Digital impressions are partially comparable to conventional impressions in terms of accuracy; the use of a larger scanning head may improve the accuracy for removable partial denture fabrication.

INTRODUCTIONIn recent years, injection-molded thermoplastic resins such as polycarbonate, polyamide, and polyester have been used as denture-base materials [1][2] . These materials have high toughness and low elasticity, and have found applications as non-metal clasp dentures, which are mainly characterized by the non-use of metal clasps [3][4][5] . Nevertheless, these materials are controversial because of various issues such as their adhesion to heat-curing resins (which are conventional base materials) and their resistance to abrasion [6][7][8][9][10][11] . With regard to the effect on the mucosa underlying the denture base made of polyamide resin, the difference in the force transmitted onto the denture base by the existence of a metal rest was reported, and the application of a metal rest with the non-metal clasp denture was recommended 12) . Therefore, simulation models of intermediate missing teeth were created using dentures made of thermoplastic resin and conventional dentures made of polymethyl methacrylate resin. The movements of the models, as well as the load applied onto the denture base, were measured and compared between the models. The aim of this study is to examine whether dentures made of injection-molded resin have sufficient stiffness in comparison with conventional dentures. MATERIALS AND METHODS MaterialsAs shown in Table 1, two types of injection-molded thermoplastic resins [the polyamide resin (Valplast; VAL) and the polyester resin (EstheShot; PET)] were used for the manufacture of dentures in this study. In addition, dentures made of polymethyl methacrylate resin (Physio Resin; PMMA) were used as controls. Fabrications of denturesThe dentures used in the experiments consisted of missing tooth models (E16-516; Nisshin, Kyoto, Japan); that is, with a missing left mandibular second premolar and first molar (Fig. 1). A load sensor was placed on the slope on the buccolingual side of the alveolar ridge, under a silicon membrane corresponding to the mucosa directly under the first molar. An impression was taken under this condition, and a denture was manufactured.The design of the denture used in the experiment is shown in Fig. 2. The dentures used in this study were designed in such a way that at the site of the missing middle teeth, a metal rest was set to the tooth adjacent to the deletion; however, inside the denture base, the pedicles of the metal rest were not connected to each other. The Co-Cr alloy (Wironit ® , Bego, Bremen, Germany) was used for making the metal rest. Table 2 shows heating temperature, heating time, injection pressure and the temperature of the flask. Five dentures made with each of the materials were manufactured and used for the measurements. MethodTo apply a load on the denture, the occlusal force was estimated using a universal testing machine (Autograph, Shimadzu Corporation, Kyoto, Japan); the experimental denture was set to a fixed position that allowed for the load to be applied on the mesial fossa of the first molar, and a pressure of 100 N was applied at ...

The plaque formation on the buccal surface is not dependent on the types of clasps. It is effective to prepare a guide plane as close to the gingival margin as possible to reduce the plaque accumulation on the distoproximal surface.

The purpose of this study was to investigate the effects of repetitive insertion/removal cycle tests on denture retainers with simulated occlusal loads on the retentive force and deformation of clasp. Abutment teeth in the form of mandibular secondary premolars and clasp in the form of Akers clasps were prepared. The retentive force of the clasp on the abutment teeth were evaluated before and after undergoing repetitive insertion/removal cycle tests with or without cyclic loading. Changes in the clasp shape were monitored using a 3D scanner and scanning electron microscope. The initial retentive force was approximately 10 N and this value later decreased due to deformation of the clasp tips. In contrast to the non-load group, the load group exhibited a reduction in retentive force during earlier stages. Therefore, cyclic loading was related to a decrease in retentive forces, specifically in the early stages of repetitive insertion/removal cycles.

The reproducibility of the SP was comparable to that of the MP. The results indicate that for sites of maintenance patients with probing depth <7 mm there was excellent agreement obtained by a single examiner using the SP compared to the MP.