David N. Firtell, D.D.S., M.A.,* Thomas W. Henberg, D.D.S.,** University of California, School of Dentistry, San Francisco, Calif.
and Jeremiah F. Walsh, D.D.S.***
he retention of roots for the preservation of supporting bone beneath complete overdentures is an accepted procedure. Retention of roots can also be advantageous for removable partial dentures, but the problem of fulcrum changes and associated lever action is introduced. The problem is not insurmountable but must be considered when designing removable partial dentures over retained roots.
base of a distal-extension removable partial denture introduces fulcrum points not normally present. The fulcrum is not at the abutment but at a point distal to it, where the denture rests on the retained root. During function the denture can rotate about that point. This relationship of the fulcrum point can be. derived logically, but it was the purpose of this bench study to demonstrate it.
ROTATION OF DISTAL.-EXTENSION REMOVABLE PARTlAL DENTURES
A major problem associated with distal-extension removable partial dentures is the stress placed on the abutment teeth and supporting tissues as the denture rotates about a fulcrum line during function. Kratochvil’ and Krol” advocated placing mesial rests and buccal I-bar clasp arms on the distal abutments of a removable partial denture with distal-extension ridges. Their recommendation was based on the premise that stressesare thereby reduced in function. There is also an improved relation with the underlying soft tissue in the distal-extension region. Miller3 advocated using distal rests on the abutment teeth. His intent was to reduce the length of the distalextension lever arm and increase the distance of indirect retainers from the fulcrum line. These recommended solutions to the problem of removable partial denture rotation are based on an average situation. However, Demer4 indicated that placing the rest on the mesial of an abutment tooth does not insure that the rest will become the fulcrum point. He stated that problems can arise due to changing fulcrum points and levers when abutment teeth are tipped or malpositioned. The retention of roots for the preservation of alveolar bone under the
*Professor and Chairman, Division of Removable Prosthodontics. **Askstant Clinical Professor, Division of Removable Prosthodontics. ***Assistant Professor, Division of Removable Prosthodontics.
C. V. Mosby
A model of a bilateral distal-extension lower jaw was fabricated with a 3 mm thickness of silicone rubber in the region of the ridges (Fig. 1). The silicone rubber was placed to allow the surface to be compressed and to simulate mucosa. The distal abutments were prepared with seats for me&al rests and distal guiding planes as suggested by Krol” so that the physiologic relief advocated by Kratochvil’ would not have to be done. Housings were prepared for three machine screws in the distal-extension part of the model. The screws could be positioned as required to simulate retained roots (Fig. 2). A removable partial denture framework was fabricated using mesial rests, distal proximal plates, and I-bar retainers (Fig. 3). Attached to the framework was a stylus with a pencil lead positioned as a lateral extension of the tip of the retentive arm on one side (Fig. 4). The framework was placed on the model, and the pencil lead of the stylus was put in contact with recording paper. For record purposes and clarity, the simulated roots (screws) were numbered I, 2, and 3 from anterior to posterior. The framework was depressed between roots 1 and 2, called point A, and again between roots 2 and 3, called point B (Fig. 5). The arcs made by the stylus were recorded as the framework moved about the fulcrums, first on the mesial of the distal abutments and then on each of the simulated roots. The framework was also depressed on the side which did not have simulated retained roots.
THE JOURNAL OF PROSTHETIC DENTISTRY
The testing model.
Fig. 2. Three machine [email protected]
are used as roots.
RESULTS The recorded arcs are depicted in Fig. 6. The stylus moved down and anteriorly when the framework rotated on the mesial rests. There wq no apparent difference between the arcs recorded when the framework was depressed at point A or at point B. When root 1 was positioned to contact the framework, the stylus moved up and slightly posterior, and them was no apparent difference in the arc when the framework was depressed at either point A or at point B. When root‘2 was positioned to contact the framework and the framework was depressed at point B, the stylus moved up and posteriorly. However, there was no apparent movement of the
is positioned on the model.
Fig. 4. The stylus is positioned as a lateral extension of the tip of the retentive arm (arrow). framework when force was applied at point A and the framework contacted root 2. Also, there was no apparent movement when root 3 was positioned to contact the framework and force was applied at points A or B. When the framework was depressed on the opposite side (the side without retained roots), the stylus pulled away from the recording paper and no distinct recordings could be made. Visually, however, the stylus appeared to rise when the framework was resting on any of the retained roots.
DISCUSSION As indicated by Demer,4 the most distal stable contact point of a distalextension removable partial
Fig. 5. Identification of roots and areas where the framework was depressed.
Fig. 7. A, A retained root is used with a removable partial denture. B, A possible solution to the problem is using 20-gauge wire retentive clasp arms.
Fig. 6. The recorded arcs. A, The arc is formed using the mesial rest as a fulcrum. B, The arc is formed using screw No. 1 as a fulcrum. C, The arc is formed using screw No. 2 as a fulcrum. (No arc was formed when screw No. 3 was used as a fulcrum because the pressure was applied between screw No. 3 and the mesial rest, thereby stabilizing the framework and eliminating the fulcrum.) denture becomes the fulcrum during function. When roots are retained distal to an abutment, the roots function as rest seats and as fulcrums. If an I-bar or any other clasp is designed to function routinely around the rest on a distal abutment where there is a distal retained root, the abutment tooth may be unduly stressed. The presence of a retained root changes the design requirements of the clasp. Because the fulcrum is now distal to the abutment,
the clasp on the side with a retained root must be designed as though it were a part of‘ a unilateral or Kennedy Class II removable partial denture. However, since there is no posterior abutment, the retention may be reduced and maximum stability is necessary. An effective guiding plane is mandatory, and the clasp design must be given special consideration. When an I-bar is used, it should be placed at the height of contour with minimai retention. If a suprabulge or combination clasp is used, a minimal undercut should be engaged with a 20-gauge wrought-wire retentive arm (Fig. 7). SUMMARY The position of a retained root for preservation of a ridge requires modification of the design of a distal-extension removable partial denture on the side of the retained root. A model with silicone rubber over the ridges and simulated retained roots was fabricated. A framework with mesial rests, distal proximal plates, and I-bar clasp arms was fitted to it. A pencil lead was attached to the framework to
record the arcs of rotation of the I-bar clasp arm. The movements and the fulcrums of the prosthesis were changed by the presence of simulated retained roots posterior to the distal abutment.
Miller, E. L.: Removable Partial Prosthodontics. Baltimore, 1972, The Williams and Wilkins Co., p 124. Demer, W. J.: An analysis of mesial rest-I-bar clasp designs. J PROSTHET DENT 36:243, 1976.
Reprint requeststo: DR. DAVID N. FIRTELL
UNIVERSITY OF CALIFORNIA SCHOOL OF DENTISTRY 738-bfED1C~L SCIENCE BLDC. SAN FRANCWCO, CALIF. 94143
1. Kratochvil, F. J.: Influence of occlusal rest position and clasp design on movement of abutment teeth. J PROSTHET DENT 13:114, 1963. 2. Krolj A. J.: Clasp design for extension-base removable partial dentures. J PRO~THET DENT 29:408, 1973.
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