Accuracy of Virtual Surgical Planning with Mandible-first Surgery and Factors that may Affect It

Abstract

Planning bimaxillary orthognathic surgery has become much easier and the workflow more streamlined with the advent of virtual surgical planning (VSP). Among its advantages, VSP allows for easy transition between performing maxillary or mandibular surgery first, as it eliminates the cumbersome process of reverse model surgery. In this experience of performing mandible surgery first, it was observed that the osteotomy gaps at the piriform and zygoma appeared to be different than what was predicted on the VSP. The present study was initiated to compare the planned movements to the actual achieved surgical position of the maxilla after correction of dentoskeletal deformity. The goal of the study was to evaluate the accuracy of a maxillary surgery-first sequence and a mandibular surgery-first sequence. Additionally, the researchers sought to determine if any surgical interventions could mitigate the under-advancement noted in observations.The authors implemented a retrospective audit of 61 patients who underwent bimaxillary orthognathic surgery from January 2017 to September 2020. Exclusion criteria included previous cleft lip/palate surgery, previous orthognathic surgery, lack of a post-surgical CBCT, non-sagittal split osteotomies of the mandible, or if there was greater than 3mm vertical deviation in planned and achieved surgical position of the central incisor based on intraoperative incisor show. Patients were divided into 2 groups based on the surgical sequencing used; 28 patients had maxillary surgery performed first and 33 underwent mandibular-first surgery. Pre-surgical and post-surgical CBCT scans were analyzed by 3D Systems engineers, with overlays made using the predicted osteotomies and the postoperative CBCT scans. A pre-determined set of skeletal landmarks were measured on the virtual surgical plan and the actual postoperative images, and the difference between the planned and actual position was calculated for each reference point for each patient. The absolute value of each difference was then calculated, and a series of 2 sample T-tests was performed to compare the average absolute difference between planned and actual position between groups.The absolute values of the linear (mm) and angular (°) measurement differences between planned and postoperative results demonstrate statistically significant differences in accuracy between the 2 groups, with greater deviation in the mandible-first group. The maxillary central incisors were under advanced in the anterior-posterior (AP) direction in both groups. Additionally, a significant difference in reference points ISU1 (AP) and A Point (AP) was noted between the groups, indicating that greater inaccuracy lies within the mandibular-first group. However, the majority of data points show deviation from the surgical plan less than or equal to 2mm and 4 degrees, meeting previously described success criteria.1,2To further evaluate the larger maxillary under advancement in the mandible-first surgery cohort, the researchers sub-divided the mandible-first surgery group to evaluate factors that may explain the difference, 1 factor being the type of fixation used for sagittal split segments, rigid or non-rigid. When comparing the average linear measurements, the non-rigid fixation group showed less accuracy when compared to the rigid fixation group, and this was statistically significant (P-value: .007).In conclusion, the findings of the present study demonstrate that virtual surgical planning can be less accurate in predicting the maxillary incisor position when mandibular surgery is performed first, but this inaccuracy is within the acceptable range and can be mitigated by more rigid fixation of the mandible.