Finite element analysis of orthodontically induced stress in the periodontal ligament of the maxillary first molar with simulated bone loss

摘要

The problems that often arises in orthodontic treatment of adult patients is the presence of periodontal disease and loss of bone support. As a consequence of this, the center of resistance of the tooth is altered. Excessive orthodontic force with advanced periodontal bone loss may traumatize the periodontium. Force dosage during orthodontic treatment is one of the most difficult problems at this stag.the purpose of the study is to simulate the effect of orthodontically induced stress in the periodontal ligament of maxillary molar at different levels of bone loss. The study is carried out to estimate the reduction in magnitude of force at different levels of bone loss (2.5 mm, 5 mm and 6.5 mm) necessary to achieve evenly distribution of stress in the periodontal ligament of the tooth as obtained without bone loss and to determine the change in counterbalancing M/F ratio to induce uniform stress distribution in the PDL. A 3-dimensional finite element model of a tooth comprises of maxillary first molar with periodontal ligament and alveolar bone with different levels of bone height was constructed. Bone loss that ranged from 2.5 thrugh 6.5 mm was simulated in a model. Necessary reduction in the magnitude of force and the increase in moment to force (M/F) ratio was determined to obtain evenly distributed stress in the periodontal ligament of a tooth. The results showed that lesser force magnitude is required in simulated 2.5 mm bone loss (80%), 5 mm bone loss (60%), 6.5 mm bone loss (35%) as compared to the initial load applied to the tooth without bone loss. The counter tipping moment to force ratio should be increased from no bone loss (9.65) through 6.5 mm bone loss (12.8) to maintain the same range of stress in the PDL as was obtained without bone loss. The counter rotation moment to force ratio should decrease from no bone loss (5.65) through 6.5 mm bone loss (4.0) to maintain the same range of stress in the PDL as was obtained without bone loss. Combination of decreased force magnitude and increased counter tipping moment to force ratio will bring about uniform stress in the periodontal ligament of a tooth with varying degree of bone loss.