Quantification of masseter muscle deformation during palpation using MRI and image-analysis procedure.

PURPOSE: The aim of this study was to quantify masseter muscle deformation during palpation and to discuss the clinical significance of muscle palpation using magnetic resonance imaging (MRI) and the image-analysis procedure. MATERIALS AND METHODS: Subjects were 10 male volunteers with a mean age of 26.8 years. MRIs were taken under two muscle conditions: compressed and noncompressed (control). By superimposing a compressed image on a noncompressed image, muscle deformation (the cross-sectional area, length of the surrounding area, thickness, circular convolution, compressed point, and anterior and posterior extension) was measured. RESULTS: The cross-sectional area of the masseter muscle was diminished by 14%, circular convolution of the muscle was reduced by 31%, the length of the surrounding area was enlarged by 11%, and the thickness of the muscle was reduced by 6.7 mm because of compressing. Thus, the masseter muscle was compressed. An uneven stretch of the muscle caused by compression was observed at the same time. The anterior portion of the masseter muscles extended on average 10%, with great individual differences. On the other hand, posterior extension (3%) and its individual differences were small. CONCLUSION: The method used made it possible to quantify masseter muscle deformation. By compressing, the masseter muscles were deformed especially in the anterior direction but with great individual differences.

New fabrication system for dental implant surgical stents: time-saving laboratory technique using a light-curing resin and transparent artificial teeth.

BACKGROUND: Surgical stents are a prerequisite for dental implant diagnosis. However, the traditional fabrication method including wax-up, investment, and resin polymerization is time consuming. PURPOSE: This article introduces a new fabrication system using a light-curing resin and transparent artificial teeth to reduce time spent in the laboratory. MATERIALS AND METHOD: The process of this system includes placing the light-curing resin and the artificial teeth on the partially edentulous dentition and alveolar ridge, making an appropriate pattern for the stent, light-curing for 5 minutes, briefly polishing, and making holes for titanium guide pins. RESULTS: This system requires only 30 to 40 minutes in the laboratory to complete a stent after mounting the casts on the articulator. Owing to the elasticity of the light-curing resin, this system eliminates the need for a blockout procedure on the undercuts of existing teeth on the casts, protects the casts from breakage, and provides the appropriate retention intraorally without any retainers. As well, titanium guide pins embedded in the stent were clearly identified by panoramic and computed tomographies. CONCLUSIONS: The new fabrication system proposed here can be time saving and, further, serve benefits for radiographic diagnosis.

Tomographic evaluation of TMJ loading affected by occlusal pivots.

PURPOSE: The purpose of this study was to clarify the direction and magnitude of condyle displacement during clenching with occlusal pivots. Temporomandibular joint (TMJ) loading and flexibility and the clinical significance of orthopedic appliance therapy are also discussed. MATERIALS AND METHODS: Participants were 16 volunteers with a mean age of 25.8 years. Maxillary and mandibular flat occlusal devices were fabricated. TMJ tomogram series were taken first with tripod support and then with maxillary clenched anterior pivot support and/or unilateral posterior pivot support. Condyle positions on the TMJ tomograms were analyzed using the digital subtraction technique. RESULTS: When clenching with the anterior pivot, the anterior reference point (A) on the mandibular condyle was translated 0.1 mm posteriorly (P > 0.05) and 0.2 mm superiorly (P < 0.01) from the tripod-support reference point. In the case of the ipsilateral posterior pivot, point A was rotated 1.2 degrees (P < 0.01) and was translated 0.4 mm (P < 0.01) posteriorly and 0.18 mm superiorly (P > 0.05). CONCLUSION: Condyle displacement during clenching with the anterior and posterior unilateral pivot was found, which supports previous findings. However, the direction and magnitude of the displacement differed from those of previous studies using jaw-tracking devices, and individual values differed greatly among subjects.

Influence of different elastic moduli of dowel and core on stress distribution in root.

