[Study on mechanical stability of maxillary LeFort Ⅰ type osteotomy after fixation with mortise and tenon and absorbable plate].

To evaluate the mechanical stability of the bone block after LeFort Ⅰ osteotomy with maxillary advancement using absorbable plates fixed with tenon-and-mortise structures. This study developed three finite element models: one for the maxillary LeFort Ⅰ osteotomy with anterior advancement fixed with absorbable plates (Model 1); another for the maxillary LeFort Ⅰ osteotomy with anterior advancement fixed with absorbable plates assisted by tenon-and-mortise structures (Model 2); and the last one for the maxillary LeFort Ⅰ osteotomy with anterior advancement fixed with titanium plates and screws (Model 3). Simulated occlusal forces were applied on the anterior and posterior teeth in each model. The displacement changes of the nasal-palatine point (NP) and posterior nasal spine point (PNS) in the finite element coordinate system were compared and analyzed. The Mises equivalent stress distributions of the metal and absorbable plates were also examined to assess the mechanical stability of the three finite element models. Clinical data from 45 patients with dentofacial deformities treated from January 2017 to January 2023 at the Stomatology Hospital of China Medical University were collected. The age of the patients was 21±3 years. Among these, 15 patients had absorbable plates for fixation, 15 had absorbable plates assisted by tenon-and-mortise structures, and 15 had titanium plates and screws fixation after maxillary advancement. All patients underwent preoperative (T0), postoperative 3 days (T1), and 6 months (T2) spiral CT scans. The CT data in DICOM format were input into digital software, which was used to calculate the distances from the NP and PNS points to the horizontal plane (HP), right sagittal plane (FZSR), and coronal plane (CP) at T1 and T2. The distances at T1 and T2 were statistically analyzed using the Wilcoxon signed-rank test with SPSS 20.0, and a P value of<0.05 was considered statistically significant. The finite element analysis showed that in the absorbable plate-only fixation group, the maximum displacement of the NP point (mm) under anterior and posterior tooth force conditions were 0.6 and 0.12, respectively, and for the PNS point, the maximum displacements were 0.5 and 0.11. In the tenon-and-mortise-assisted absorbable plate fixation group, the displacement of the NP point was 0.40 and 0.02 mm, and the displacement of the PNS point was 0.5 and 0.015 mm. In the titanium plate-screw fixation group, the NP point displacement was 0.33 and 0.055 mm, and the PNS point displacement was 0.16 and 0.1 mm. The Mises equivalent stress on the absorbable plates with tenon-and-mortise structure was significantly lower than that in the absorbable plate-only fixation group, while the titanium plate experienced the highest Mises equivalent stress. The clinical data analysis showed that in the horizontal direction, the postoperative stability of the three fixation methods was similar. However, in the vertical and anterior-posterior directions, the absorbable plate-only fixation group showed significant differences in the distances of PNS-HP, PNS-CP, and NP-CP between T1 and T2 (=0.018, =0.009, =0.017), suggesting significant postoperative bone displacement. In contrast, the tenon-and-mortise-assisted absorbable plate fixation group and the titanium plate-screw fixation group showed no significant differences in displacement during surgery and postoperatively(all >0.05), demonstrating higher stability. The tenon-and-mortise-assisted absorbable plate fixation provides comparable stability to titanium plate fixation in clinical results, and it is more stable than absorbable plate-only fixation. In the mechanical study, when force was applied on the anterior teeth, the stability of the tenon-and-mortise-assisted absorbable plate fixation was slightly less than that of titanium plate fixation, but when posterior teeth were used, its stability exceeded both titanium plate fixation and absorbable plate-only fixation. The tenon-and-mortise-assisted absorbable plate fixation serves as an effective alternative to titanium plate fixation after LeFort Ⅰ osteotomy.