Stress Comparison Between Orthodontic Miniscrews Using Finite Element Analysis

Objectives: The aim of this study was to investigate the impact of different orthodontic miniscrews and alveolar bone stiffness on stress distributions using finite element analysis(FEA).

Methods: Models of a posterior left maxillary dentoalveolar complex were constructed from µCT data. Computer aided design models of the TOMAS® and IMTEC® orthodontic miniscrews were inserted buccally between the second premolar and first molar at 60° relative to the cortical bone surface. An FE mesh with 91,500 tetrahedral elements was generated for each model, a retraction load of 150grams was placed on each miniscrew, Young's modulus was varied, interfaces between materials were rigidly bonded, and FE models were solved using ANSYS (10.0). Maximum principle stress (MPS) and von Mises stress for each model were compared.

Results: MPS was greater for the TOMAS (33.64MPa) than the IMTEC (17.68MPa) miniscrew model while Von Mises stress was similar for both (24.76MPa to 23.12MPa) at a low cortical bone Young's Modulus (10,000MPa). At a higher cortical bone Young's Modulus (18,000MPa), MPS and von Mises stress in the TOMAS model decreased slightly (29.95MPa, 22.62MPa) while they increased in the IMTEC model (18.11MPa, 25.09MPa). For all models, stress patterns were similar in location and magnitude on both miniscrews and cortical bone. The greatest MPS values were located distal to the miniscrews; the greatest von Mises values were diffused around the miniscrew/bone interface.

Conclusions: MPS and von Mises values generated in cortical bone were below the ultimate tensile strength of bone (>100MPa) and titanium (900MPa) in all models. Differing miniscrew specifications may account for differences in the stress patterns seen in alveolar bone. No clinically appreciable static mechanical difference exists between the miniscrews evaluated. Clinical fractures are likely to be preceded at low stress levels by a slow sub-critical process that may involve fatigue of materials and tissues.