Purpose : Because of the shape of mandibular complete dentures, stress is the greatest along the midline. These dentures frequently fracture at or near this area. In order to contribute to the database of information available to dental professionals, the present study was undertaken to evaluate the distribution of stress and the fracture strength of denture bases, both before and after repair of a midline fracture.
Materials and Methods : Twelve denture bases (each 3 mm thick) were constructed of polymethyl methacrylate acrylic resin. The stress pattern of each base was characterized by measuring changes in thermal emission from the surface of the denture base. The fracture strength of six of the bases was measured by bending them with a compression-bending tester until they fractured.
Results : Stress concentration along the midline of the denture bases was observed. The mean fracture strengths of unbroken bases, repaired bases, and bases repaired with both resin and wire were 9.36, 2.28, and 4.34 N, respectively.
Conclusion : Greater stress was observed along the midline in bases repaired with autopolymerizing resin only than in unbroken bases. In addition, the fracture strength of the repaired bases was only about 1/4 that of unbroken bases. It was observed that stress was less concentrated along the midline of bases repaired with autopolymerizing resin and a reinforcement wire compared to either repaired bases or unbroken bases.

抄録全体を表示

Purpose : This study examined differences in the viscoelasticity of the periodontium between Japanese and Chinese subjects and the influence of their living environment on the periodontium.
Materials and Methods : We selected 93 and 113 upper central incisors, 100 and 100 upper canines, and 104 and 98 upper first molars of the Japanese and Chinese subjects, respectively, for examination in this study. The viscoelasticity of the periodontium was measured using an automatic diagnostic system that measured tooth mobility. The evaluated parameters were viscosity c₁, c₂ and elasticity k. An analysis of variance was used to compare the upper central incisors, canines, and first molars in the Japanese and Chinese subjects. An unpaired t-test was used for comparison between sexes and between Japanese and Chinese.
Results : The viscoelastic parameters showed significant differences among central incisors, canines, and first molars in both the Japanese and Chinese. Almost all periodontal viscoelastic parameters were significantly different between Japanese males and females, while only a few such parameters showed significant gender differences in the Chinese. All parameters except c₁ of the central incisors in men and all parameters in women showed significant differences between the Japanese and Chinese subjects.
Conclusion : There were differences in the viscoelastic properties of the periodontium between Japanese and Chinese subjects.

抄録全体を表示

Purpose : The purpose of the present study was to examine the relationship of tongue pressure and changes in occlusal vertical dimension to swallowing.
Materials and Methods : The volunteer subjects were four young male dentulous adults and eight elderly complete denture wearers. The tongue pressure to the palate was simultaneously recorded with electromyography (EMG) measurements of the suprahyoid muscles as the occlusal vertical dimension changed. The subjects were instructed to perform two actions : swallowing without any liquid or food (dry swallowing) and swallowing of 2 ml water (wet swallowing).
Results : During dry swallowing, the maximum pressure decreased significantly as the vertical dimension increased in the young subjects. In the elderly, the maximum tongue pressure decreased monotonically as the vertical occlusion increased. The same results were clearly found for wet swallowing. Overall data suggested that the tongue pressure of the elderly group was lower and the duration of tongue pressure in wet swallowing was longer than that of the younger group.
Conclusion : An increase of the vertical dimension decreased the tongue pressure to the palate, whereas a decrease of the vertical dimension affected the tongue pressure less. Tongue pressure is an important factor for the evaluation of tongue function, especially swallowing.

