Takayoshi Miyata

Comparative strengths of metal framework structures for removable partial dentures

STATEMENT OF PROBLEM The framework of removable partial dentures often breaks where the metal retention for the resin denture base attaches to the clasp and major connector. PURPOSE To study these failure factors, various types of three-dimensional simulated denture frameworks were designed to measure their ultimate strengths and stiffnesses under static load. This study also sought to determine which could be recommended for long-term clinical service. MATERIAL AND METHODS Three groups of three-dimensional frameworks (double, T-shaped, and trussed structure) were designed, and two groups of classic framework designs (uniform and nonuniform skeleton) were used with an acrylic resin block as a control (n = 5 per group). Fatigue strength, stiffness and amount of strain, and free-end displacement under dynamic loads were measured for each framework. The fatigue strength was determined with a constant-loading fatigue test by counting the number of loading cycles required to induce catastrophic failure. RESULTS With 20 kgf of load, four T-shaped samples and three trussed structure samples failed at 30,000 to 60,000 loading cycles. No samples of double structure group failed until 100,000 loading cycles. CONCLUSIONS Three-dimensional frameworks, especially the double structure, have significant differences in fatigue strength and can provide long-term clinical service.

An Experimental Study on the Relationship between the Denture Structural Designs and Pressure Distribution. The Effect of Retainer Designs.

Removable partial dentures should be designed to ensure efficient distribution of masticatory loads over the abutment teeth and alveolar ridge. The relationship between the denture stiffness and the various clasp designs on the pressure distribution to a single abutment tooth and the alveolar ridge was investigated in this study.The results were as follows:1. Regardless of the retainer designs, the pressure on the alveolar ridge was less when using a double structure than a skeleton type, and the pressure on the abutment tooth in the vertical direction was greater when supported by a double structure than a skeleton type.2. When a Konus telescope was applied as a retainer the pressure on the abutment tooth in the vertical direction was greater than an Akers or an RPI clasp.3. When an RPI clasp was applied as a retainer the amount of pressure on the alveolar ridge was maximum.4. The pressure on the abutment tooth in the lateral direction showed a greater tendency when an Akers clasp was applied.5. The displacement and strain of the denture were the least when a double structure design was applied.6. When a double structure design with Konus telescope was applied, the displacement of the denture was the least.

Fatigue Strength of the Metal Framework Structures for Removable Partial Dentures.

The structural design of the metal alloy framework for removable partial dentures is critical. A double structure framework was the representative of these designs, which enables the mechanical strength and durability to improve and the abutment teeth and alveolar ridge to protect. In this study, several structural designs (double structure, T-shapes, trussed structure, rectangular column, nonuniform section) including resin block as control were subjected to a load-controlled fatigue test. The benefits on fatigue strength of five types of structures were evaluated to record the number of cycles at catastrophic failure, to measure continuously displacement and strain under loads repeatedly, and to observe the fracture patterns.The results were as follows:1. Maximum stiffness of double structure and T-shapes was approximately 20 times that of rectangular column and nonuniform section by theoretical calculations.2. Fatigue strength of double structure was the highest, followed by that of trussed structure and T-shapes.3. Double structure was found to be the significantly lowest of both displacement and strain.4. Packed resin fracture was observed in all specimens except double structure.5. The interface of fractured specimen frameworks were revealed inherently with fatigue.