Iwao Hayakawa

Compositional characteristics and hardness of acrylic and composite resin artificial teeth

STATEMENT OF PROBLEM New types of artificial teeth are commercially available; however, evidence-based information regarding composition and properties is lacking. PURPOSE The purpose of this study was to qualify the compositional characteristics and hardness of new commercially available types of acrylic resin and composite resin artificial teeth. MATERIAL AND METHODS Twelve brands of 3 types (2 conventional acrylic resins, 3 cross-linked acrylic resins, and 7 composite resins) of artificial teeth were examined. The filler geometry and composition were observed using scanning electron microscopy and energy disperse x-ray analysis, respectively. Vickers hardness was determined for each layer of the polished cross-sectioned teeth. The inorganic content of the enamel layer was determined by thermogravimetric analysis with an ash method. The swelling behavior of the artificial teeth after 12 hours of methyl methacrylate immersion was observed to determine the cross-linking structure. The data were statistically analyzed using 1-way analysis of variance and Tukeys multiple comparison (alpha=.05). RESULTS Examined teeth were composed of 2, 3, or 4 layers of resin. Different sizes and shapes of filler were found but were composed only of silicon oxide. Vickers hardness ranged from 17.4 to 47.0 kgf/mm(2). The inorganic content ranged from 0 to 42.8 mass%. The enamel layer of all teeth, except for 3 products, produced negligible swelling, and the base layer of all the teeth, except for 2 products, produced obvious swelling. A significant linear correlation was observed between hardness and inorganic content. CONCLUSIONS Within the limitations of this study, differences in size, shape, distribution, and content of the silica filler and the cross-linking nature of the resin matrix were found among the commercial brands of artificial teeth evaluated.

A new polyisoprene-based light-curing denture soft lining material

OBJECTIVES The purpose of this study was to investigate some clinically relevant properties of Clearfit LC, a newly developed polyisoprene-based light-curing lining material. Its properties were compared with those of other four commercial products. MATERIALS AND METHODS Five soft lining materials, Clearfit LC, two plasticised acrylics (Super-soft, Soften) and two silicones (Molloplast B, Sofreliner) were used to determine water sorption, solubility, staining resistance, Shore A hardness and shear bond strength to denture base. Five specimens for each test were fabricated. The results of each test among the five materials were compared by one-way ANOVA and Scheffes post-hoc test at alpha=0.05. RESULTS One-way ANOVA of each test showed that the differences among five materials were significant (P<0.05). Clearfit LC showed the intermediate water sorption and solubility (10.07; 1.72 microg/mm(3)). It also showed the lowest color change in beta-carotene/olive oil solution and instant coffee solution (DeltaE=1.19, 5.48, respectively). The shear bond strength of Clearfit LC was 1.56MPa, and no adhesive failure was found. Its Shore A hardness value (56.2) was located between two acrylics and silicones. It was also found that the softer materials tend to failure cohesively. CONCLUSIONS The polyisoprene-based lining material showed low water sorption and solubility, moderate softness, high staining resistance and satisfactory shear bond strength to denture base resin. It also provides clinicians sufficient working time due to its light-curing property. It would be an attractive alternative as a relining material.

Soft denture liner of fluoroethylene copolymer and its clinical evaluation

The acrylic resins are acrylic copolymers to which plasticizers are added occasionally. Inasmuch as the monomers used are generally solvents for the denture base resins, reasonably good adhesion of the liner to the denture should be expected. On the other hand, acrylic soft resins absorb water, swell, and eventually deteriorate; the plasticizer molecules are leached out by the saliva, and the liners gradually harden. Their intraoral efficacy is short lived, and they are not satisfactory for extended clinical use. The silicone rubbers excel in their resiliency and their initial resistance to water absorption. However, in time they also will absorb water and begin to deteriorate. The liners are attached to denture base resins by an adhesive; but the bond is a poor one, and separations between the two materials often occur. Furthermore, they cannot be polished or trimmed properly. Another objection to the silicone lining materials is the possibility of microbial degradation. We developed fluoropolymer soft liners (Kurepeet, Kurecha Co., Tokyo, Japan) that are chemically stable and show good resistance to solvents and abrasion. In addition, their water absorption is low. We have used the liners clinically and observed their behavior in vivo more than 2 years. The objective of this

A new approach to evaluating occlusal support by analyzing the center of the bite force

Abstract. The aim of this study was to create a standard for occlusal support in the diagnosis and evaluation of prosthetic treatment. In experimental occlusion, the maximal bite force of eight normal dentates was measured by pressure-sensitive film over the whole dentition using splints divided into nine pieces. The occlusal contact was altered by exchanging splint pieces according to the shortened dental arch concept. The occlusal load center (OLC) was located on a graph set up with reference to the size of the individual dental arch. The occlusal supporting index (SI) and the rehabilitation index (RI) were calculated from the locations of the OLC corresponding to each occlusal contact. Differences in numbers and distribution of occlusal stops could clearly be distinguished by the location of the OLC. Though the SI showed a strong positive correlation with maximal bite force, it is proposed that the number of occlusal stops, or occlusal units, provides the most effective index for quantitative parameters of occlusal support in clinical use. From the standpoint of bite force, analysis of the RI is suggested as a useful method of objectively evaluating the recovery of occlusal support with prosthetic treatment.

A method to remold worn acrylic resin posterior denture teeth and restore lost vertical dimension of occlusion

A method is described for remolding and correctly restoring abraded posterior denture teeth. By adding a layer of paraffin wax for jaw registration, the dentist can repeat the registration procedure until the proper jaw relation is established. This wax occlusal record is used as a guide during the remolding, and is incrementally replaced by autopolymerizing resin.