抄録全体を表示

Purpose : The purpose of this study was to evaluate the reliability of video images of occlusal contacts (VIO), a method that was developed for occlusal analysis at the intercuspal position.
Materials and Methods : Images were obtained by computer imaging techniques applied to silicone bite records. Photographic images of the occlusal surfaces were displayed with occlusal contacts superimposed on them. Levels of intra- and inter-examiner agreement for detection of occluding teeth were evaluated for VIO, articulating film, shimstock and silicone bite records. Two examiners performed occlusal examinations twice for 20 subjects using the four methods. Percent agreement and Kappa statistics were used for data analysis. The percent agreements were divided into positive (contact) and negative (no contact) agreement in order to evaluate their distribution.
Results : Based on the Kappa statistics, articulating film showed the lowest levels of agreement, and shimstock, silicone bite records and VIO showed approximately equal levels of agreement. From the percent agreements, VIO showed the most unbiased distribution of positive and negative agreements while the other methods tended to show higher proportions of positive agreements.
Conclusion : The VIO and silicone bite methods were as reliable as shimstock for evaluating occlusal contact. Articulating film was the least reliable. The most unbiased distribution of positive and negative agreement was found for VIO, while the other methods exhibited higher proportions of positive agreement.

抄録全体を表示

Purpose : This study examined the functional efficacy of complete dentures with full balanced occlusion or lingualized occlusion in patients with different degrees of residual ridge resorption.
Materials and Methods : To compare the residual ridge condition among the five subjects, the alveolar ridge height and supporting area under the denture were measured. Experimental upper and lower complete dentures were fabricated with interchangeable molars and were used by the study subject for two months prior to the functional analyses. To estimate denture function, electromyography (EMG), mandibular kinesiography (MKG), and pressure distribution under the denture while tapping and masticating peanuts were investigated.
Results : The EMG results suggested that subjects with poor residual ridge condition may benefit from lingualized occlusion for more efficient mastication. The MKG results suggested that, in subjects with poor ridge condition, mastication tends to be unstable with a full balanced denture, but that widening of the mandibular path that destabilizes the denture is reduced with a lingualized denture. The results of pressure measurement suggested that when lingualized occlusion is provided for patients with a poor residual ridge condition, the occlusal pressure is transmitted mostly at the lingual side of the residual ridge, leading to efficient denture function.
Conclusion : A sufficient lingualized occlusion effect was obtained in subjects with a poor alveolar ridge condition. The following indexes were examined as the criteria for applying lingualized occlusion. The ratio of the residual alveolar ridge must be 0.5 or less, and the area of the supporting base of the denture must be 2, 000 mm² or less.

抄録全体を表示

Purpose : The aim of this study was to investigate the relationship between the changes in surface texture and the imbibition and volatilization of home reliners.
Materials and Methods : The home reliners used in this study were Tafu Grip^®, Polydent^®, New Liodent^®, and Cushion Collect^®. Soft-Liner^® was used as a control. The changes in the surface texture of the materials were observed using a stereoscopic microscope. Weight changes were measured to evaluate the imbibition and volatilization of these products after being kept in distilled water (37°C) and ambient air. After immersion, regression analysis was done. The relationship between the changes in surface texture and both imbibition and volatilization of the home reliners was investigated by canonical correlation analysis.
Results : The surface texture of the home reliners tested changed rapidly after soaking when holes and grooves appeared. Soft-Liner^® exhibited few changes. The amount of imbibition of the home reliners, except for Polydent^®, increased with time, but the amount measured for Soft-Liner^® decreased only slightly. The amount of volatilization of both the home reliners and Soft-Liner^® increased with time. Results of the regression analysis showed that the coefficients of determination were highly significant in the quadratic equation. The result of the canonical correlation analysis indicated that there was a relationship between the changes in surface texture and volatilization of home reliners.
Conclusion : Within the parameters of this study, it was learned that many grooves appear on a small number of ethanol-containing home reliners.

抄録全体を表示

Purpose : This study investigated the joint strengths of two frameworks for removable partial dentures (RPD) embedded in a denture base resin.
Materials and Methods : Six types of framework structures were cast conventionally from a Co-Cr alloy. To connect two castings, six types of joints were designed : Type A (butt joint), Type B (soldered butt joint), Type C (soldered lap joint), Type D (10 mm overlapped T-shape), Type E (20 mm overlapped T-shape), and Type F (Type D with two pins). Two castings were embedded in denture base resin and finished to a size of 10.0×10.0×65.0 mm. As a control, the same size resin block without a framework was prepared. Joint strengths were determined using a three-point bending test. To evaluate the strength change after rejoining, the broken specimens were reconnected using the same resin and tested as in the first test.
Results : Both the breaking load and maximum stiffness of Types D, E, and F were significantly (p<0.01) greater than those of Types A, B, and C. Using the same resin for repair, the joint strengths of every type were not significantly (p>0.05) different between the before- and after-repair state.
Conclusion : Because of the design of the three-dimensional RPD frameworks, greater joint strengths were obtained than for the conventional soldered framework design.

