Orthodontic Forces at the Root Apex With Simulated Periodontal Ligament

Objectives: To compare forces at the apex of the maxillary central incisor between different directions of orthodontic tooth movement using a new simulation system with a soft periodontal ligament.
Methods: A new orthodontic force simulation system with a maxillary central incisor made of stainless steel and a periodontal ligament made of a silicone-based denture liner (0.33 mm thickness) was used to measure the force at the apex of the maxillary central incisor using a six-axis sensor. Lingual and intrusion movements were simulated with orthodontic force levels of 20, 50, and 100 gf. To simulate lingual movement, orthodontic forces were applied to a bracket placed on the labial surface of the central incisor and forces were measured at the root apex only in the lingual direction. To simulate intrusion movement, orthodontic forces were applied to the edge of the central incisor and forces were measured at the root apex only in the root apex direction. Means and standard deviations were calculated from the results of five measurements performed for each orthodontic force level. For the two directions of orthodontic movement, Pearson's correlation coefficients between forces applied and measured for fifteen measurements (five measurements at three orthodontic force levels) were individually calculated. The effects of the direction of tooth movement and applied force levels on the force measured at the root apex were analyzed by two-way analysis of variance and Bonferroni’s multiple comparison tests (P<0.05).
Results: Significant correlation coefficients between applied and measured forces were observed for lingual (0.88, P<0.01) and intrusion movements (1.00, P<0.01). For all applied force levels, measured forces for the simulated lingual movement were significantly smaller than measured forces for the simulated intrusion movement (P<0.01).
Conclusions: Measured forces at the root apex differ according to the direction of the simulated tooth movement.