The purpose of this research was to prospectively determine the ratio of 2 soft-tissue landmarks, pogonion (sPg) and menton (sMe), to their hard-tissue counterparts (Pg and Me) in the sagittal and vertical directions for mandibular lengthening surgeries.
We drew a sample from the prospective Orthognathic and Orthofacial Surgery Research study, consisting of patients who underwent surgical mandibular lengthening (alone or in combination with maxillary osteotomy) without genioplasty. We digitized landmarks using Facewizz software and determined the relationships between the hard- and soft-tissue changes by correlation analysis.
Pearson correlation test showed a significant correlation between the type of surgery and the sPg:Pg and sMe:Me ratios. The sPg:Pg ratio was 87% for mandibular lengthening only and 102% for mandibular lengthening in combination with maxillary surgery. The sMe:Me ratio was 85% and 96% for upward and downward movements, respectively.
The average ratios presented in this study for the pogonion and menton can aid in preoperative planning by providing estimates for soft-tissue behavior. Further stratifications will be possible after the Orthognathic and Orthofacial Surgery Research database is enriched with more inclusions.
We prospectively measured sPg:Pg and sMe:Me ratios on lateral cephalograms.
The influence of sex, age, soft-tissue thickness, and the amount of lengthening was studied.
The sMe:Me ratio was 85% for upward and 96% for downward movement.
The downward movement reduces submental thickness, and the upward does not change it.
The overall sPg:Pg ratio was 97%.
The chin contributes to facial attractiveness. Chin prominence, height, and width; the depth and vertical position of the mentolabial fold; chin position and symmetry along with its transition with the lateral mandibular border; and the cervicomental angle and submental fullness are all features to be considered in orthofacial surgery (the “face-first approach” in orthognathic surgery). , An array of osteotomies are available to achieve the desired form and position of the chin. Knowledge of the integumental changes that accompany repositioning by osteotomy increases the effectiveness of preoperative planning and helps surgeons to communicate with patients about the facial profile changes that can be expected. ,
Currently, estimates of soft-tissue changes after chin repositioning are based on 2-dimensional radiographic images, which is the traditional and (although still limited) most evidence-based approach to plan chin profiles. Three-dimensional planning software also starts with a profile view, but its usefulness expands to asymmetrical cases in which rotations and translations in the coronal or axial plane are necessary, notwithstanding the absence of evidence to estimate the 3-dimensional soft-tissue response to surgery. A disadvantage of using reference soft-to-hard tissue ratios in the profile view is that they are averages based on limited evidence; therefore, there are large deviations from the reference values among individual cases. We conducted a systematic review in 2015 and concluded that with osseous genioplasty, the most accurate average ratio of soft-tissue pogonion (sPg) to boney pogonion (Pg) movement was 0.9:1 for protrusive movements and –0.52:1 for retrusive movements, and the average ratio of soft-tissue menton (sMe) to boney menton (Me) movements was 0.95:1 for chin extrusion (down-grafting) and 0.43:1 for chin impaction. Only 3 of the included studies (2 on advancement, 1 on impaction) were prospective.
In 2010, a systematic review of soft-to-hard tissue profile changes after mandibular base advancement, which also included only 3 prospective studies, found that the average sPg:Pg ratio for horizontal movement was 1:1. Another systematic review conducted in 2010 by the same group found that the average sPg:Pg ratio for retrusive mandibular base movement (setback) was also 1:1. There are currently no systematic or narrative reviews of soft-to-hard tissue ratios for displacement in the vertical direction.
Our objective was to determine sMe and sPg changes after mandibular base advancement. We investigated the direction and amount of repositioning, age, sex, soft-tissue thickness, and monomaxillary or bimaxillary surgery as factors that might influence the changes in sMe and sPg. Our null hypothesis was that the soft-tissue changes of the chin are not precisely equal to the amount of advancement of the hard tissues in the vertical and horizontal dimensions. To test this hypothesis, we measured the soft-tissue changes of the chin (ratios with linear and multiple regression analyses) after mandibular advancement by orthognathic surgery and performed analyses to identify factors that influence the changes.
Material and methods
This study is a part of the international Orthognathic and Orthofacial Surgery Research (OSRES) prospective registry, an ongoing project to investigate the response of facial soft tissues to bone movement in orthognathic surgery, and was approved by the Ethics in Research Committee of Universidad de La Frontera (protocol number 066/13). The internal ethics committees of the 4 participating institutions independently reviewed and approved patient participation. The patients included in the registry had a full understanding of their participation and provided written informed consent.
Patients of any age, sex, and ethnicity were included. Patients were excluded if they had any malformation syndromes, previous facial surgeries, history of facial trauma, or facial asymmetry with chin deviation >5 mm. We selected from the OSRES database 73 patients (26 males; 47 females) who had bimaxillary surgery or mandibular advancement without genioplasty between July 2015 and January 2018 at the Universidad de la Frontera in Temuco (Chile), the European Face Centre at the Universitair Ziekenhuis Brussel (Belgium), the Hospital Sant’Anna e San Sebastiano in Caserta, Italy, or the Amphia Ziekenhuis in Breda, The Netherlands. The sample included all patients who had undergone the abovementioned surgeries and excluded all whose records showed breaches against the protocol. Power statistics were not performed because the differences between the subpopulations were not considered of paramount clinical relevance. The patients were treated by a variety of surgeons with different surgical techniques and fixation methods.
Preoperative lateral cephalograms were taken within 1 month before the date of surgery (T1). Postoperative cephalograms were taken at least 6 months after surgery (T2) and always after the removal of the orthodontic brackets. The same craniostat and setup were used for the preoperative and postoperative cephalograms of each patient in every participating institution. The patients were wax-bite guided into centric relation and instructed to relax their lips for the cephalograms.
Cephalometric analysis was performed using Facewizz (version 1.2.4; Orthoface R&D BVBA, Sint-Martens-Latem, Belgium) and a set of landmarks ( Fig 1 ). The positions of the soft and hard tissue landmarks were marked in a Cartesian system. The x-axis was the Frankfort horizontal, and the y-axis was a line perpendicular to the Frankfort horizontal through the porion. The x-axis and y-axis were automatically drawn by the Facewizz tool after the porion and orbitale were selected as landmarks. The software assigned x- and y-coordinates to every landmark. By superimposing the anterior cranial base of the preoperative and postoperative cephalograms, we could observe the changes in a single reference frame. Before embarking on the study series, assessor AB digitized the landmarks until all intraclass correlation coefficients were > 0.90 and none of the 95% confidence limits had a lower boundary <0.85.
Statistical analysis was performed using SPSS (version 25.0; SPSS, Chicago, Ill). We used the Kolmogorov-Smirnov test with P <0.05 as the threshold of significance to check whether the continuous variables were normally distributed.
We calculated the measurement error by digitizing 10 duplicated random data sets measured by the same investigator (AB) at least 2 weeks after the initial measurements were taken. We used the Dahlberg formula to calculate the measurement error: