Objective:
Approximation of periodontal ligament (PDL) by orthodontic anchorage screw (OAS) causes root resorption. The purpose of the current study is to describe the relationship between the proximity of OAS to PDL and stress in the PDL as the potential factor behind root resorption.
Materials and methods:
Sixteen subject-specific finite element models of the region of right first molar and second premolar were developed using CT images of 4 patients. For each patient, a micro-CT derived OAS surface model was placed at 4 different distances from premolar periodontal ligament; arbitrarily between 0 and 2 mm. Finite element analysis was conducted with mesial load on the OAS increasing from 1 to 4 N. Peak absolute compression stress was calculated at each 1 N step. Linear fit was used to present the relation between compressive stress and proximity to the premolar PDL.The protocol of this study was approved by the ethics committee of Kyushu University Faculty of Dental Science (registration number 25–279).
Results:
The absolute regression slope of stress to proximity relationship doubles when the load on screw doubles. When the load on screw is 2 N or less, it is predicted that stress will be less than 30 Pa regardless of proximity. When the load is 3 N or more and the proximity is less than 1 mm the stresses are predicted to be more than 30 Pa.
Conclusions:
A minimum distance of 0.5 mm between the OAS and adjacent periodontal ligament is recommended to preserve the adjacent roots when the screw is loaded with 2 N force. Increased forces requires minimum 1 mm distance.