Displacement and stress distribution of mandibular incisors after orthodontic treatment in the presence of alveolar bone loss under occlusal loads: A finite element analysis.

INTRODUCTION: This study evaluated the initial stress produced in the periodontal ligament (PDL) and the displacements of mandibular incisors under masticatory force in patients with alveolar bone loss (ABL) after orthodontic treatment. METHODS: Four horizontal absorption models (zero, one third, one half, and two thirds of root length) and 2 labiolingual absorption models (labial two thirds, lingual one third of root length, and vice versa) of the mandibular anterior segment were constructed. A total force of 285.3 N was applied vertically to the edges of incisors. The tooth displacement and principal stresses in the PDL were evaluated in a finite element analysis. RESULTS: In all models, the labial movements of the central incisors ascended more significantly, whereas there was obvious compressive stress and tensile stress concentrated in the labial and lingual cervical margins of the PDL, respectively. For the lateral incisors, augmentation of the distal motions was more evident. Compressive stress was apparent in the labial-distal margin, and tensile stress was concentrated in the lingual-mesial cervical margin. With the same proportion of ABL, more significant displacement and stress concentration in the PDL occurred in the central incisors. In labiolingual absorption models, labial ABL caused greater incisors displacement and periodontal stress concentration. When horizontal ABL extended from one half to two thirds of the root length, mobility of the central incisors and stresses in the PDL increased significantly. CONCLUSIONS: Mandibular incisors follow the different movement and stress distribution patterns under occlusal loads. Special consideration should be given to the retention of mandibular incisors when horizontal ABL exceeds half of the root length.