posterior sector of the mouth. (]rum and Loiselle x2 also mentioned work by Molnar and others who studied regions such as the floor Of the mouth, the m a n d i b u l a r alveolar tubercle, and the buccinator pockets, where three-dimensional localization could barely be determined. Berry 13 tested oral awareness to removable partial denture design. A two-point discriminatory test in 14 patients showed that in the mouth the anterior part of the palate, the tip of the tongue, and the lingual gingivae behind the mandibular incisors are the sites of greatest discrimination. From the studies on function one becomes aware that, once a significant n u m b e r of teeth have been lost, masticatory efficiency cannot be re-established with removable partial dentures to a level:near 100%. This would lead the dentist to advise the patient of a need to alter his or her masticatory habits to compensate somewhat for the expected decrease in efficiency. In evaluating which type of prosthesis (fixed or removable) would be indicated for a given situation, it is likely that factors other than masticatory efficiency would more heavily influence the final decision. Regions in the oral cavity identified as being particularly sensitive were the tip of the tongue, the anterior portion of the palate, and the lingual gingivae behind the mandibular incisors. When feasible these tissues should not be impinged upon when designing a removable partial denture. Conversely those areas that have been shown to have a low level of discrimination should be utilized b y the replacement prosthesis if at all possible. IMPRESSIONS Lytle 14 measured soft-tissue displacement beneath removable partial and complete denture bases. Partially edentulous ridges in 25 patients were stud-

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led. Gross tissue displacement was observed when the bases were underextended and when the denture teeth contacted prematurely. The author concluded that partial dentures should be designed with strategically placed occlusal rests on the remaining natural teeth. Strain gauges were used by Metty 1~ to record the movements of four free-extension removable partial denture bases during loading. Alginate (irreversible hydrocoll0id ) and fluid wax were the impression materials used. Metty summarized that the records indicated that an impression technique which allows controlled displacement of transient tissue bulk (in his situation the fluid-wax technique) will provide the most stable partial denture base. Holmes 16 measured partial denture movement (mandibular Class I) in three patients to determine which impression technique permitted the least amount of movement at the time of insertion. It was concluded that movement of partial dentures, resulting from occlusal loading, is related to the impression technique and impression material used. The altered-cast technique provided, the least amount of movement from occlusal loading for 11 of the 12 partial dentures tested. Korecta wax IV* impressions were associated with the least amount of denture base movement. A comparative study of impression procedures for distal-extension removable partial dentures was performed by Leupold. 1~ Contour tracings of the supporting ridges of five patients indicated that a more favorable relationship (denture base to ridge) was obtained by use of the altered-cast procedure than by use of the stock-tray irreversible hydrocolloid impression and the one-piece cast. Pressure exerted on the supporting ridge by freeextension removabl e partial denture bases has been observed clinically:aS Clear acrylic resin bases with no artificial teeth attached to them were utilized. These bases were processed on master casts altered from various impression materials (irreversible hydroeolloid, regular- and light-body rubber base, zinc-oxide/eugenoI paste, and Koreeta wax IV). Observations of the edentulous ridge subjacent to the clear bases were made at insertion of the test framework and after 48 hours. Tissue blanching was evident (to a greater or lesser degree)at both periods regardless of the type of impression material used for the corrective impression. *Kerr Mfg. Co., Romulus, Mich.

