(44) What is the Relationship Between Maxillary Movement and Incisal Display in Orthognathic Surgery?

Abstract: Maxillary incisal display (MID) is a key component of orthodontic and orthognathic surgical planning, that critically impacts facial esthetics, and forms the basis for the vertical positioning of the maxilla. In orthognathic surgical planning, several variable individual empiric approaches have been proposed to position the maxilla with an expected ideal MID, but little evidence exists regarding the relationship between maxillary movement and MID (1, 2). The purpose of this study was to analyze the relationship between MID and maxillary movements in maxillary orthognathic surgery.

A retrospective cohort study was designed to assess the impact of maxillary movement on incisal display. Subjects were pooled from patients presenting to the University of Illinois Hospital and Health Sciences System, Department of Oral and Maxillofacial Surgery, for maxillary orthognathic surgery between July 2017 and January 2019. Inclusion criteria included: 1) presence of full-field cone-beam computed tomography scans (CBCT) at preoperative and long-term postoperative timepoints, 2) age between 12 and 64 years, 3) presence of maxillary incisors. Exclusion criteria included: 1) distortion of the perioral soft tissues in CBCT (eg, lip straining, chin cup use), 2) long-term postoperative CBCT less than 6 months from surgery, 3) scarring of soft tissues around upper lip that is unrelated to those expected from routine maxillary orthognathic surgery, 4) clefting or syndromic craniofacial deformities. The predictor variables included maxillary impaction (IMP, primary): defined as the vertical movement of the right central maxillary incisor (U1) from preoperative to long-term postoperative (mm, scalar measurement) and maxillary advancement (ADV, secondary): defined as the horizontal movement of the U1 from preoperative to long-term postoperative (mm, scalar measurement). The outcome variable was MID Change (Δ MID): defined as the difference between preoperative and long-term postoperative MID (mm, scalar measurement). Covariates included age (at long term follow up), sex, and use of V-Y closure. Data collection was carried out using preoperative and long-term postoperative CBCT using regional voxel-based registration (R-VBR) technique. The data were analyzed using multiple linear regression to assess correlation between the predictor and outcome variables. Partial correlation analysis was used to analyze the correlation controlling for V-Y closure use with a statistical significance set at P < .05.

After application of inclusion and exclusion criteria, 19 subjects were included in this study (21% male 79% female), mean age of 20.36 ± 2.9 years). Mean IMP and ADV were 0.90 mm (range -3.70 – 10.80 mm) and 1.98 mm (range -1.60 – 6.30 mm), respectively. Mean Δ MID was 1.17 mm (range -3.80 – 12.20 mm). Regression analysis showed a statistically significant correlation between IMP and Δ MID (P < .001) only, with no correlation shown for ADV (P = .574). A significant regression model was found (P < .001), with R-square of 0.906, where Δ MID is equal to 0.915*IMP – 0.808*ADV + 0.508, where IMP is the only significant predictor of Δ MID. Partial correlation analysis controlling for V-Y closure did not impact the correlations.

The results of this study shows that MID is determined by IMP, explaining 90.6% of MID. ADV or use of V-Y closure did not influence MID. These findings can help guide the clinician with surgical planning to control MID while balancing other surgical goals for the maxilla.

1. Khamashta-Ledezma L, Naini FB. Prospective assessment of maxillary advancement effects: maxillary incisor exposure, and upper lip and nasal changes. Am J Orthod Dentofacial Orthop. 2015 Apr;147(4):454-64. doi: 10.1016/j.ajodo.2014.11.028. PMID: 25836005

2. Louis PJ, Austin RB, Waite PD, Mathews CS. Soft tissue changes of the upper lip associated with maxillary advancement in obstructive sleep apnea patients. J Oral Maxillofac Surg. 2001 Feb;59(2):151-6. doi: 10.1053/joms.2001.20485. PMID: 11213983