Effect of abutment tooth location on the accuracy of digital impressions obtained using an intraoral scanner for removable partial dentures.

PURPOSE: To verify the effect of abutment tooth location on the accuracy of digital impressions obtained using an intraoral scanner (IOS) for removable partial dentures (RPDs). METHODS: The target abutment teeth included the left first premolar (#34), second molar (#37), and right second premolar (#45) in a mandibular Kennedy class II model and the left and right second molars (#37, #47) in a class III model. Only #37 was isolated from the remaining teeth by the mucosal area in both models. Digital impressions were obtained using a desktop scanner (reference data) and an IOS (IOS data; scanning origin #37; n=10). The general trueness based on the entire model superimposition (TG), local trueness (TL) of an individual tooth, and dimensional accuracy (coordinate and linear accuracy) of the IOS data of the target abutment teeth were compared (α=0.05). RESULTS: In both models, #37 showed significantly inferior TG (P<0.01), superior TL (P<0.01), and mesial coordinate displacement (P<0.01 and P<0.05 in class II and III models, respectively). Intra-model comparisons showed that #45 in the class II model and #47 in the class III model had significantly inferior linear accuracy (P<0.05 and P<0.01, respectively) and buccal coordinate displacement (P<0.05 and P<0.01, respectively) compared with the other target teeth. CONCLUSIONS: In digital impressions of RPDs, isolation of abutment teeth by mucosal areas can reduce general trueness based on the entire dental arch and mesial tooth displacement, whereas increased distance from the scanning origin can adversely affect local trueness and dimensional accuracy.

Effect of tension and compression on dynamic alveolar histomorphometry.

Here, we tested the hypothesis that tensile and compressive stresses generated in the alveolar bone proper regulate site-specific cellular and functional changes in osteoclasts and osteoblasts. Thirty-two 13-week-old male mice were randomly divided into four groups: two experimental groups with vertical loading obliquely from the palatal side to the buccal side of the maxillary molar (0.9 N) 30 min per day for 8 or 15 days and unloaded controls (n = 8). Calcein and alizarin were administered 8 and 2 days before euthanization, respectively, to detect the time of bone formation. Undecalcified sections parallel to the occlusal plane were prepared on the palatal root and the surrounding alveolar bone in the middle of the root length. The alveolar perimeter was divided into 12 equal regions for site analysis, and the bone histomorphometric parameters were obtained for each region. Data from in vivo microfocus computed tomography were used to construct animal-specific finite element models. 2D stress distribution images were overlain on histology images obtained from the same location. Significant differences in the total perimeter between groups and between loading and unloading in each region were statistically analyzed (α = 0.05). Osteoclast counts and the alizarin label ratio were significantly higher in the loaded group than in the unloaded group in regions where the maximum von Mises and principal tensile stresses were the highest along the perimeter. The label ratio of calcein was significantly lower in the 8-day loaded group than in the unloaded group, indicating that the calcein-labeled surface was resorbed by osteoclasts that appeared during the loading period. The effect of loading was mitigated by an increase in the maximum principal compressive stress. We conclude that bone resorption and bone formation are functions of site-specific tension and compression in the alveolar bone proper, confirming our hypothesis. This finding is critical for the advancement of diagnosis and treatment planning in clinical dentistry.

Load limit of mini-implants with reduced abutment height based on fatigue fracture resistance: experimental and finite element study.

PURPOSE: The primary objective of this study was to investigate the fracture resistance of experimental mini-implants with a reduced abutment height. The secondary objective was to assess the effects of implant diameter and bone level on the load limit, using finite element simulations. MATERIALS AND METHODS: Two Ti-6A1-4V 1.8-mm-diameter implants were subjected to monotonic bending testing and fatigue tests incorporating 5 x 10(6) cycles (ISO 14801): a commercially available implant (c18), and an experimental implant with a reduced abutment height (e18). The load limit was estimated using the finite element models based on the maximum stress at failure in the experiments. For simulations, implants with increased diameters of 2.1 and 2.4 mm were also modeled, and the load limit was estimated for all models in a bone model. RESULTS: In the bending test, e18 revealed a higher mean load at yield stress than c18, and this was attributed to the reduced height of the former. An endurance limit of 140 N was detected for both c18 and e18 in the fatigue test, while the load limit of e18 was higher than that of c18. The estimated load limit increased as the implant diameter or the bone level increased, with the highest value of 510 N observed at a diameter of 2.4 mm. CONCLUSION: A higher load limit was evident in the experimental mini-implant with a reduced abutment height. The simulations indicated that the load limit increased with increased implant diameter and higher bone levels.