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Research indicates that a more stable removable partial denture base (at the time of insertion) can be obtained by use of a corrective impression/alteredcast technique rather than a single impression. When choosing the material to accomplish the corrective impression, one has to take into consideration not only base stability but also resultant subjacent tissue pressure. Each dentist must evaluate these two factors prior to choosing a corrective impression material. RETENTION A laboratory study of enamel abrasion by removable partial denture clasps was reported in a 1953 article by Phillips and Leonard? 9 Circumferential cast clasps (chrome-cobalt and gold) engaging 0.02 inch undercuts were tested. Clasps did not cause enamel abrasion regardless of the metal used. Severe abrasion was noted, however, when an amalgam restoration (disto-occlusal) provided the bearing surface for either type of clasp. Bates 2~evaluated three possible causes of retention loss with chrome-cobalt removable partial dentures. He stated that, due to the inheren~ errors in production technique, high modulus of elasticity, and lack of a safety factor, it is preferable to use gold as the clasping material. A wrought-wire retention clasp arm is recommended if use of gold is not possible. FirtelF a made a comparison of the ability of a limited number of retentive Clasp arms to resist vertical displacement. A metal cast was used to simulate an ideal denture situation, and a 0.03 inch undercut was engaged. O f the nine clasp designs tested the U clasp afforded the highest degree of retention, followed closely by the ring clasp:About the same amount of force was required to dislodge the Akers clasp, the Akers clasp plus wroughtZwire retentive arm, and the T-bar clasp. An investigation of the efficiency of indirect retainer designs was done by Fisher and Jaslow. z2 During negative loading (moving the base away from ridge) of the test partial denture they recorded movement of the mandibular canine teeth on which the indirect retainers rested. After evaluating their data they concluded that the efficiency of indirect retainers is greater when a positive rest seat is used than when the retainers rest on the inclined lingual surfaces of anterior teeth. Dunny and King 23reported a preliminary study of minor connector designs for the attachment of acrylic resin bases. Large openings in a retention

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design provided greater bulk and therefore supplied stronger attachment possibilities than small openings. Labial projections of a retentive framework over the crest of residual ridges were found to reduce the strength of the resin attachment. FORCE ANALYSIS (IN VITRO) Removable partial denture design. In 1956 Frechette 24 first investigated the effect of removable partial denture design on forces imparted to abutment teeth. Results indicated that the loading and movement of abutment teeth are strongly influenced by (1) the n u m b e r and location of occlusal rests, (2) contour and rigidity of major connectors, and (3) extension of the denture bases. T h e latter two, rigidity and base extension, were also found by Kaires2,-,. 28 and Henderson and Seward 27 to significantly affect force transmission to a b u t m e n t teeth. A very thorough evaluation and analysis of factors governing load distribution and transmission by removable partial dentures was done by Hekneby.28, 29 Major connector flexibility was again shown to exert an important influence on distribution of load to the supporting teeth. In addition it was experimentally demonstrated that the point of application of vertical pressure on a horizontal plate influenced both load transmission to the tooth and distribution of load by the base. Denture base movement was found to be altered by the inclination o f the base. Sagittal inclination of the residual ridge (i.e., angle of the base) was confirmed in another study as affecting abutment tooth movement. ~~ In this study the type of loading was identified as another factor acting on abutment tooth movement. D a t a analyzed showed that bilateral loading of the partial denture consistently caused less a b u t m e n t tooth movement than unilateral loading. In another study by these same investigators it was demonstrated that the fit of the removable partial denture base was capable of altering not only the magnitude of abutment tooth movement but also the direction of that movement. 31 Kratochvil and Caputo, 3~ in a photoelastic analysis, demonstrated that fit of the partial denture framework affected forces exerted on the supporting teeth. T h e y concluded that the adjusted casting keeps the line of force within the long axis of the a b u t m e n t teeth. Clasp assembly. Clayton a n d Jaslow 33 used strain gauges to measure clasp forces exerted on a b u t m e n t teeth. PGP wrought-wire retentive clasp arms were

