1 Professor Dentofacial Deformities Research Center, Research Institute of Dental Sciences and School of Dentistry, Shahid Beheshti University of Medical Sciences

2 Postgraduate Student of Orthodontics, Orthodontic Department, Shahid Beheshti University of Medical Sciences, Dental School, Tehran, Iran

3 Orthodontist



Background and aim: The objective of this study was to investigate the relationship between the gonial angle changes and occlusal plan steepness in patients underwent Bilateral Sagittal Split Osteotomy (BSSO) surgery.
Materials and method: In this analytical retrospective clinical trial, 18 males and 12 females were studied who had been underwent BSSO surgery for mandibular advancement in. Cephalometric landmarks were calculated on the patients' lateral radiographs taken before and 1-6 months postsurgery using Dolphin software. The patients assigned into 3 groups according to different occlusal plan steepness. Landmarks' changes before and after surgery were analyzed by Paired t-test. The changes occurred regarding the normal and flat facial types assessed by Student t-test while the relationship between the presurgical occlusal plan and the landmark changes was evaluated using the Pearson Coefficient.
Results: After BSSO, the gonial angle had increased in all patients with the mean value of 6.83ᴼ (P˂0.0001) but occlusal plan steepness increased to an insignificant value (mean 2.82ᴼ). Increased values of the gonial angle were similarly reported in the patients with normal occlusal plan steepness and flat occlusal plan while no significant relationship was found between occlusal plan steepness before the surgery and the gonial angle changes after surgery. However, fair more increase occurred in patients with normal steepness than lower ones. 
Conclusions: The results showed that there is no significant relationship between the gonial angle changes after surgery and the occlusal plan steepness prior the surgery of BSSO for mandibular advancement.