Akihiko Kadokawa

A comparison of stress relaxation in temporary and permanent luting cements

PURPOSE The stress relaxation and compressive strength of resin, resin-modified glass ionomer, glass ionomer, polycarboxylate, and zinc oxide eugenol cements were measured to determine the characteristics of these materials after setting. METHODS A total of 19 luting cements including 12 permanent cements and 7 temporary cements were used. Cylindrical cement specimens (10mm long and 6mm in diameter) were obtained by chemical setting or light curing. The specimens were stored for 24-36 h in water at 37°C and were then used for the stress relaxation and compression tests. The stress relaxation test was carried out using three constant cross-head speeds of 5, 50, and 100 mm/min. Upon reaching the preset dislocation of 0.5 mm, the cross-head movement was stopped, and the load was recorded for 60s. Fractional stress loss at 1s was calculated from the relaxation curves. The compressive strength and modulus were measured at a cross-head speed of 1mm/min. Data were analyzed with the Kruskal-Wallis test and Holms test. RESULTS A zinc oxide eugenol cement [TempBOND NX] exhibited the largest fractional stress loss. A resin cement [ResiCem] showed the largest compressive strength, while a glass ionomer cement [HY-BOND GLASIONOMER CX] showed the largest compressive modulus among all tested cements (p<0.05). CONCLUSION The fractional stress loss could not be classified by the cement type. Two implant cements [Multilink Implant and IP Temp Cement] showed similar properties with permanent resin cements and temporary glass ionomer cements, respectively. Careful consideration of the choice of cement is necessary.

A Study on Porcelain Laminate Veneer Crown. Part 2. Setting Characteristics of Light-Curing Luting Materials.

On the assumption that porcelain laminate veneer crown had been bonded to the abutment tooth with the light-curing luting material, setting characteristics of four light-curing luting materials were evaluated for linear shrinkage from the start of exposing, water absorption, Vickers hardness and compressive strength after exposing the materials to visible light. The results were as follows : 1. The light-curing luting materials rapidly shrank as soon as exposed to light passing through a cover glass or a plate made of porcelain. When exposed to light through the laminate veneer plate, the shrinkage of materials was less than that of materials exposed to light through a cover glass. The result suggested that the curing process was delayed because of a shade effect of the laminate veneer plate. 2. The compressive strength was shown to have a tendency to increase with the elapse of time. The compressive strength of luting materials stored in air at 37•Ž was larger than that of materials immersed in distilled water at 37•Ž, and such a tendency was remarkable from 1 to 4 weeks after exposing. 3. The values of Vickers hardness of light-curing luting materials were almost the same from 1 to 4 weeks after exposing, and the hardness values of the materials in air at 37•Ž was larger than those of materials immersed in distilled water at 37•Ž. 4. The amounts of water absorption (wt%) increased rapidly after the luting materials were immersed in distilled water at 37•Ž, but were hardly increased 8 weeks after exposing the materials to visible light.

Study on Effects of an Extracoronal Direct Retainer's Arm to the Periodontal Ligament of an Abutment Tooth Utilizing Finite Element Method. Part 1. The Introduction of Three-Dimensional Finite Element Analysis.

Direct retainers arm is placed on an abutment tooth when an edentulous space is restored by prosthetic treatment with a removable partial denture. Various attempts have been made to investigate the effects of the retainer of a partial denture on an abutment tooth during functional loading. However, no analysis seems to have been reported about the effects of retainers on the abutment teeth when the denture base rotates toward and away from an edentulous ridge. Attention was paid to the lower second premolar tooth, and the three-dimensional finite element method was used.The results were as follows:1. When only a retentive force was applied, the von Mises stress in the periodontal ligement increased.2. When both retentive and reciprocal forces were properly applied, the von Mises stress in the periodontal ligament was less than that with the retentive force only.3. When both retentive and reciprocal forces were properly set, and a reciprocal force was one-third of a retentive one, the value of the von Mises stress was within 2.4 MPa. Thus, it was suggested that the reciprocation effectively released the stress in the periodontal ligament of an abutment tooth.4. Effective factors for stress relaxation of the periodontal ligament were suggested to exist in the design of a retainers arm.