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tested along with various cast bar clasps. The authors f o u n d that all clasps tested exerted forces against the tooth when at rest. Wrought-wire clasps were shown to exert a greater force against the abutment tooth than the cast bar clasps when the base was seated, at rest, positively loaded, and removed. Shohet a~ studied the relative magnitudes of stress exerted on teeth by four different retainers: (I) the circumferential cast clasp, (2) the back-action clasp, (3) internal precision attachments , and (4) the C and L attachment. The greatest degree of destructive stress was registered with the precision and conventional cast clasps. Under most of the testing conditions the back-action clasp caused less displacement and stress than the conventional clasp in all directions except mesially. With few exceptions the C and L attachment caused less displacement and stress than the precision attachment in all directions. Four different clasp assemblies were tested in a 1971 inquiry to measure their effect on the magnitude and direction of a b u t m e n t tooth movement. 3~ The clasp assembly consisting of a mesio-occlusal rest, distal guide plane, and buccal I-bar retentive clasp arm caused significantly greater a b u t m e n t movement than the other assemblies t6sted. Three of the clasp assemblies that were identical except for the type of buccal retentive clasp arm did not differ significantly in their effect o n supporting tooth movement. Direction of a b u t m e n t tooth movement was unaffected by clasp designs. An analysis or clasp assembly modifications and their effect on a b u t m e n t tooth movement was reported by the same investigators. 36 Tooth movement was measured with the clasp assemblies intact, with the lingual clasp a r m removed, a n d finally with the buccal retentive clasp arms eliminated. Abutment tooth movement was not significantly altered when the clasps were removed. This thereby established that forces created when a partial denture is loaded are transmitted to a b u t m e n t teeth primarily through the occlusal rests. When the a b u t m e n t tooth and the load applied to the partial denture were on the same side of the arch, tooth movement was greater than when these were on opposite sides. Nally 3~ evaluated methods of handling a b u t m e n t teeth in Class I mandibular removable partial dentures. Both vertical and horizontal displacements of the supporting tooth were measured. In the first experimental series (eight different clasp system s on eight different frameworks) the least a m o u n t of tooth

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movement was obtained with a mesial connector/ mesio-occlusal rest assembly, or the back-action clasp. Tooth m o v e m e n t was measured after the indirect retainers were removed from four of the test frameworks. Without the indirect retainers an increase in abutment movement was obtained. A study into the effect of rest design on transmission of forces to a b u t m e n t teeth was published in 1974. 38 Statistical analysis of the data showed that precision rests and deep rests affect tooth movement in a similar manner. Rests with gingival seats at m a x i m u m depth in abutment teeth were found to decrease movement of those teeth. Two studies pertaining to stressbreakers were found in the literature) 7' 39 In one of these, a b u t m e n t tooth and ridge displacements were measured) 9 These movements were recorded while the stressbreakers were active and after they had been deactivated (made rigid). There was no evidence to demonstrate any advantage to the use of stressbreakers as far as tooth m o v e m e n t and ridge displacement were concerned. Nally 3~ tested two stressbreakers and their effect on a b u m e n t tooth movement and concluded that their use was not recommended. A research project involving guide planes and abutment tooth movement has been described? ~ It demonstrated that guide planes could affect abutm e n t tooth movement depending upon the type of clasp assembly utilized. Major connectors. Craig and Peyton 4' and Craig 42 studied strains in a typical mandibular free-extension removable partial denture framework. T h e y found that the amount of strain observed was a function of the length o f the bases, bulk of the framework, relation of the site of the strain gauge to the position of the loading, the presence of an indirect retainer, and the position of the fulcrum for the loading conditions in use. Concentrations of stress were found in the central portion of the lingual bar and in the lateral regions where the lingual bar terminated and the retention mesh began. Perhaps the most important contribution of this group of research articles is the firm foundation that they have established from which accepted (though unproven) concepts of removable partial denture design can, and should he, questioned. Even though several conflicting conclusions from these studies are evident, an analysis of the individual research reports would resolve the. apparent discrepancies. Without a doubt all laboratory studies are prelim-

