UCSF

Objective: To compare the alveolar bone thickness of mandibular incisors in individuals with mandibular low-, average-, and high- angle skeletal patterns BEFORE orthodontic treatment, to measure alveolar bone loss and root resorption of mandibular incisors in the three skeletal patterns AFTER orthodontic treatment, and to link condylar morphology to the different skeletal patterns, using cone-beam computed tomography.

Methods: Pretreatment CBCT images of 75 non-growing individuals, 25 in each of 3 groups: low-angle (SN-MP , average- angle (30-37 , and high-angle (≥39 , were analyzed. Buccal-lingual bone thickness was measured at the root apex, mid-root, and at the alveolar crest of the mandibular right central incisor. Post-treatment CBCT images of 11 low-angle, 20 average-angle, and 27 high-angle patients were compared to their pre-treatment images to determine changes in the alveolus and mandibular incisor after orthodontic treatment. 20 pre-treatment CBCTs in each skeletal group were also evaluated to characterize their pre-existing temporomandibular joint morphology.

Results: ANOVA test showed that the buccal-lingual width at the apex of the mandibular incisor was significantly different between the three groups (p-value=0.000). A Bonferroni correction showed that the bone supporting the mandibular incisor in low-angle individuals was significantly thicker than that of average and high-angle individuals (p-value=0.000), but that the difference was not significant between those with average and high angles (p-value=0.059). The pretreatment TMJ evaluation indicated that high angle subjects had a higher prevalence of signs of degenerative joint disease, but this difference was not statistically significant (Fisher's exact, p = 0.09). Evaluation of post-treatment changes showed that high-angle subjects had more external root resorption when compared to those with average and low-angles, but the buccal-lingual bone thickness changes were minimal (ranged from 0.24mm to 0.58mm).

Conclusions: Negative sequelae of orthodontic treatment is more frequently found in individuals with high-mandibular plane angle, and could be linked to their thin pre-existing mandibular buccal-lingual widths. Individuals with a narrow alveolar ridge have a narrow envelope in which teeth can be moved safely.