Functional genioplasty in growing patients
To evaluate the role of age as a moderator of bone regeneration patterns and symphysis remodeling after genioplasty. Fifty-four patients who underwent genioplasty at the end of their orthodontic treatment were divided into three age groups: younger than 15 years at the time of surgery (group 1), 15 to 19 years (group 2), and 20 years or older (group 3). Twenty-three patients who did not accept genioplasty and had a follow-up radiograph 2 years after the end of their orthodontic treatment were used as a control group. Patients were evaluated at three time points: immediate preoperative (T1), immediate postoperative (T2,) and 2 years postsurgery (T3). The mean genial advancement at surgery was similar for the three age groups, but the extent of remodeling around the repositioned chin was greater in group 1, less in group 2, and still less in group 3. Symphysis thickness increased significantly during the 2-year postsurgery interval for the three groups, and this increase was significantly greater in group 1 than in group 3. Remodeling above and behind the repositioned chin also was greater in the younger patients. This was related to greater vertical growth of the dentoalveolar process in the younger patients. There was no evidence of a deleterious effect on mandibular growth. The outcomes of forward-upward genioplasty include increased symphysis thickness, bone apposition above B point, and remodeling at the inferior border. When indications for this type of genioplasty are recognized, early surgical correction (before age 15) produces a better outcome in terms of bone remodeling. (Angle Orthod. 2015;85:360–373.)ABSTRACT
Objective:
Method:
Results:
Conclusion:
INTRODUCTION
Inferior border osteotomy of the mandible for chin augmentation was first reported by Trauner and Obwegeser1 in 1957 and has become widely used as an isolated procedure or in combination with other maxillo-mandibular osteotomies. Although the chin can be repositioned in any direction with this procedure, simultaneous advancement and upward movement to correct both a horizontal deficiency and vertical excess is the most common. Precious and Delaire2 defined forward-upward repositioning of the chin as a “functional genioplasty” because it provides a beneficial change in lip function and helps to obtain lip competency at repose.2 It also tends to reduce lip pressure against the lower incisors.3 When orthodontic treatment has created mandibular incisor protrusion, improving the relationship between the chin and mandibular incisors (the Holdaway ratio in cephalometric analysis) is thought to improve the chance of incisor stability—and one way to do that is to advance the chin rather than retracting the incisors. This can be particularly helpful when the improvement in occlusion in Class II patients was achieved largely by tooth movement because of minimal or unfavorable mandibular growth.
These functional and stability benefits stand, of course, in addition to the esthetic improvement enjoyed with genioplasty. Facial appearance can be a serious psychosocial handicap, even early in life,4 and functional genioplasty offers a means to improve esthetics, function, and stability in conjunction with orthodontic treatment.
Although a number of publications on inferior body osteotomy for genioplasty have appeared, only a few studies have data for this procedure in adolescents, and none include follow-up of a control group who were evaluated as potentially benefiting from functional genioplasty but rejected it. The optimum age for genioplasty has been somewhat controversial. The positive psychosocial reaction to improved facial appearance would suggest earlier treatment for severely affected patients4,5; concerns about possible negative effects on growth and decreased stability would be the major reason for waiting until little or no growth remained.6 Martinez et al.7 reported in 1999 that there is better regeneration of symphysis thickness in patients younger than age 15 than in older nongrowing individuals. More recently, Frapier et al.8,9 suggested that early genioplasty could improve the direction of mandibular growth and might increase nasal breathing because of improved lip function, but these assertions were based on samples that were too small and diverse for broad generalization.
An isolated lower border osteotomy requires general anesthesia, but not overnight hospitalization, and is commonly done as a day-op procedure either in a hospital or a free-standing surgical center. In the United States, genioplasty is usually part of a larger orthognathic surgery plan because medical insurance will almost never cover the cost of an isolated procedure. This is not the case in Canada, where medical coverage is provided. The aims of the study were to clarify the optimal time for functional genioplasty from evaluation of (1) the pattern of bone remodeling at the chin after functional genioplasty and (2) the pattern of postsurgical stability in growing and nongrowing patients.