抄録全体を表示

Purpose : This study established a method of evaluating the susceptibility to staining of artificial teeth and clarified whether the surface texture of artificial teeth influences this staining tendency.
Materials and Methods : Conventional resin teeth (R), resin teeth with a special acrylic polymer (FX), conventional composite resin teeth (EF), and composite resin teeth with an enhanced amount of fluor monomer (PX) were used. To evaluate the influence of surface texture on the staining tendency, the following polishing methods were used : no polishing, barrel-polishing and buff-polishing. The surface texture was examined using a scanning electron microscope. To evaluate direct staining, the artificial teeth were immersed for 1 week in curry and fuchsin basic solutions. To assess protein adsorption, they were immersed for 1 week in a human parotid saliva solution and a solution containing bovine serum albumin. They were subsequently immersed in a plaque-disclosing agent for 24 hours and washed in water. Color changes after direct staining and protein adsorption testing were evaluated. Multiple comparison of the color difference values (ΔE^*ab) were evaluated using a Bonferroni test (p<0.01).
Results : Protein adsorption testing showed that EF was remarkably stained for all surface textures. The barrel-polished surfaces were coarser than the other surfaces. EF was remarkably stained compared to the other teeth.
Conclusion : It was demonstrated that the staining tendency of the conventional composite resin teeth is due to their high level of protein adsorption and that protein adsorption testing is a useful evaluation method. Also, it was recognized that surface roughness is an influencing factor on the staining tendency.

抄録全体を表示

Purpose : Determining the minimum opening position during chewing when food particles are being fractured and the variability of this minimum opening, which also affects the efficiency of the motion, are crucial for dentistry. This paper investigated estimates of the minimum opening position during gum-chewing movements and compared the variability of different mandibular reference points (i.e., lower incisal point, right and left molars). The variability between the maximum intercuspal position and the minimum opening position was also compared.
Materials and Methods : Twenty-eight young female subjects participated in this study. The gum-chewing motion of each subject was recorded using an optoelectronic analysis system with six degree-of-freedom. Minimum opening position was defined as the position where the lower incisal point reached its uppermost position between two maximum opening positions.
Results : The three reference points at the minimum opening position were located more posteriorly compared with their position at the maximum intercuspal position. The minimum opening position of the molars was more stable than that of the lower incisal point, and the molar had less variability on the working side than on the balancing side. In the lateral direction, the mandibular position at the maximum intercuspal position was less stable than at the minimum opening position during chewing.
Conclusion : The minimum opening position was not coincident with the maximum intercuspal position. In addition, its reproducibility was relatively higher than the maximum intercuspal position, especially in the lateral direction, suggesting that closer attention must be paid to lateral deviation of bite registration at the maximum intercuspal position.

抄録全体を表示

Purpose : Recently, many kinds of accelerated-type investments were introduced into the market. With this kind of material, the time spent on the casting procedure is greatly shortened compared with the time required for casting in conventional investments. The purpose of this study was to investigate the fit and surface roughness of crowns made from two kinds of alloys cast in the accelerated gypsumbonded investments and to compare the results with results using a conventional investment.
Materials and Methods : Four kinds of accelerated investments and one kind of conventional investment, and two kinds of alloys (12% gold-silver-palladium and Type III gold alloy) were used in this study. The mean surface roughness, maximum height and marginal discrepancy of the crowns were measured and statistically analyzed using a t-test.
Results : The fit and surface roughness of crowns for both alloys made with the accelerated investments were not statistically different compared with the conventional investment. The values for fit and surface roughness of the Type III gold alloy crowns were less than for the 12% gold-silver-palladium alloy crown (p<0.05). There were no significant differences between alloys for any of the four kinds of accelerated investments.
Conclusion : These results suggested that the casting performance of the accelerated investments was similar to that of the conventional investment.

