A Cone Beam Computed Tomography Research On Displacement Of The Proximal Segment Caused By Mandible Setback Via Bilateral Sagittal Split Osteotomy

The bilateral sagittal split osteotomy(BSSO) is the most common procedure used to correct mandibular deficiencies. There is growing support for the belief that post surgical changes in the proximal segment can lead to remodeling of the glenoid fossa and condylar surface which results in surgical effect instability and even relapse.The purpose of this study is to use cone beam computed tomography (CBCT) to identify position changes of the proximal segment caused by mandible setback via bilateral sagittal split osteotomy.Part one:Visual Observation of The Proxinal Segnent Displacenent Caused by Mandible Setback via BSSO Objective: To identify displacement of the proximal segment caused by BSSO using visual tool. Methods:Pre surgical and post surgical cone beam CT images were taken on eight patients who had undergone BSSO to setback the mandible. The Invivo Dental software package was used to achieve the volume reconstruction of the skull hard tissues, and changes in the proximal segment were roughly observed in the superimposition windows. Results:Extensively superimposition was achieved after visual modeling. It was easy to distinguish the location of the presurgical and postsurgical proximal segment. All the patients were obviously found position changes in the gonion of the mandible. Conclusion:Visual modeling of the skull superimposition lays the foundation of relative quantitaive analysis.Part two:Semiquantitative Visual Analysis of The Proximal Segment Displacement Caused by Mandible Setback via BSSOObjective:To identify postsurgical changes in the proximal segment by using graphical overlays. Methods:Pre surgical and post surgical cone beam CT images were taken on eight patients who had undergone BSSO to setback the mandible. The Valmetl.1tool was used for both visual and semiquantitative assessment of the location and magnitude of the surface displacement of the proximal segment. Results:The gonion regions mostly had outward surface displacement and the maximum value of which was greater than2mm. Conclusion:Torque of the proximal segments changed in three dimensional space. Outward surface displacement of the gonion regions can be observed nearly in all the patients. Part three:Quantitative Analysis of The Proximal Segment Displacenent Caused by Mandible Setback via BSSOObjective:To identify post surgical changes in the proximal segment caused by mandible surgery via BSSO. Methods:Pre surgical and post surgical cone beam CT images were taken on eight patients who had undergone BSSO to setback the mandible. Using Invivo Dental, digital landmarks including the lateral pole of the condyle, the medial pole of the condyle, the coronoid process, and anatomical gonion were placed on the proximal segments. Right and left proximal segments were evaluated independent of each other. Wilcoxon signed ranks test was performed on the presurgical and postsurgical landmarks. Results:There was significant changes of the y and z coordinate in coronoid process (p=0.001for y coordinate,0.000for z coordinate) and gonion (p=0.001for x coordinate, p=0.000for y coordinate,0.000for z coordinate). The mean posterior displacement of coronoid process was1.0±0.6mm while the mean inferior displacement was2.0±0.4mm; the mean lateral displacement of antomical gonion was3.0±1.6mm while its posterior and inferior displacement was2.6±1.4mm and1.2±0.7mm. there was no significant displacement at the level of the condyle. Conclusion:Clockwise rotation of the proximal segment and lateral diasplacement of the gonion occurred in BSSO setback surgery.Part four:Relationship between the Amount of Mandibular Setback and the Displacement of Proximal SegmentObjective:To determine the relationship between the displacement of the proximal segment caused by BSSO and the setback amount. Methods: Pre surgical and post surgical cone beam CT images were taken on eight patients who had undergone BSSO to setback the mandible. Using Invivo Dental, digital landmarks were placed on anatomical points on the proximal segments. A landmark was also placed on the supramental in order to measure the amount of setback. Spearman’s rho analysis was performed to see if there was a correlation between the measurements of mandibular setback and the displacement of the proximal segment landmarks. Results:A positive correlation was found between the lateral displacement of the anatomical gonion and the setback amount (r=0.632, p=0.004).The mean setback amount was5.7mm while the lateral displacment of antomical gonion was3.0mm. Conclusion:The results indicates a significant relationship between the setback amount and both lateral displacement of the anatomical gonion which means the greater amount we setback the mandible, the greater lateral displacment of its gonion produces.