MATERIALS AND METHODS
Patient Sample
All participants in this research project were treated in the private orthodontic practice of the senior author. The surgery patients had lateral cephalometric radiographs at three time points: T1, immediately prior to genioplasty, which was done at the end of their orthodontic treatment; T2, immediately after the genioplasty; and T3, at 2-year follow-up. The initial sample was all of the 59 patients who had this surgery between June 1992 and December 2012; five were excluded because of missing radiographs, for a final sample size of 54. This group was divided into three age groups: younger than 15 years at time of surgery (group 1, n = 28), 15 to 19 years (group 2, n = 16), and 19 years or older (group 3, n = 10; Table 1). Skeletal age (maturation of vertebrae) was not used because chronologic age is more likely to predict the peak of adolescent growth,10 and age 15 was the cutoff point in the only previous report of age-related changes in symphysis remodeling after genioplasty.7
It is difficult to assess exactly how many patients were offered a genioplasty and declined it, but we were able to find 23 patients who did not accept genioplasty and had a follow-up radiograph 2 years after the end of their orthodontic treatment. This control group (group 4) had only two observation time points: at the end of orthodontic treatment and 2-year follow-up. They were similar to the younger surgical patients (group 1) in age, percentage female, a-p and vertical chin position, and symphysis thickness at baseline and therefore are comparable to that group. Five of the control group eventually joined the surgery group because they decided to accept genioplasty after their 2-year postorthodontics records had been obtained.
Surgical Procedure
A mandibular lower border osteotomy was performed with the patient under general anesthesia at Hôpital l'Enfant-Jésus, Québec, Canada, by the same oral-maxillofacial surgeon, following the technique described by Precious and coworkers.11 Anterior and superior repositioning of the chin was achieved by sliding the chin to its new position. For 49 of the 54 patients, wire osteosynthesis was achieved with at least three transosseous double strands of 28-gauge stainless-steel wire. The other five patients had bone screws. Neither the wire nor screw fixation was removed.
Cephalometric Data
Lateral cephalometric radiographs for patients participating in this study prior to mid-2008 were taken on an Orthophos Ceph machine (Siemens, Beinsheim, Germany); afterward, an OP100 (Instrumentarium, Tuusula, Finland) unit was used. All radiographs were traced by the senior author with Quick Ceph Studio (Quick Ceph Systems, San Diego, Calif). Magnification was calibrated for both the older scanned films and newer digital radiographs. An x–y cranial base coordinate system was constructed through sella with the x-axis drawn 7° to the sella-nasion line and the y-axis passing through sella, perpendicular to the x-axis (Figure 1).
Citation: The Angle Orthodontist 85, 3; 10.2319/030414-152.1
For all subjects, the recommendation for genioplasty was based on clinical evaluation of the prominence and vertical position of the soft tissue chin relative to the lips and midface. Cephalometric data for pretreatment a-p chin deficiency relative to the lower incisors, the vertical distance from the incisors to the bottom of the chin, and the mandibular plane angle are shown in Table 1. To evaluate postsurgical changes in the chin, the focus was on four measurements (Figure 1): symphysis thickness, vertical height of the chin relative to the lower incisors, and remodeling above and behind the chin.
Statistical Analysis
The distribution of the sample was evaluated and judged to be close enough to normal to use mean, standard deviation, and range as descriptive statistics. The study design involved comparison among the three age groups who underwent genioplasty (groups 1, 2, and 3) and comparison of the youngest group (group 1) to an age-matched control group (group 4) with the same characteristics. For both comparisons, changes scores between time points were analyzed with multivariate analysis of covariance, in which gender effect was evaluated as a covariate. Although gender did not contribute to the differences, we kept this effect in the model to adjust the conclusions for gender. One-sample t-tests were used to evaluate the chance that data for each time point were different from zero; pairwise comparisons with Bonferroni adjustments for multiple comparisons were used to evaluate the change between groups. Unlike the Tukey adjustment, the Bonferroni method does not need correction because of the unbalanced sample size between groups. The level of significance was set at P < .05. All of these analyses were conducted with IBM SPSS Statistics (version 21).
To assess the method error, 15 cephalograms were redigitized. An analysis of variance showed that there was no significant difference between the first tracing and the redigitized tracing. The coefficient of fidelity for all variables was .9997. The coefficient of fidelity for the symphysis thickness change and remodeling of the inferior border (PGP) was .9231. The analysis of the method error was conducted with SAS 9.4 (SAS Institute Inc, Chicago, Ill).
