Shigeo Yokozuka

Stress analysis of tooth and supporting tissue using the finite element method —Effect of properties of periodontal membrane—

We investigated changes in natural teeth under loading and the correspondence of simulated models to these results using the finite element method. Four kinds of models were analyzed according to loading conditions at the periodontal membrane. Two models for linear analysis and two models for nonlinear analysis were established. The influences of these 4 models on stress distribution were compared. The following results were obtained. 1) The differences Young’s modulus at the Periodontal membranes greatly affected the structure analysis; 2) a loading-deviation curve approximating actual values was observed under the loading condition of NL-2 at the periodontal membrane; and 3) comparison of stress distribution at the alveolar bone in linear and nonlinear analysis models indicated that the neutral axis appears only in the linear analysis models, suggesting that it is important to define and establish the periodontal membrane as the nonlinear elastic body.

Dynamic studies of luting cement layer on full cast crown

This study was undertaken to elucidate the dynamic behavior of the cement layers of the full cast crowns under the following conditions: 1) four marginal designs 2) two material properties of the cement. Stress distribution and the three values at the margin of the cement layer were analyzed using the three-dimensional finite element model.The results can be summarized as follows:1)When the model crown was vertically loaded, compression tended to decrease on the crown and there was no high stress concentration at any site.2)When the crown model was obliquely loaded, compression tended to decrease at the margin of the crown on the side of the loading, and there was floating of the margin of the crown and a marked tensile stress concentration on the non-loaded side.3)The range of tensile stress values varied with the marginal designs of the abutment tooth.4)The stress values were only slightly affected by the material properties of the cement. These results suggest that the stress values of the cement layer at the tooth margin are influenced by differences in marginal designs of the abutment tooth, rather than differences in material properties of the cement.

Influence of thickness on color stability of body and incisal porcelains

Knowledge of the relationship between thickness and shade of body porcelain and incisal porcelain is essential for producing esthetic porcelain restoration. We thus performed an experimental study to clarify the effect of thickness on shade of body porcelain and incisal porcelain. The main results were:1)In a two-layered restorstion of opaque porcelain and body porcelain, the values of L*, a* and b* decreased with increasing thickness of body porcelain. The effect of the thickness of body porcelain on shade was strongest in A1 and weakest in C4.2)The effect of the thickness of two-layered restoration on shade was stronger as the color number of the restoration was decreased.3)With a three-layered restoration of opaque porcelain, body porcelain and incisal porcelain, the values of L*, a* and b* decreased with the increased thickness of inisal porcelain. The effect of incisal porcelain on shade was greater in C4 than in A1.4)For three-layered restoration, and the thickness of incisal porcelain had a greater effect on shade than that of body porcelain.

Three-Dimensional Photoelastic Stress Analysis on Posterior Hybrid Ceramics Bridge Restorations.

Hybrid ceramics meet high esthetic requirements. However, more information on application to the bridge still needed. This three-dimensional photoelastic study was conducted to analyze the stress distribution of hybrid ceramics bridge.Two types of bridge were compared. Type I was consisted of hybrid ceramics alone, while Type II had a metal frame inside. The bridge models were cut into three parts, buccal, central and lingual. The results were as follows:1. Maximum tensile stress appeared in the distal connector in Type I.2. Tensile stress appeared in the distal connector decreased in Type II.3. In both types, compressive stress closely appeared in both connectors.This means that the connectors have complicated distributions and different types of stress.4. In all three parts, the gingival area showed higher maximum shearing stress concentration than in the occlusal surface where loading applied.

Influence of Firing Condition for Fracture Toughness, Color Stability and Firing Shrinkage of Porcelain-fused-to-titanium.

An experiment was conducted to determine fracture toughness, color stability, and firing shrinkage of porcelain-fused-to-titanium in relation with firing conditions. Titanium (VT), Duceratin (DD), Titanbond (OT) and Titan (NT) were examined under firing temperatures of 10, 20, and 30°C higher than the reference temperature, respectively, with either 1, 3, or 5 rounds of firing.The results were as follows:1. With the rise in firing temperature, the fracture toughness level was decreased in VT and increased in NT, while the level remained constant in DD and OT. Repeat firing also influenced the fracture toughness level, which was reduced in VT and OT with increasing number of firings, though the level remained constant in NT and DD.2. When the color obtained at the reference firing temperature was regarded as the standard color, the color difference ΔE* from the standard was observed in all except for VT at 10°C higher than the reference temperature. In addition, when the color obtained by one firing was taken as the standard color, the color difference ΔE* from the standard was recognized in all repeat firings.3. The degree of linear shrinkage was increased in VT and decreased in NT, with the rise in firing temperature and increasing number of rounds of firing, but it remained constant in DD and OT.