抄録全体を表示

Purpose : The goal of this study was to clarify the frequency and reasons for crown failure, based on the longevity and quality of full coverage crowns evaluated in 226 patients.
Materials and Methods : Of a total of 529 patients who received full coverage crowns between 1980 and 1987 at the student clinic, 226 (42.7%) responded to a request to attend a recall examination. The crowns were fabricated and delivered in a uniform way according to the students' clinical manual used in the Department of Fixed Prosthodontics, School of Dentistry, Iwate Medical University.
Results : The 226 participants (61 men and 165 women with a mean age of 41.6 years) attended a recall examination between 1996 and 1997. Periodontal pocket depth was 2.2±0.1 mm (mean±S.D.) at the initial examination and 2.2±0.8 mm (mean±S.D.) at the recall examination. The gingival index at the initial visit indicated that 90.3% of recall subjects had either slight gingival inflammation or none at all. A 10-16 year follow-up examination on the 388 crowns revealed that 38 crowns (9.8%) were lost or replaced due to periodontal disease, caries, and/or fracture. Of the failed crowns, 19 abutments were extracted, 11 crowns were removed and remade as an abutment of a fixed bridge, and 8 crowns were replaced by new crowns. The survival rates of the crowns at 5, 10 and 16 years after placement were 96.9, 93.3, and 84.9%, respectively.
Conclusion : Although the recall rate of this follow-up study was low, it is suggested that the periodontal status at the initial stage of treatment played an important role in achieving a high survival rate of the crown. Following an approved procedural manual together with a fabrication manual and careful checking by a senior instructor were also thought to contribute to the excellent survival rate of the crowns.

抄録全体を表示

Purpose : A study was done to investigate the use of a newly developed esthetically and mechanically superior all-ceramic zirconium post and core system (CosmoPost^®, Ivoclar, Liechtenstein). Fabrication techniques and clinical cases are discussed.
Materials and Methods : CosmoPost^®s are cylindrical conical-shaped posts made of ZrO₂-TZP and are marketed with corresponding rotary drilling and trimming instruments. The ceramic cores can be fabricated by either direct or indirect methods.
Results : The authors have used zirconium all-ceramic posts and cores since 1997 to improve esthetics and to treat cases of dental metal allergy. Results at achieving natural tooth color and translucency have been evaluated as excellent. No failures of crowns, post and cores have been observed up to the present.
Conclusion : Good esthetic results and improvements in the oral soft tissue in cases of dental metal allergy can be achieved using the newly developed all-ceramic post and core.

抄録全体を表示

Purpose : There is a wide variety of patterns for retainers used in partial dentures. In these patterns, most of the retentive mechanisms apply friction, mechanical interlocking force or resilience. In contrast to these methods, there is a mechanical retention system that utilizes a retentive projection to latch onto another projection without applying any force. This system is the riegel precision attachment. However, a more complicated technique using high-precision devices is required to fabricate this riegel precision attachment, which is more troublesome to work with during laboratory operations. The new Suginaka riegel system was designed to facilitate laboratory operations and simplify laboratory processes and clinical handling.
Methods : The Suginaka riegel consists of a sheath case in which an easily retractable lever is placed in advance. The retentive section of the lever is connected to a projection on an abutment tooth. The denture is inserted or removed from the mouth by protracting or retracting the lever.
Results : The Suginaka riegel can avoid the most difficult laboratory process of incorporating the lever that retracts or protracts the hook smoothly, such as is found with the conventional riegel attachments. Because it is different from retainers that apply friction or interlocking force and resilience, no prosthetic injuries are sustained by the abutment tooth even during functioning and placement or removal of the denture because no detrimental force was applied to the abutment tooth.
Conclusion : The use of the Suginaka riegel simplifies laboratory processes, and the latch mechanism works reliably without the necessity for complicated laboratory techniques.

抄録全体を表示