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inary. Although m a n y facets of partial denture design can be validly examined in the laboratory, in the final analysis clinical investigations are necessary to determine biologic and physiologic responses to the prostheses. CLINICAL STUDIES Force analysis. Kaires 43' 44 undertook to electronically study the effect of removable partial denture design on functional force distribution to the supporting tissues. There was no observable significant relationship between design and masticatory performance. However, a reduction in the size of the occlusal table decreased the forces acting on the partial denture, thereby lessening stresses on the abutment teeth and the supporting tissues. It was found that a rigid design is generally more desirable than a flexible one. Matsumoto and Goto 4~ measured the r e l a t i v e excursion of a b u t m e n t teeth when known forces were applied to three types of removable partial dentures. Forces applied directly to a b u t m e n t teeth were decidedly reduced by the wearing of a partial denture. When the partial denture was loaded the abutment teeth nearest the denture base generally received the greatest part of the load. The findings also suggested that the fulcrum of rotation varies according to the site and a m o u n t of applied force as well as according to the design of the removable partial denture. Dynamic investigations of palatal and lingual bars were undertaken by Sekine and associates? ~ 47 Strain gauges were used to measure the stresses on the connector during function. Under varying occluding conditions only a slight stress appeared in the central part of the bar. The greatest strain in the center was far less than the greatest strain at the bordering parts (where the bar and retention mesh join). Indications were that the magnitude of stress occurring in the palatal bar depended on various factors including jaw form, position of the artificial tooth in relation to the residual ridge, and the size and form of the ridge: Experimental procedures for analysis of lingual bars were carried out in a similar manner. T h e greatest strain on the lingual bar was concentrated at the bordering sections (junction of bar and base). Strain at the central section was much less than that at the lateral regions. The same factors that could affect stress in the lingual bar could affect stress in the palatal bars. A design of a lingual bar applying these and other

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research findings has been presented in the literature. 4~ That bar was positioned in the alveololingual fold to maximize rigidity, minimize patient awareness, and avoid impingement on attached gingival tissues. Kydd, Dutton, and Smith 19 measured intraorally the lateral horizontal forces applied to the buccal and lingual surfaces of removable partial denture abutment teeth during swallowing and mastication. Data were obtained on five subjects. Amplitude, frequency, and duration of force were considered in the evaluation. When computed on a daily basis forces exerted on abutment teeth during swallowing were almost twice the force exerted during mastication. Lowe, Kydd, and Smith 5~ determined forces exerted by the tongue on lingual partial denture flanges of varying thicknesses during involuntary swallowing. Results suggested that on a daily basis the resting posture of the tongue contributes more lateral force to a lower dental prosthesis than do swallowing forces. Thickness of the lingual flange was shown to be a factor capable of increasing forces applied to the partial denture. Mobility. Changes in mobility of abutment teeth supporting removable partial dentures were studied by Fenner, Gerber, and M u h l e m a n n ? 1 Clinical measurements were made on mandibular canines during a 7 month period. The four subjects with maxillary complete dentures had only their anterior mandibular teeth. Cast circumferential clasps and wrought clasps with no rests were utilized. The authors summarized that significant acute or gradual tooth mobility resulted from treatment with these partial denture prostheses. Hofmann 52 investigated abutment tooth kinematics in 10 patients. Unilateral free-end dentures were supported by means of adapted cast clasps. It was observed that the abutment teeth moved in all three dimensions. The rate and direction of these movements depended mainly on point of load application, direction of stress exerted, and the angle between the axis of a given tooth and its occlusal plane. Goto ~3 also studied changes in abutment tooth mobility resulted from removable partial dentures. Unilateral partial dentures (all of the same design) were made for 16 patients. Mobility was checked at intervals of up to 2 years. Results indicated that changes in abutment tooth mobility were greater in the second premolar (with a direct retainer) than in the first premolar (with an indirect retainer). The