Pantographic reproducibility of mandibular border movements in patients with craniomandibular disorders

The change of the reproducibility of maximum border movements in patients with CMD has been studied in the United States. Clayton and others reported a change of PRI, pantographic reproducibility index. There is no information on this in the Japanese population. Also, there is found no study on each section of a scribed line, pantographic reproducibility value (PRV). The purpose of this study is to analyze each section’s characteristics with a Japanese population. 30 craniomandibular disorder patients were used in study. The Denar pneumatic pantograph has used. The average PRI before treatment was 35.5 and after treatment was 14.8 Statistically significant correlation (r=0.56) was found in PRI between before and after treatment. All 12 sections showed a stasistically significant difference in PR value between before and after treatment. Sensitive sections were: 1. posterior horizontal rotating path early part; 2. posterior horizontal orbiting path later part; 3. posterior horizontal rotating path later part.

Statistics for the analysis of tenderness score summation in patient with craniomandibular disorders

Digital palpation has been used to evaluate patients with craniomandibular disorders. Tendemess at each site is classified on a 0 to 3 scale and evaluated as total summation. Tendernes were ranked on the scale. Nonparametric statistics were applied to analyze these data. 34 kind 64 sites makes 192 steps in total summation. Increase of step yielded similer results in parametric and nonparametric statistics. The purpose of this study is to clarify the difference in results between nonparametric and parametric statistics. 100 female CMD patient data were used to make group of 13, 25, 50 and 100 subject group. 16 kind 30 sites, 23 kind 44 sites and 34 kind 64 sites were anlysed as summation. The Wilcoxon test and t-test were used to analyse before and after treatment data. The Spearman’s correlation coefficient and Pearson’s correlation coefficient were analyzed. In the probability, nonparametric statistics always showed more conservative result than parametric statistics. Discrepancy between nonparametric and parametric statistics decreased with an increase of the number of subjects. With 50 subjects, the maximum discrepancy was 0.0011. Discrepancy between correlation also decreased with an increase of the number of subjects. With 50 subjects, the maximum discrepancy was 0.0095. There found very little difference if the number of subjects is more than 50.

Stress analysis of porcelain laminate veneer on lower central incisors with three-dimensional photoelastic experiments

We performed stress analysis by the three-dimensional photoelastic experiments in order to elucidate dynamic behavior in porcelain laminate veneer restoration of the lower central incisors. We employed two models in preparing the crown: Model 1 (incisal edge not covered with laminate shell) and Model 2 (incisal edge covered with laminate shell).We used two conditions of loading: Loading 1 (load applied to the labial mesial angle near the incisal edge from the side at an angle of 45° against the tooth axis) and Loading 2 (load applied to the incisal edge in parallel to the tooth axis).1)Below the loading point during Loading 1 in Model 1 suggesting the possbility of laminate shell detachment.2)In a wide range in the distal part of shell during Loading 1 in Model 2, suggesting the possibility of laminate shell detachment.3)The results of our examination suggested that, in performing porcelain laminate veneer restoration in the lower central incisor, a more stable stress condition can be achieved when the abutment shape is used without reducting the incisal edge.

Pantographic reproducibility of mandibular border movements in subjectively normal subjects

The Pantographic Reproducibility Index (PRI) has been developed to evaluate mandibular dysfunction through the reproducibility of lateral border movements. There exist no data on the Japanese population regarding PRI. The purpose of this study is to determine the normal range of PRI in normal subjects. Those data will be compared to the patient data and an evaluation of the treatment outcome. Ten male and ten female Japanese who reported no signs and symptoms of Craniomandiubular Disorders were used in this study.1.The average PRI in males was 6.95 (5.93). In females the average was 7.30 (4.45). There was no statistical difference between sexes.2.One male subject showed a PRI at 25 and was classified to the slight dysfunction category suggested by Clayton where as other 19 subjects were classified as normal.3.Within the 24 divided section, the largest PR value was 2 and average varies from 0.03 (saggittal orbiting path early part) to 0.58 (saggittal rotating path later part).

Tenderness in patients with craniomandibular disorders before and after the splint therapy part IV: Intraoral sites

In this study, 7 intraoral sites were analyzed. The results were compared with previous studies (high to low).1.The order of pain was: 1. Lateral pterygoid muscle 2. Medial pterygoid muscle 3. Posterior part of tongue and Anterior belli of digastrics 5. Temporalis insertion and middle part of tongue 7. Anterior part of tongue. This order was constant before and after the splint therapy.2.Intraoral sites showed significantly higher tenderness compared to corresponding intraoral sites. Temporalis showed 2.29 times or more in before treatment and 10.85 times or more after treatment. Medial pterygoid shows 1.47 times higher before and 5.32 times higher after treatment. Anterior belli of digastrics shows 1.34 times higher before and 2.49 times higher after treatment to the Posterior Belli of digastrics.