Investigation of stress distribution within an endodontically treated tooth restored with different restorations.

Background/purpose: Recently, metal-free restoration has become the standard in prosthetic treatment. However, it is still unclear which combination is most effective in preventing root fracture and secondary caries. The purpose of this study was to evaluate the influence of different post systems, crown materials, crown thickness and luting agents on the stress distribution around the crown margins, cervical dentin and the tip of the post. Materials and methods: Ninety-six mandibular first premolar models were developed and analyzed using finite element analysis (FEA). Two designs of crowns, six kinds of crown materials, four types of post and core systems and two kinds of luting agents were included and evaluated for the stress distribution within the abutment teeth. The Von Mises stress magnitudes were compared among all models. Results: The stress at the tip of the post decreased as the young's modulus of luting agent decreased; The stress concentrated more at the cervical area (dentin and crown), as the physical properties of the crown material increased. Conclusion: Crowns fabricated using polyetheretherketone (PEEK) can reduce the stress concentration at the cervical area, so it may be possible to reduce the amount of tooth reduction during abutment tooth preparation. The stress distribution around the post tip is affected by the post and core systems and luting agent, regardless of crown materials and thickness. When inserting a post of the higher Young's modulus such as zirconia post, methyl methacrylate luting cement can reduce the stress concentration at the tip of the post.

The investigation of the stress distribution in abutment teeth for connected crowns.

BACKGROUND/PURPOSE: With the advancement of an over aging society, the average number of remaining teeth has increased. However, these remaining teeth do not always have sufficient alveolar bone support, and sometimes fabricated connected crowns are applied. This study evaluated the influence of crown material, crown thickness, and alveolar bone resorption on the stress distribution within the abutment teeth of connected crowns. MATERIALS AND METHODS: Using structural analysis software, a premolar crown model was fabricated. Three kinds of crown materials, two types of crown thickness, two types of post and core systems, and two levels of alveolar bone were assumed and evaluated for the stress distribution within the abutment teeth. RESULTS: The higher material properties crown was, the more stress was concentrated at the marginal area. The composite resin core showed larger stress values around the marginal area, and the metal core showed larger stress values at the tip of the post. Alveolar bone resorption progressed, the marginal area stress value increased. CONCLUSION: The low elastic modulus crown material polyetheretherketone (PEEK) prevented stress concentrations at the marginal area of the crown and dentine, even with alveolar bone resorption. However, the amount of bone resorption has a great influence on the stress distribution around the tip of the post compared to the type of crown material.

Evaluation of the occlusal contact of crowns fabricated with the bite impression method.

In prosthodontic treatment, reconstruction of a proper occlusal contact relationship is very important as well as reconstruction of a proper interproximal relationship and marginal fitness. Unfortunately, occlusal relationships are sometimes lost in the process of occlusal adjustment of crowns. The purpose of this study was to compare the occlusal contacts of single crown fabricated by two different types of impression techniques. Nine subjects, whose molars required treatment with crown restoration, were enrolled in this study. Full cast crowns were fabricated using two types of impression techniques: the conventional impression method (CIM) and the bite impression method (BIM). The occlusal contacts of crowns were precisely evaluated at the following stages: after occlusal adjustment on the articulator (Step 0), before occlusal adjustment in the mouth (Step 1), after occlusal adjustment at the intercuspal position (Step 2), and after occlusal adjustment during lateral and protrusive excursions (Step 3). The number of occlusal contacts of the crowns on the functional cusps fabricated with BIM was significantly greater than that with CIM after occlusal adjustment. For this reason, the crowns fabricated with BIM might have a more functionally desirable occlusal surface compared to the crowns fabricated with CIM.

The main occluding area in patients with fixed partial dentures.

It has become clear that food crushing is performed in the circumscribed region between functional cusps, that is, at "the main occluding area". However, the position of the main occluding area in patients with fixed partial dentures (FPDs) has not been investigated quantitatively yet. There is a possibility that the load onto the abutment teeth of FPDs is loaded excessively depending on the position of the main occluding area at the early stage of mastication. Therefore the position of the main occluding areas of the FPDs was investigated. The purpose of this study was to evaluate the position of the main occluding area in patients with FPDs quantitatively. We compared the position of the main occluding area between the control subjects who had complete dentition and the patients who wore FPDs for replacing missing first lower molars. Pieces of temporary filling material were used as test material to decide the position of the main occluding area. The centroid coordinate of the test material was calculated and projected onto the reference axis in order to evaluate the position of the main occluding area. Moreover, we recorded the occlusal contact areas at the intercuspal position in order to evaluate the relationship between the position of main occluding area and occlusal contacts.As a result, the position of the main occluding areas in the FPD group were significantly posterior to those in the control group and tended to be less stable. Besides occlusal contact area, existence of periodontal ligaments in the pontic region may cause the difference of the positions of the main occluding area between the FPD group and the control group.