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magnitude of the changes in mobility correlated with initial mobility. The transmitting force from the denture to the abutment tooth gradually decreased, depending upon the denture-wearing period. Another mobility study was reported by Goodkind? 4 Removable partial dentures of similar design were constructed for five patients. A force of 500 gm was applied at right angles to the long axis of the abutment tooth, and movements were measured in buccal and lingual directions. It was concluded that the isolated abutment (mandibular second premolar) became more mobile in a buccai direction. Lingual mobility was not altered. Plotnick 5~measured changes in tooth mobility and chewing efficiency, as related to variations in the opposing dentitions in partially edentulous mandibles. Results indicated an increase in mobility of abutment teeth following placement of a removable partial denture. After 18 months mobility returned to the original level. Comparisons of chewing efficiency indicated no particular differences between subjects with unilateral or bilateral distal-extension removable partial dentures or different types of opposing dentitions. Masticatory efficiency approximated 50% of normal with the use of the partial dentures. Rudd and O'Leary ~ authored a preliminary report on the stabilization of periodontally weakened teeth and guide-plane removable partial dentures. They reported that data collected on 12 subjects (from several months to 2 years) indicated that carefully planned, designed, fabricated, and fitted removable partial dentures with guide planes may be effective in stabilizing mobile teeth. Various clinical surveys of removable partial denture patients have been reported in the literature. 5~-~~In these studies it was found that a tissueborne partial denture design covering both gingiva and mucosa and fabricated in acrylic resin always resulted in retrograde oral changes. Personal factors identified as being important to the success of a partial denture were oral hygiene, motive for wearing the denture, and general health of the patient. Investigators have observed that prostheses should be designed as simply as possible so as not to interfere with t h e possibility of good oral hygiene. Seeman 61 studied the relationship of periodontal disease and the wearing of removable partial dentures in 129 patients. Results showed that the index of periodontal disease was higher in those teeth

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involved in the design. H e s u m m a r i z e d that both gingival coverage a n d i n a d e q u a t e oral hygiene were found to be significant factors in the p r o d u c t i o n of periodontal disease. Carlsson, Hedegard, a n d K o i v u m a a 62 reported on a 4 year l o n g i t u d i n a l investigation of d e n t o g i n g i v a l l y supported removable partial dentures in 88 patients. A significant n u m b e r of a b u t m e n t teeth (18%) became loose d u r i n g the follow-up period, a n d a n a d d i t i o n a l 25% showed a t e n d e n c y for increased mobility. Patients n o t w e a r i n g their partial dentures had no significant changes in tooth mobility. Sixtyeight percent of the a b u t m e n t teeth showed a significant increase in gingival i n f l a m m a t i o n . T h e r e was a 25% increase in pocket d e p t h a r o u n d the a b u t m e n t teeth. A strong correlation was observed between the presence of local pathologic alterations a n d both the use of the partial dentures a n d poor oral hygiene. Serial radiographs were used in a study by Plotnick, Beresin, a n d Simkins 63 of the effects on m a n d i b u l a r bone of variations in the opposing dentition. All subjects were in need of m a n d i b u l a r freeextension removable partial dentures. M e a s u r e m e n t s were m a d e at 1, 6, 12, a n d 18 m o n t h intervals. T h e greatest a m o u n t of change was in the group with m a n d i b u l a r removable partial d e n t u r e s opposing complete dentures. I n the control group (who did n o t wear the partial dentures) almost no change was noted. In all groups w e a r i n g the p a r t i a l d e n t u r e s there was a large increase in a b u t m e n t tooth movement, More t h a n 60% of the subjects in every group showed no bone loss, T h e wearing of r e m o v a b l e partial d e n t u r e s has been implicated as a c o n t r i b u t i n g factor in a b u t m e n t tooth mobility a n d gingival i n f l a m m a t i o n . Some factors of design that can m i m m i z e these u n t o w a r d reactions have been identified in l a b o r a t o r y studies a n d s u b s t a n t i a t e d in clinical investigations. C o n t i n u ing clinical inquiries to identify a n d confirm more of these factors are imperative. SUMMARY T h e a u t h o r has studied as m a n y r e m o v a b l e partial d e n t u r e texts (13), articles ( a p p r o x i m a t e l y 265), a n d abstracts (approximately 75) as were available to h i m these past 10 years. Sixty-three of those articles pertained to research. This review has abstracted those investigations. REFERENCES 1. McCracken, W.: A survey of partial denture design by commercial laboratories. J PROSTHETDENT 12:1089, 1962.

