Evaluation of residual bone mass of the mandibular third molar and the risk of mandibular fracture.

Background: A quantification of the residual bone mass of the mandible (B/A) was utilized in this study to examine the correlation between mandibular fracture and residual bone mass. To improve the clinical utilization rate and reduce the incidence of iatrogenic mandibular fractures, the B/A ratio calculation should be simplified.

Material and methods: Data were collected from the Yanbian University Hospital on 175 cases of mandibular fracture with third molar (M3), 67 normal cases without fractures and 20 cases of impacted teeth extraction. Twenty cases of iatrogenic mandibular fracture were collected, and the case records and panoramic radiographs of the patients were recorded.

Results: The average B/A ratio of mandibular angle fracture group was 0.61±0.10.The value of B/A was found to be statistically significant in terms of whether M3 emerged from alveolar bone (P = 0.001), location (horizontal P < 0.001, vertical P < 0.001), the degree of impaction (P < 0.001), the number of roots (P < 0.001), the difference in impaction (P < 0.001), and the fracture type (P = 0.002). The average B/A ratio of normal group was 0.62±0.10. In the statistical results of the B/A value of normal patients, M3 involving alveolar bone (P < 0.001), position classification (P < 0.05), degree of impaction (P < 0.001) and presence or absence of a root (P < 0.05) were statistically significant. The average B/A ratio of iatrogenic mandibular angle fracture group was 0.28±0.08. The average B/A ratio of the extraction group for impacted teeth was 0.62 ± 0.09.

Conclusions: There is a high risk of mandibular angle fracture when the (B/A) value of the residual bone height (B) in the mandibular M3 area compared to the mandibular bone height (A) in the M3 area is less than 0.4.