Morphologic and biomechanical determinants in the selection of orthognathic surgery procedures

Purpose: This study examined which features of craniofacial morphology are most important in the selection of an orthognathic surgery procedure by 1) producing a small number of composite variables representing multiple measures of craniofacial morphology and craniofacial biomechanics, and 2) looking for correlations between these composite variables and selected orthognathic surgery procedures. Patients and Methods: Lateral cephalograms of 201 adult subjects (71 men and 130 women) were used to generate 47 standard cephalometric measures and 30 biomechanical measures. Of the 201 subjects, 121 were scheduled for orthognathic surgery to treat a variety of dentofacial deformities. Factor analysis reduced the number of variables by identifying underlying latent composite variables, thereby strengthening correlations among the reduced number. Weightings for each factor were than compared among the orthognathic surgery procedures, indicating which factors may have influenced the selection of that procedure. Results: Factor analysis determined 12 factors (explaining 93% of the variance) for the morphologic measurements and 6 factors (explaining 90% of the variance) for the biomechanical measurements. However, only 6 of the morphology factors (accounting for 53% of the morphologic variance) and 5 of the biomechanics factors (accounting for 69% of the biomechanics variance) significantly separated any of the 10 treatment groups. The separating morphology factors were generally related to relative maxillary and mandibular position or dental relationships. Of these, relative maxilla/mandible anteroposterior position was most important for defining the surgery groups. The biomechanics of the lateral pterygoid muscles did not contribute to separation of the groups. Conclusions: Only a subset of available morphologic information was used to select surgical treatment. The most important factors in treatment selection were difference in maxillary and mandibular lengths and differences in anterior and posterior facial height. Standard morphology factors accounting for 40% of the total morphologic variance apparently played no role in selection of treatment. Several biomechanical factors differentiated treatment groups as well as or better than some standard morphology factors.