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2. Sykora, O., and Calikkocaogle, S.: Maxillary removable partial denture designs by commercial dental laboratories. J PROSTHETDEter 29:633, 1970. 3. Frantz, W.: Variability in dentists' designs of a removable maxillary partial denture. J PROSTHETDENT29:172, 1973. 4. Frantz, W.: Variations in a removable maxillary partial denture design by dentists. J PROSTHETDENT34:625, 1975. 5. Atkinson,R., and Elliott, R.i Removable partial dentures designed for laboratory fabrication by recent dental school graduates. A survey. J PROSTHETDENT22:429, 1969. 6. Trainor, J., Elliott, R., and Bartlett, S.: Removable partial dentures designed by dentists before and after graduate level instruction: A comparative study. J PROSTHETDENT27:509, 1972. 7. Yurkstas, A.: The effect of missing teeth on masticatory performance. J PROSTHETDENT4:120, 1954. 8. Yurkstas,A., and Gurby, W. A.: Force analysis of prosthetic appliances during function. J PROSTHETDENT3:82, 1953. 9. Abel, L., and Manly, R.: Masticatory function of partial denture patients among Navy personnel. J PROSTHETDENT 3:382, 1953. 10. Yurkstas, A., Fridley, H., and Manly, R.: A functional evaluation of fixed and removable bridge work. J PROSTHET DENT 1:570, 1951. 11. Nagasawa, T., and Hiromichi, T.: A comparative evaluation of masticatory efficiencyof fixed and removable restorations replacing mandibular first molars. J PROSTHETDENT30:263, 1973. 12. Grum, R., and Loiselle, R.: Oral perception and proprioception: A review of the literature and its significance to prosthodontics. J PROSTHETDENT28:215, 1972. 13. Berry,D.: Oral awareness and the design of partial dentures. Proc R Soc Med 62:608, 1969. 14. Lytle, R.: Soft tissue displacement beneath removable partial and complete dentures. J PROSTHETDENT 12:34, 1962. 15. Metty, A. G.: Obtaining efficient soft tissue support for the partial denture base. J Am Dent Assoc 56:679, 1958. 16. Holmes,J.: Influence of impression procedures and occlusal loading of partial denture movement. J PROSTHETDENT 15:474, 1965. 17. Leupold,R,: A comparative study of impression procedures for distal extension removable partial dentures. J PROSTHET DENT 16:708, 1966. 18. Ceceoni, B., and Jaslow, C.: Impression techniques for removable partial dentures and resultant pressure on the edentulous ridge. Mich State Dent J 53:112, 1971. 19. Phillips,R., and Leonard, L.: A study of enamel abrasion as related to partial denture clasps. J PROSTHETDENT6:657, 1956. 20. Bates, J.: Studies on the retention of chrome-cobalt partial dentures. Br Dent J 125:97, 1968. 21. Firtell, D.: Effect of clasp design upon retention of removable partial dentures. J PROSTHETDENT20:43, 1968. 22. Fisher, R., and Jaslow, C.: The efficiency of an indirect retainer. J PROSTHETDENT34:24, 1975. 23. Dunny, J., and King, G.: Minor connector designs for anterior acrylic resin bases. J PROSTHET DENT 34:496, 1975. 24. Frechette, A.: Influence of partial denture design on distribution of forces on abutment teeth. J PROSTHETDEYr 6:195, 1956.

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25. 26. 27.

28.

29. 30.

31.

32.

33. 34. 35.

36.

37. 38. 39. 40. 41.

42.

43.

44.

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FEBRUARY 1978

VOLUME39

NUMBER2