Minoru Abe

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.

A simple technique for impression taking of teeth and functionally generated paths

The objective of this case report is to introduce a simple technique for simultaneously taking a closed-mouth impression and functionally generated path (FGP) for a full coverage crown restoration. A monolithic zirconia crown was the restoration of choice. An alginate impression of the abutment tooth was taken to fabricate a custom-made closed-mouth impression tray covering the abutment tooth and the adjacent teeth. The tray had an FGP table and an abutment tray in cameo and intaglio surfaces, respectively. The impression was taken with silicone impression material after adjusting the abutment tray and inscribing the FGP using self-curing acrylic resins. Plaster casts were made from the impression, and a zirconia crown was fabricated. The crown was cemented to the abutment tooth with minimal adjustments. This simple technique resulted in a well-fitting crown that accounted for mandibular movements. Using the custom closed-mouth impression tray incorporating an FGP table simultaneously aids in fabricating an accurately fitting restoration that incorporates harmonious mandibular movements using a single impression capture.

Occlusion accuracy of restorations and removable partial dentures fabricated using the impression under occlusal force with functionally generated path recording

To evaluate the occlusion accuracy of a novel impression technique, excessively high occlusion and the occlusal contact area during lateral movements were compared between metal restorations (restorations) and removable partial dentures (RPDs) fabricated using conventional and novel techniques. Both restorations and RPDs were fabricated on the simulation model with the precise displacement of the remaining teeth and soft tissue. For the novel technique, functionally generated path (FGP) recording and impression under occlusal force were simultaneously performed using a custom tray with an FGP table. For the two conventional techniques, definitive casts were mounted on an average value articulator and a semi-adjustable articulator in the typical manner. Prostheses were placed on the simulation models, and excessively high occlusion in the intercuspal position and occlusal contact areas during lateral movements were measured. Statistical analyses were performed using Kruskal-Wallis and Steel-Dwass tests (α = 0.05). For both prostheses, conventional techniques showed significantly higher occlusion in the intercuspal position than the novel technique. Moreover, the new technique demonstrated better guidance contact during lateral movements than conventional techniques. This novel technique can be recommended for the fabrication of highly accurate prostheses with appropriate occlusal contact without corrections at delivery.

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.