Removable partial denture design: a review and a challenge

The strength of multiple major connector designs under simulated functional loading

STATEMENT OF PROBLEM

The design of a removable dental prosthesis (RDP) must balance functional strength, comfort to the patient, and the health of the tissue. While research has been conducted to enhance the strength of major connectors, little has been done to determine if the dimensions of major connectors can be reduced in order to enhance patient comfort and tissue health.

PURPOSE

The purpose of this study was to determine the minimum major connector dimensions of 1 mandibular and 2 maxillary major connectors that would provide adequate functional strength.

MATERIAL AND METHODS

Sixty chromium-cobalt alloy (Vitallium) RDP frameworks were fabricated. The major connector designs were: a mandibular lingual bar, a maxillary palatal strap, and a maxillary anterior-posterior (A-P) palatal strap. Four groups of 5 frameworks with diminishing dimensions were fabricated for each major connector design. The lingual bar was tested at 4, 3, 2.5, and 2 mm in height, occlusogingivally, and 1.6 mm in thickness; the palatal strap at 8, 6, 4, and 2 mm, anteroposteriorly; and the A-P palatal strap at 10 x 6, 8 x 4, 6 x 2.5, and 4 x 2 mm, anteroposteriorly. All maxillary frameworks were 0.65 mm in thickness. The frameworks were of a Kennedy Class II Mod I design with 3 widely separated vertical reference points to measure deformation. Two tests were conducted to evaluate the functional strength for each framework. The first test was masticatory simulation, or torsional force. The second test was a drop test from a height of 3 feet. Permanent deformation was then determined after each test. The Cochran-Armitage test (alpha=.05) was used for both the torsion test and the drop test.

RESULTS

A statistically significant difference in permanent deformation was found for the palatal strap design among the 4 different dimensions for the compressive test (P=.015) and the drop test (P=.044).

CONCLUSION

It is safe to reduce the dimensions of some major connectors under normal loads. The reduced size of the connectors places the removable partial denture at increased risk for deformation when dropped from a height.

Influence of abutment selection in maxillary Kennedy Class II RPD on elastic stress distribution in oral mucosa: an FEM study

PURPOSE

The aim was to study the influence of abutment selection on elastic stress distribution in oral mucosa in a maxillary removable partial denture (RPD) by means of 3-dimensional finite element models.

MATERIALS AND METHODS

Four RPD framework models of an equal size (by area) and underlying oral mucosa were produced for a Kennedy Class II arch. Each framework included an occlusal rest as part of a clasp assembly on one of four abutments (canine, first, and second premolars, and first molar) on the side contralateral to the edentulous ridge (tooth-supported side). Movement of the alveolar surface of the mucosa and the occlusal rest on the abutment adjacent to the ridge were fixed in a vertical direction. Movement of the rest on the tooth-supported side was restricted in all directions. Vertical or buccally oblique biting force was applied simultaneously on each of the locations representing three missing teeth.

RESULTS

The frameworks with the contralateral side rest on the canine or the first premolar were less resistant to lateral forces than other framework designs, showing greater saddle displacements under the oblique force than the vertical force. The framework with the rest on the second premolar demonstrated relatively good resistance to deflection; however, both vertical saddle intrusion and the maximum equivalent stress in mucosa shown in all the models were within small ranges.

CONCLUSION

The saddle movement was influenced by the abutment selection on the tooth-supported side, although resultant stress in the mucosa was insensitive to the abutment location.

The effect of design modifications on the torsional and compressive rigidity of U-shaped palatal major connectors

STATEMENT OF PROBLEM

Rigidity is a requirement of removable partial denture frameworks. Although commonly used by dentists, the U-shaped (horseshoe) maxillary major connector lacks the rigidity of other maxillary major connector designs.

PURPOSE

The purpose of this study was to determine the effects of changing width, thickness, and shape on the rigidity of U-shaped maxillary major connectors.

MATERIAL AND METHODS

Five nickel-chrome alloy frameworks were fabricated from the same master cast for each of 4 different U-shape removable partial denture designs. The designs were an 8-mm wide U-shaped strap with a 6-mm posterior strap (A-P strap), a 13-mm-wide U-shape strap (Wide), an 8-mm-wide U-shape strap that widened to 13 mm at the midline (Notch), and an 8-mm-wide U-shaped strap that was twice the thickness of the other straps (Thick). A fifth group of 5 frameworks was made by removing the posterior strap from the A-P strap frameworks (A strap). Two testing points were marked on each framework corresponding to the first premolar and second molar positions. The frameworks were mounted in a universal testing machine, and vertical (torsional) and horizontal (compressive) loads were applied using a 10-kN load cell at a cross-head speed of 2 mm/min until a deflection of 1 mm occurred. A force-deflection curve was generated for each test, and slope of the curves (N/mm) was compared with analysis of variance and Scheffe's F test (alpha=0.05).

RESULTS

The Thick group was found to be significantly more rigid (P<.05) than the other frameworks when torsional loads were applied to both the premolar (22.42 N/mm) and molar (10.88 N/mm) areas, and when a compressive load was applied to the premolar (232.85 N/mm) area. The A-P strap group was significantly more rigid (P< or =.05) than the other designs when a compressive load was applied to the molar (69.56 N/mm) area. Both the Thick and A-P strap groups were significantly more rigid (P<.05) than the Notch and A strap groups in all tests.

CONCLUSION

Doubling the thickness of the anterior strap of a U-shaped maxillary major connector improved the rigidity of the framework to torsional loads. A posterior strap became more effective in maintaining framework rigidity to compressive forces as the length of the arch increased.

Preparing teeth to receive a removable partial denture

The success or failure of a removable partial denture is dependent on many factors. To achieve success, the practitioner must develop and sequence a sound treatment plan based on clinical and radiographic evidence. These findings must be carefully considered in prosthesis design and mouth preparation. Particular attention must be given to the proper placement of guiding planes and well-made rest seats and the use of surveyed crowns on abutment teeth. This article describes the rationale, importance, and clinical procedures for abutment preparation for removable partial dentures.

Clinical comparison of two mandibular major connector designs: the sublingual bar and the lingual plate

Two mandibular removable partial dentures, one with a sublingual bar and one with a lingual plate as the major connector, were constructed for 10 patients. Both were made using the same impression and were identical except for the design of the major connector. The two major connector designs were evaluated by the patients at the conclusion of the study. Eight of the 10 patients preferred the lingual plate and nine of the 10 found both designs satisfactory. One patient indicated a strong preference for the sublingual bar over the lingual plate and another found both designs equally acceptable. We have concluded from the final data that the sublingual bar compares favorably with the lingual plate in patient acceptance and should be considered as a viable design alternative when a lingual plate is not indicated.