[Influence of different spacer thickness and spacer coverage area on the adaptation of Cercon CAD/CAM copings].

OBJECTIVE: To evaluate the accuracy of Cercon CAD/CAM system and the influence of different spacer thickness and spacer coverage area on the internal and marginal fit of the copings. METHODS: A stainless steel master model of first molar was replicated. After scanning, designs of copings were made with different spacer thickness and spacer coverage area. There were six groups: S10C70, S10C90, S30C70, S30C90, S60C70, S60C90. Measurements of the adaptation were performed with silicone impression material and an image analysis system. RESULTS: Univariate ANOVA showed that variation in the internal fit of copings was related to measuring location, spacer thickness and spacer coverage area. There was significant difference between different spacer coverage area groups, and significant difference between spacer thickness 10 microm and 30 microm groups, 10 microm and 60 microm groups, no significant difference between spacer thickness 30 microm and 60 microm groups. Mean marginal fit was (27.7+/-7.6) microm. There was statistically significant difference between both mesial, distal marginal gaps and both buccal, lingual marginal gaps. No difference between mesial and distal marginal gaps, and between buccal and lingual marginal gaps. CONCLUSION: The accuracy by Cercon CAD/CAM system for the marginal and internal fit of zirconium dioxide copings is well within the range of clinical acceptability. Spacer thickness and spacer coverage area have statistically significant influence on the internal fit.

Relationship between upper and lower molars during mastication.

OBJECTIVES: This study investigated the relationship between upper and lower molars in the early stage of mastication. MATERIALS AND METHODS: We measured the occlusal contact areas during chewing test foods using the three-dimensional shape measurement system and the six-degree-of-freedom mandibular tracking device. The reference points corresponding to "A" and "B" occlusal contacts were selected from the surface of upper dental cast (UA, UB) and lower dental cast (LA, LB). The distance between reference point UA and LA, UB and LB were calculated (/A/, /B/). For evaluation of relationship between upper and lower molars during chewing gum, the ratios of /A/ / /B/ (AB ratio) were calculated. First 10 consecutive cycles of chewing gum were divided into two phase, early five strokes group (Ge) and last five strokes group (GI). Paired t-test for AB ratios of Ge and GI was performed. RESULTS: Paired t-test showed a significant difference between AB ratios of Ge and that of GI at 0.5 mm away from the terminal position of masticatory movements (p < 0.05). CONCLUSION: This result suggests the possibility that the path of mastication is influenced by the change of texture of foods with the progress of mastication in early stage of mastication.

Stress distribution in roots restored with different types of post systems with composite resin.

Although composite resin core is used with various types of prefabricated posts, it remains unclear which kind of material is most suitable for the post. The aim of this study was to evaluate the influence of prefabricated posts on the stress distribution within the root by finite element analysis. Posts and cores were built up with composite resin and four types of prefabricated posts: two types of glass fiber posts (GFP1, GFP2) with low and high Young's moduli, a titanium post (TIP), and a stainless steel post (SSP). In all models, stress distribution during function was calculated. There were differences in stress concentration at the root around the end of posts. The magnitudes of stress for GFP1, GFP2, TIP, and STP were 8.7, 9.3, 11.7, and 13.9 MPa respectively. Given the results obtained, GFP1 was the most suitable material for post fabrication since this model showed a lower stress value. It would therefore mean a lower possibility of root fracture.

Accuracy of temporary laser welding of FPDs by Nd:YAG laser in the oral cavity.

The purpose of this study was to investigate the accuracy of temporary fixation with laser welding for fixed partial dentures (FPDs). Five kinds of experimental FPD with different welding/soldering gaps were fabricated (0, 20, 50 microm for welding; 300 microm for soldering). Then, FPDs were temporary-fixed by laser welding or with a self-curing resin. Fixation accuracy was evaluated by the change in distance and the angular deformation between two retainers. The change in distance and the angular deformation between two retainers of the FPD without welding/soldering gap were significantly larger than the other FPDs (p<0.05). With due consideration to the displacement of teeth or implants especially in the mesiodistal direction, and by taking into account the inevitable errors of the indirect method, it seemed reasonable to provide a welding space of approximately 20 microm.