Noriaki Takahashi

Analysis of stress on a fixed partial denture with a blade-vent implant abutment.

Using a two-dimensional finite element method, a study was made that compared the behavior of a model mandibular posterior fixed partial denture constructed on the second premolar abutment and a blade-vent implant imbedded at the site of the second molar with the behavior of a fixed partial denture constructed on the second premolar and second molar abutments. The following were the results: 1. Deflections of the implant fixed partial denture were less than those of the natural tooth fixed partial denture in vertical and inclined loads. 2. Stress concentration was markedly found in the pontic and the mesial and distal parts of the premolar retainer in both restorations and the implant neck in the implant fixed partial denture. 3. In the implant fixed partial denture, stresses induced in the surrounding bone became higher around the posterior abutment and became lower around the premolar retainer than the stresses produced with the natural tooth fixed partial denture. 4. Therefore it was suggested that, to relieve stress to the surrounding bone around the implant abutment, occlusal forces loaded to the implant fixed partial denture have to be more concentrated on the premolar abutment than do forces loaded to the natural tooth fixed partial denture.

Evaluation of Thermal Change in Pulp Chamber

Thermal irritation to the tooth is one of the causes of pulp inflammation and necrosis. Thermal irritations can result from hot and cold drinks, tooth preparation, finishing and polishing of filling materials, thermo-elastic impression materials, etc. Most studies of interpulpal thermal change employ thermocouples. With the thermocouples, it is possible to measure thermal changes of one specimen under various applied temperatures, but it is very difficult to compare changes of various specimens under the same measuring conditions. In the present study, the finite element method was used to measure interpulpal thermal changes in order to evaluate the relative thermal conductive effects of various restorative procedures. The most significant advantage of this method is that a standard measuring condition can be employed. A lower first molar was used as the model. Figure 1 shows the finite element model divided into 251 triangular elements with 148 nodal points. Comparisons were made of teeth restored with an inlay, an amalgam, a metal crown, and a temporary resin crown. Since teeth are exposed to a temperature range of about 15 to 55 C, the models were subjected to temperatures of 15 C and 55 C applied to the coronal two-thirds; the root was maintained at 36 C. The data were examined according to a 2-dimensional steady state heat conductive analysis utilizing a NEAC 2200 computer in Osaka University Computer Center. Table 1 shows the thermal conductivity of each material. Table 2 shows the thermal changes at the pulpal horn when the two temperatures were applied to the natural and restored tooth detailed. Assuming that the interpulpal thermal change in the natural tooth reflects the physiological situation, then the resin crown was the most favorable restoration for protection from thermal irritation. Thermal conductivity of the metal differed from that of the amalgam; but no marked differences were found between the thermal changes under the inlay and amalgam metal restorations. Under each condition, when the dentin thickness increased 0.5 mm at the internal surface of the pulp chamber, the pulp temperature changed ca. 2 C toward a favorable state. If the dentin thickness increases 1.0 mm under the metal crown, inlay, and amalgam restoration, thermal changes should become almost normal as compared with the thermal change in the natural tooth. Therefore, it is suggested that the dentin thickness is more important for the interpulpal change than the thermal conductivity and the types of restorations. In a future study, thermal change in the pulp chamber will be analysed under the more detailed conditions, while various thermal irritations are applied on the tooth.

Effects of Pin Hole Position on Stress Distributions and Interpulpal Temperatures in Horizontal Nonparallel Pin Restorations

Biomechanical evaluation was made of horizontal nonparallel pin position effects on stresses of tooth structure and interpulpal temperatures with a stress analysis and heat conductive analysis, following finite element methods. The results were that it is adequate to use geometry to decide horizontal pin position in a routine clinic.