The Influential Bony Factors and Vectors for Predicting Soft Tissue Responses After Orthognathic Surgery in Mandibular Prognathism

Abstract

We sought to identify the hard tissue points and vectors that have the greatest effect on soft tissue movement after orthognathic surgery in patients with mandibular prognathism. The present retrospective study involved patients who had undergone mandibular setback surgery with or without maxillary advancement. Multiple linear regression models were adapted to evaluate the association between the 8 hard tissue landmark (predictor variables) changes and 11 soft tissue responses (outcome variables) using the x and y coordinates assessed from superimposed pre- and postoperative 3-dimensional computed tomography images. A total of 50 patients (42 patients had undergone 2-jaw surgery; 8 patients had undergone 1-jaw surgery; mean age 23±4years) were included in the present study. Our statistical models demonstrated that the horizontal hard tissue changes had a greater influence on the soft tissue responses than did the vertical changes, and these changes were more notable in the lower facial area (lower lip contour and chin profile) than the midfacial area (nasal profile, upper lip contour, upper lip length, and nasolabial angle). In the horizontal soft tissue response model, the soft tissue A point/A point ratio was 0.86:1 (95% confidence interval [CI] 0.674-1.049); the soft tissue B point/B point ratio was almost 1:1 (95% CI 0.919-1.071); and the soft tissue pogonion/pogonion ratio was 0.88:1 (95% CI 0.805-0.963). Horizontal or vertical bone tissue changes affected both the horizontal and vertical soft tissue changes in most areas. Our study demonstrated that the soft tissue response is not linear but a more complicated and dynamic reaction.