Apical Stress Distribution of Orthodontic Forces in Maxillary Central Incisors with Abnormal Root Shapes: A Finite Element Analysis

External apical root resorption (EARR) is a common problem observed during orthodontic tooth movement. Maxillary incisors with abnormal root shapes have been found to be at higher risk compared to those with normal root shapes. Objectives: the purpose of this finite element analysis is to determine the stress distribution in the apical region of a maxillary central incisor with four different root shapes (normal, blunted, pointed, and pipette shaped) under simulated orthodontic loading. Methods: a 3-dimensional finite element model of a maxillary central incisor was created with 20852 nodes and 11924 elements. For each root shape, a force of 1 Newton was applied to simulate intrusion, extrusion, tipping, and rotation. Material properties of enamel, dentin, pulp, cementum, periodontal ligament, and alveolar bone were derived from average values of the literature. Boundary conditions were fixed on the outermost layer of the alveolar bone on the mesial and the distal aspects. The finite element analysis was performed with NASTRAN for Windows (MacNeal-Schwendler Corp, Costa Mesa, Calif). Results: tipping forces and extrusive forces consistently displayed higher stress on the lingual aspect of all root shapes: stress was minimum for the normal shape (0.00146 N/mm2) and maximum for the pipette shape (0.02530 N/mm2). Intrusive forces displayed higher stress on the buccal aspect of all root shapes: stress was minimum for the normal shape (0.00333 N/mm2) and maximum for the pointed shape (0.00899 N/mm2). Rotational forces displayed minimal stress in the apical region for all root shapes, ranging from 0.00004 N/mm2 (pipette shape) to 0.00013 N/mm2 (pointed shape). Conclusions: apical root shape influences the stress distribution in the apical region, under different force configurations. Normal roots and blunted roots presented the lowest stress, whereas pipette roots and pointed roots presented the highest stress. This finding is in agreement with recent clinical reports.