Comparison of facial midline landmark and condylar position changes following orthognathic surgery

Background: CBCT imaging offers numerous advantages in orthognathic surgery case diagnosis, treatment planning and post-surgical evaluation. In maxilla-mandibular combination surgery, regions of the craniofacial complex are altered in all three dimensions of space. The upper jaw, lower proximal segment and bilateral distal segments must be separated, reoriented and then secured in the desired position. The movements of the surgical segments must be made in concert with each other in order to ensure a stable result. ❧ Purpose: To evaluate the relationship between midline lateral cephalometric landmark changes with condylar translation and rotation displacement, relative to the glennoid fossa, following bi-maxillary surgery. ❧ Materials and Method: Post-surgical condyle translation and rotation values were measured for 32 patients in two groups (sample A, n=17 and sample B, n=15) by segmenting pre- and 2 weeks post-surgery DICOM files in ITK snap. Each pair of condyles was re-oriented to the glenoid fossa, cropped to the same size and then superimposed with Geomagic software. Lateral cephalometric images were generated from these same pre- and post-surgical CBCT images, oriented to natural head position, and several midline landmarks were identified. The linear or angular change between the pre- and post-surgical landmarks was then calculated and analyzed for statistical significance. The condylar translation and rotation changes were then compared to the post surgical lateral ceph changes. Also, Virtual Surgical Planning (VSP) data for 31 of the 32 patients (one VSP report was not available) in the sample were collected and evaluated. Pogonion values from the post-surgical positional change and pre-surgical VSP predicted movement were calculated and evaluated for statistically significant correlation. ❧ Results: For sample A (n=17) the average condylar displacement was 0.78 mm on the x-plane, -3.80 mm on the y-plane and 0.15 mm on z-plane. The average rotation was 0.06° on the x-axis, 0.31° on the y-axis and -0.28° on the z-axis. Translation along the y-axis (anteroposterior) and rotation on z-axis (superoinferior), were the only movements that were statistically significant (p<0.008). In sample B (n=15) the average condylar displacement was 0.05 mm on the x-plane, -0.39 mm on the y-plane and 0.01 mm on the z-plane. The average rotation was -0.69 mm on x-axis, 1.69 on the y-axis and -2.43 on z-axis. The movements that were statistically significant were translation on the y-axis and rotation on the y-axis (p<0.008). Lateral cephalometric landmark samples for SNA, SNB, AFH, SN-GoGn and Occl-SN were all statistically significantly (p<0.05), while PFH (posterior facial height) was not significant (t(31)=1.719, p=0.096). No significant correlation was found between lateral ceph midline landmarks SNA, SNB, AFH, PFH, SN-GoGn, or Occl-SN with age of patient at time of surgery, gender of patient, condyle rotation, or condyle translation. Pre- and post-surgical lateral ceph pogonion change and the VSP pogonion movement estimates were strongly correlated (correlation coefficient = 0.751). ❧ Conclusions: We saw an agreement between pre- and post-surgical changes for sample A and sample B for all rotation and translation in the X direction; however, there was less conformity between samples in translation for Y and Z directions. The strong correlation between pre- and post-surgical lateral ceph pogonion movements and the VSP pogonion movement estimates is to be expected, as surgical movements are completed as planned using pre-fabricated intraoral surgical splints to aid in obtaining planned skeletal movements.