Appropriate diameter for screws to fix the maxilla following Le Fort I osteotomy: An investigation utilizing finite element analysis

Background: After Le Fort I osteotomy, there is sometimes a secondary deformity (relapse), with the lower segment deviating from the intraoperatively fixed position. It is hyopothesized that the structural stability of the reconstructed maxilla is affected by the diameter of the fixation screws. The present study aims to elucidate the relationship between the diameters of the screws and the structural stability of the maxilla after Le Fort I osteotomy. Methods: 3D models were produced on a workstation from 20 dry skulls and a Le Fort I operation was simulated on them. The upper and lower segments of the divided maxilla in each of the 20 models were connected using four plates and 16 screws. Five different diameters of the fixation screws were tested. Thus altogether 100 models were produced. A 180 N load was applied to the molar region for each model. Using finite element analysis, the resultant stresses and deviations of the lower segments were calculated. Finally, referring to these values, the relationships between screw diameters and stability of the lower segment were evaluated. Result: The stability of the lower segment was greatest when the diameter of the fixation screws was equal to the thickness of the bone at each fixation site. Conclusion: In performing Le Fort I osteotomy, it is recommended that bone thickness is measured at each fixation site in advance, and the diameter of the fixation screws matched accordingly; thereby optimum stability of the reconstructed maxilla can be anticipated.