title: 3D Assessment of the skeletal stability of the surgery-first approach for orthognathic correction of maxillary deficiency
Creators: name: Saghafi, Hamideh
Publisher: name: University of Glasgow
Year: 2024

description: Introduction:
Stability is one of the important criteria for determining the success of orthognathic surgery. Stability is defined as the maintenance of the skeleton and associated dental structures in the intended postoperative position over the long-term. The stability following surgery-first and orthodontic-first approach might be affected by the quality of occlusion, a fact which is not yet confirmed. Skeletal, dental and surgical factors can affect the post-operative stability (Jackson &amp; Golden, 2016). These factors include degree of incisor inclination, overjet, overbite, depth of the curve of Spee, mandibular and maxillary plane angles, occlusion, as well as rotation of maxilla-mandibular complex, condylar position, alterations of masticatory muscle, the magnitude of jaw movement and method of fixation (Jackson &amp; Golden, 2016; Peiro-Guijarro et al, 2016) One of the main objectives of orthognathic surgery is skeletal stability, which can be categorized into short and long-term stability. Short-term stability can be defined as physiological adaptation, directly related to the post-surgical healing and orthodontic treatment. The long-term stability is influenced by surgical or patient-related factors and orthodontic treatment. The pre-operative orthodontic alignments maximize the optimal surgical repositioning of the jaws, while the postoperative orthodontic treatment ensures refinement of occlusion and retention (Proffit et al, 2007). In the surgery-first approach, orthodontic dental alignment and decompensation are deferred until after surgery, therefore, the surgical occlusion is different from the final occlusion and expected to be unstable due to premature occlusal contacts. The unstable occlusion may hinder the long-term skeletal stability, leading to skeletal relapse (Nagasaka et al, 2009; Soverina et al, 2019). Nadjmi et al. (2010) stated that a stable dental occlusion is one of the key goals in orthognathic surgery planning, since it defines the postoperative position of the maxilla and mandible (Nadjmi et al, 2010). Meanwhile, the quality of postoperative occlusion and its influence on skeletal stability has not been fully understood. There are no consistent evidence or criteria to define the post-operative occlusion. Some studies have reported no statistically significant differences in postoperative stability between surgery-first and conventional approach (Choi et al, 2015; Choi et al, 2016; Park et al, 2016), while others stated that surgery-first was less stable than conventional approach due to lower quality of post-operative occlusion (Kim et al, 2014b; Ko et al, 2013).

Aims:
The aim of this project was the 3D assessment of the skeletal stability in skeletal class III patients who underwent Le Fort I osteotomy by surgery-first approach. The second aim of the study was to assess the relationship between immediate postoperative occlusion and skeletal stability.

Methodology:
The study was carried out on the pre- and post-operative CBCT images of 25 patients who received their orthognathic surgery treatment at Glasgow dental hospital and school, Glasgow, UK. Skeletal class III patients who underwent Le Fort I maxillary osteotomy by surgery-first approach and had complete CBCT records were included. Ethical approval was obtained from the NHS Greater Glasgow &amp; Clyde (R &amp; D reference: GN20OD634, REC reference: 21/NE/0019). Patients were selected from the Glasgow Dental School database from 2012-2022. All patients were diagnosed with maxillary retrognathism, which required orthognathic surgical correction. Patients who had previous facial surgery, cleft lip and palate cases, or history of dentofacial trauma were excluded. The CBCT scans taken within one week prior to surgery (T0), at 1 week (T1) and 6 months following surgery (T2) were used for the analysis. The pre- and post-operative CBCT DICOM images of each patient were segmented at HU=276 to generate the STL files. Total of 3 STL files were generated for each patient. The 3D STL models were assessed with VRMesh software (Virtual Grid, Seattle City, U.S.A), to measure the surgical and skeletal changes. The anterior cranial base, zygomatic arches, and forehead unaffected by surgery, were used for the registration of postoperative (T1, T2) 3D models to the preoperative (T0) 3D model using Surface-based registration (SBR). The maxillary right and left greater palatine foramen and incisive foramen were selected to measure 3D surgical movement (T0-T1) and skeletal changes (T1- T2).The translation and rotation of the coordinates of these 3 landmarks were reported in six degrees of freedom along x, y, z axis; left/right (L/R), anterior/posterior (A/P) and superior/inferior (S/I) and pitch, roll and yaw. The dental study models taken at one day before surgery, were scanned using 3Shape intraoral scanner (TRIOS3, 3shape A/S, Copenhagen, Denmark) and imported to IPS Case Designer® software (KLS Martin, Tuttlingen, Germany) for replacement of the defective dentition of postoperative CBCT scans (T1). The occlusal colour map representing the distances between the maxillary and mandibular dentition was generated within the VRMesh software (Virtual Grid, Seattle City, U.S.A). The interocclusal distance of -0.5 to 0.5mm was defined for visualisation of the occlusal contacts. The anterior region from the right canine to left canine and posterior regions extending from the Premolars to 2nd molars of the right and the left sides were defined. The distribution of occlusal contacts was divided to three regions, two regions, and the one region group. The overjet, overbite, and number of teeth in occlusal contacts were recorded. The statistical significance of mean surgical movements (T0-T1) and skeletal relapse (T1-T2) were measured with Paired t-test or Wilcoxon signed test for normal and non-normal data, respectively. Pearson’s or Spearman’s correlation analysis was applied for assessment of the relationship between skeletal relapse at 6-month following surgery and the quality of the occlusion immediately following surgery. The correlation between magnitude of surgical movements and the stability at 6 months was measured. Probabilities of 0.05 or less were accepted as significant.

Results:
 The result showed no statistically significant difference between the repeated measurements of skeletal changes (T0-T1) and relapse (T1-T2). The maxilla was positioned in forward direction by (6.79 ± 2.3 mm), (1.28 ± 1.09 mm) in vertical direction and (0.71 ± 0.79 mm) medio-lateral movement. The absolute mean relapse of maxilla was 0.72 ± 0.43 mm backward, 0.57 ± 0.43 mm vertical and 0.30 ± 0.33 mm in medio-lateral direction. Between T0-T1, there was a significant difference in overjet (mean = 6.94 ± 2.42, P: &lt; 0.001), overbite (mean = 2.56 ± 2.58, P: &lt; 0.001) and the category of the occlusal contact (mean = 0.53 ± 1.27, P: 0.041). The Spearman correlation coefficient showed a weak correlation between the magnitude of maxillary advancement and the relapse at T2. There was a weak correlation between the Roll, Yaw, and Pitch of the surgical movements and the detected relapse at T2. A weak negative correlation (r: -0.434, P: 0.030) between number of the teeth in contact and the relapse of the maxillary roll was detected.

Conclusion:
Le Fort I maxillary advancement is reasonably stable, the measured relapse ranged between 1 mm and 1 degree, which is clinically insignificant. There was a weak correlation (r:0.204) between relapse at 6 months and the magnitude of maxillary surgical advancement. There was no strong correlation between the quality of immediate post-operative occlusion and the relapse at 6 months following surgery.