Removable partial denture design for a few remaining natural teeth Ali Bolouri, D.M.D., D.D.S.* Baylor College of Dentistry, Dallas, Texas
This article presents a partial denture design utilizing the hinge-lock apparatus. The patient for whom this design was used is classified as a Kennedy Class II, mod. 1.1 The mandibular arch is bilaterally edentulous, and only four unilaterally posterior teeth are remaining (Fig. 1). The hinge-lock design was used to reduce torque and tooth movement and to provide maximum retention, support, and bracing. 2 In light of these goals it must be remembered that the axis of rotation, i.e., the line around which the denture tends to rotate (Fig. 1), is not bilateral; it is a unilateral line. This presents a problem in providing indirect retention. The ideal location for providing indirect retention is on the side opposite the free-end extension a as far from the axis of rotation as possible. As seen in Fig. 1, the location for indirect retention in this instance lies on the buccal portion of the natural teeth. With the hinge-lock design presented, it is possible to obtain the needed retention through the buccal arms of the hinge-lock apparatus (Fig. 2). The length and location of the buccai arms are important factors in determining the magnitude of stress on the abutment teeth. PROCEDURE 1. Obtain a diagnostic cast using irreversible hydrocolloid impression material? 2. Survey the diagnostic cast to determine if it is necessary to recontour, reduce, or restore any of the remaining natural teeth. 3. Obtain a master cast after all necessary modifications have been performed and after preparations for the rest seats have been accomplished (Fig. 1). 4. Design the framework. The specifications given *Assistant Professor, D e p a r t m e n t o f R e m o v a b l e P r o s t h o d o n t i c s .
M A R C H 1978
Fig. 1. The partially edentulous upper and lower casts, demonstrating the axis of rotation for the removable partial denture. to the dental laboratory technician for the length of each arm from the labial bar of the hinge-lock should be precise, since flexibility and therefore retention are proportional to the length of the clasp arm from point A to point B (see Fig. 6). 5. Adjust the framework to the mouth (Fig. 3). 6. Replace the casting on the master cast. 7. Construct an impression tray over the meshwork (Fig. 4). 8. Border-mold the tray in the mouth. 9. Record an impression of the soft-tissue-bearing area (Fig. 5). The principle behind the altered cast is to obtain support for the hinge-lock partial, as in any
0022-3913/78/0339-0346500.30/0 © 1978 The C. V. Mosby Co.
REMOVABLE PARTIAL DENTURE DESIGN
Fig. 2. Partial denture framework in position on the final cast with the hinge-lock closed.
Fig. 4A. Individual acrylic resin impression tray is attached to the framework on the cast.
Fig. 3. Partial denture framework is tried in the mouth. extension-base partial denture? During steps 4 and 5, the occlusal rests and indirect retainers must be properly seated to the teeth. During insertion of the framework to complete the border molding and impression procedures, the hinge-lock must be in the closed position (Fig. 3). 10. Complete the restoration, following the same procedure as with conventional extension-base partial dentures (Fig. 6). 6 DISCUSSION In this instance the effects of the forces of occlusion and consequently of the rest clasps are different than from when similar hinge-locks are used for remaining anterior teeth. There are similarities between the hinge-lock design and the rest, proximal plate, I-bar retainer
THE JOURNAL OF PROSTHETIC DENTISTRY
Fig. 4B. The individual acrylic resin impression tray is attached to the framework off of the cast. (RPI) concept of partial denture design. Since the R P I design favors an extension-base (tissue-borne) removable partial denture, 2 the hinge-lock can serve the purpose better than a conventional design. Another advantage of the hinge-lock design is the minimal coverage of tooth structure and tissue that results in improved oral hygiene and reduced plaque accumulation under clasps and other components of the partial dentures. Fig. 7 shows the fulcrum line AB, around which the partial denture, as designed, will rotate with the application of masticatory forces. In an alternative design, the length of arm R can be shortened to reduce the effects of these forces. This is accomplished by setting the artificial teeth on the edentulous side as far lingually as possible and moving the rest seats on the natural dentition from
J 0 Fig. 5. Impression of the tissue-bearing part of the removable partial denture after removal of assembly from the mouth.
Fig. 7. Masticatory functions of the denture are improved by moving denture teeth and rest seats lingually. See text for description.
c o n n e c t e d to t h e o c c l u s o l i n g u a l rest a n d act as a n indirect retainer. REFERENCES
Fig. 6. Upper and lower partial dentures in place in the mouth. the m e s i a l surface o c c l u s o l i n g u a l l y . As a result o f these t w o a d j u s t m e n t s t h e l e n g t h o f a r m R is r e d u c e d to R " a n d t h e effects o f m a s t i c a t o r y forces a r e r e d u c e d . T h e f u l c r u m line A B n o w b e c o m e s CD. A n o t h e r rest at t h e o c c l u s o b u c c a l p o i n t N c o u l d be
1. Henderson, D., and Steffel, V. L.: McCracken's Removable Partial Prosthodontics, ed 4. St. Louis, 1973, The C. V. Mosby Company, p 15. 2. Krol, A. J.: RPI (rest, proximal plate, I-bar) clasp retainer and its modifications. Dent Clin North Am 17:631, 1973. 3. Applegate, O. C.: Essentials of Removable Partial Denture Prostheses, ed 3. Philadelphia, 1966, W. B. Saunders Company, p 232. 4. Dykema, R. W., Cunningham, D. M., and Johnston, J. F.: Modem Practice in Removable Partial Prosthodontics, ed 1. Philadelphia, 1969, W. B. Saunders Company, pp 14-28. 5. Henderson, D., and Steffel, V. L.: McCracken's Removable Partial Prosthodontics, ed 4. St. Louis, 1973, The C. V. Mosby CompAny, pp 274-275. 6. Henderson, D., and Steffel, V. L.: McCracken's Removable Partial Prosthodontics, ed 4. St. Louis, 1973, The C. V. Mosby Company, p 267. Reprint requests to: DR. ALl BOLOURI BAYLORCOLLEGEOF DENTISTRY 3302 GASTONAVE. DALLAS,TEXAS75246