This study was carried out to investigate the influences of elastic moduli of the dowel-core combination on the stress distribution in the root by the use of 2-dimensional finite element analysis. The peak stress at the dowel-cement interface was influenced strongly by a change of elastic modulus of the post (from 20,000 kg/mm2 as a hard prefabricated post to 8,000 kg/mm2 as a custom cast post) for both vertical and 45 degree oblique loading (rho > 90%). Peak dentinal stress adjacent to the luting cement layer depended only on the post material for vertical loading (rho [symbol: see text] 99%). In contrast, the post and core materials (from 8,000 kg/mm2 as cast core to 300 kg/mm2 as composite resin core) acted in cooperation on the stress magnitude for oblique loading, but the influence of the core material was stronger than that of the post (rho of core [symbol: see text] 41% and rho of post [symbol: see text] 26%). On the other hand, at the marginal region the effect of the core material contributed more than 86% to the peak stress value for both loadings, and the post material affected at most about 11% of the bending resistance.

Castability of a commercial castable glass ceramics.

This study was carried out to determine the design and thickness of the cervical margin of a castable ceramic restoration from the point of view of its castability. Castability of castable ceramics was lower compared to Ag-Pd-Au and Co-Cr alloy with the use of the mesh pattern and rod pattern test (p < 0.05). It was more dependent on the pattern thickness than alloys (p < 0.05), and perfect reproducibility with non-measurable variability could be estimated to reach 1.0 mm thickness of the original pattern (p < 0.001). Marginal shape was certainly more reproducible in a right angle shoulder than in 30, or 45 degree bevels (p < 0.05). From these results, it is suggested that a right angle shoulder margin with 1.0 mm thickness is more suitable to castable glass ceramic restoration than any other design.

Developments of the new instruments for TMJ arthroscopic surgery.

A large and clearly visible operating area is essential for successful arthroscopic surgery of the temporomandibular joint. The keys to a successful operation are the safe and accurate positioning of a large scope and multiple cannulations, overcoming blind areas. We developed some instruments to resolve these problems; i.e., scopes with a large diameter for high resolution, a triangulation instrument for multiple cannulations, a needle set-up jig for disk traction suture, a step cannulation system and a two-channel cannula for operating in the narrow lower joint space and a fixing jig for cannulas in the upper and lower joint space to observe the same portion of the discal tissue from both joint space during disk suturing. From our experience in applying systematic procedures using these instruments in 37 arthroscopic surgeries, it is possible for this procedure to be done under a visual field and the surgical time considerably shortened.

Clinical application of a static splitting agent for removing the denture from its cast after curing.

A very effective method for removing a denture from its cast after curing has been developed using a static expansive agent for hydration. Three filling hole designs for packing the expansive agent into a stone cast were examined to obtain the most effective method of splitting the cast for clinical application. The dynamic strain in the resin denture base was measured during static splitting one filling hole design was the safest for handling and the most effective for removing the denture. The upper tolerance limits for the maximum strain were estimated to be 2.9 x 10(-3) in the upper denture and 3.4 x 10(-3) in the lower denture. These values were lower than one-third of the critical strain corresponding to the proportional limit of the denture resin.

[Study of strengthening and stress distribution of porcelain jacket crowns constructed with twin foil technique. 2nd report].

McLean and Sced have reported (J Dent Res, 56: 1067-1069, 1977) a procedure for a reinforcement of the porcelain jacket crown, which improved the resistance to a fracture as a result of inhibiting the propagation of cracks into brittle materials, so called the twin foil technique. The purpose of this study is to evaluate the clinical application of this technique. The FEM analysis revealed that the same magnitude of surface tensile stress and no difference of the stress pattern were observed between porcelain jacket crowns constructed with a conventional and the twin foil technique. As the internal tensile stress concentrated in a platinum matrix of the twin foil technique, the stress distribution in a core layer reduced to about 20-35% compared to that of the conventional technique. This is explained by the mechanical and chemical bonds between a platinum matrix and a core porcelain and also by extreme high young's ratio of platinum. We can assume the similar finding from the fractography of the conventional porcelain jacket crown. But contrary above results, no significant differences of the strength were observed in both the presence and the absence of a platinum matrix in the fracture test. From the fact describe above, we may conclude that the tin coated platinum matrix contributes slightly to the strengthening of the porcelain jacket crown.