[Influence of proximal two-wall bone defect on periodontal ligament stresses under normal occlusal forces]

Objective: To study the influence of two-wall bone defect on periodontal ligament stresses under normal occlusal forces, and to analyze the influence of depth and width of bone defect to periodontal ligament stresses. Methods: Three-dimensional finite element models of teeth, periodontal ligament and alveolar bone were created based on cone beam CT images. Proximal two-wall bone defect with different depths (bone defect occupies one third, two thirds, and full length of root) and widths (bone defect occupies one fourth, two fourths, three fourths and full width of buccal lingual width) were simulated by modifying the elastic modulus of elements within defect areas. Occlusal forces with magnitudes of half of the maximum occlusal forces were applied to the model at an angle of 45° to the long axis of tooth, and equivalent stresses of periodontal ligament were analyzed. Results: In the model of no bone defect, the equivalent stresses of periodontal ligament of incisors, canines, premolars and molars were 2.88, 2.31, 8.67 and 7.53 MPa respectively. The equivalent stresses of periodontal ligament increased with the enlargement of depth and width of bone defect. The equivalent stresses of periodontal ligament with maximum bone defect in both depth and width for incisors, canines, premolars and molars were 4.47, 3.62, 11.66 and 8.72 MPa respectively. In the model of width of bone defect was consistent and bone defect develops vertically, the increments of equivalent stresses of periodontal ligament were significantly greater in the early stage bone defect model (from no defect to one third of root length bone defect) than that in the later stage bone defect model (from two thirds to full length of root length bone defect). In the model of bone defect depth was consistent and bone defect develops transversely, the increments of equivalent stresses of periodontal ligament in the early stage bone defect model (from no defect to one fourth of buccal lingual width bone defect) were significantly smaller than that in the later stage bone defect model (from three fourths to full width of buccal lingual width bone defect). Conclusions: Bone defect with shallow depth and that with large width would increase periodontal ligament stresses. Therefore, more attention should be paid to occlusion evaluation and adjustment for teeth with shallow and wide two-wall bone defect.