RESULTS
Change at Surgery (T1–T2)
Changes at surgery for a typical functional genioplasty patient are shown in Figure 2, and the data are summarized in Table 2. There were no significant differences in genial advancement or vertical reduction between the three age groups (Table 2). The changes were highly statistically significant (<.0001 for both).
Citation: The Angle Orthodontist 85, 3; 10.2319/030414-152.1
Change from Surgery to Follow-up (T2–T3)
Changes in symphysis thickness
Symphysis thickness increased significantly for all three surgical groups and showed a small but significant decrease for the controls (Table 3; Figure 3). Pairwise comparisons between groups 1 and 3, controlling for different sample size, showed a significant difference (P = .004) between these two groups for the symphysis thickness change.
Citation: The Angle Orthodontist 85, 3; 10.2319/030414-152.1
When there is considerable variability in treatment outcomes, as there often is, the percentage of patients with clinically significant change can provide a better understanding of the data.12,13 Figure 3 shows that 39% of the youngest patients (group 1) had a 2- to 4-mm increase in symphysis thickness during the 2 years postsurgery, and 28% had a >4-mm increase. Therefore, two-thirds of the youngest patients had a more than 2-mm increase in symphysis thickness. The percentage with change >2 mm was smaller in group 2, but two of those patients (7%) had a >4-mm increase. No patients in the oldest group (group 3) had a >4-mm change, and only two (20%) had a >2-mm change. In contrast to the genioplasty groups, no patient in the control group had an increase in symphysis thickness from the end of treatment to 2-year recall, and seven (30%) had a 1-mm or greater decrease.
Changes in coordinate positions and dimensional relationships
Data for changes in coordinate positions for points B, Pg and Me are displayed in Tables 4 and 5, and the changes for Pg and Me are shown graphically in Figures 4 and 5. It is important to keep in mind that these changes are due to a combination of mandibular growth and surface remodeling at and near the chin.
Citation: The Angle Orthodontist 85, 3; 10.2319/030414-152.1
Citation: The Angle Orthodontist 85, 3; 10.2319/030414-152.1
Horizontal growth change at Pg after genioplasty (T2–T3) of group 1 was less than the control group, but the difference was not statistically significant. Group 1 showed a significant forward growth change, while for groups 2 and 3, the horizontal change at Pg was not significant (Figure 4). Vertical growth change at Me after surgery was similar to the control group, and the vertical change was significant for group 1, group 2, and controls (Figure 5). Figure 6 shows the pattern of vertical dentoalveolar change postgenioplasty. All changes were statistically significant from zero for each group, but for group 1, the T2–T3 change was significantly different from groups 2, 3, and 4. One should keep in mind that this vertical change at Me was balanced by posterior facial growth: the mandibular plane angle change for group 1 was not significant. Pairwise comparisons are shown in Table 6. This confirms that remodeling in group 1 is different from remodeling in groups 3 and 4.
Citation: The Angle Orthodontist 85, 3; 10.2319/030414-152.1
Three variables were significantly correlated to the postsurgical change in symphysis thickness: the amount of genial advancement, the amount of vertical dentoalveolar growth, and the age at surgery. The R value of these three variables taken together was .47 (r2 = .22), and their influence was significant at the P < .05 level (P = .03). When the predictor variables were ranked by the standardized coefficient beta, the result clearly showed that the younger the age at surgery and the greater the dentoalveolar growth as incisors erupted, the more the symphysis would increase in thickness due to bone apposition. The amount of genial advancement was not a determinant.
Remodeling changes
Remodeling of the symphysis after genioplasty involves bone apposition above the repositioned chin, with changes leading up to and even beyond point B, and removal of bone adjacent to the notch in the lower border of the mandible that is present after the chin has been moved (Figures 7 and 8). Figure 8 illustrates the typical pattern of remodeling in the younger patients. Statistical analysis showed that the decrease in the depth of the notch at the inferior border was significant for groups 1 and 2, but no significant change was noted for the adult group (Figure 7; Table 5). There was no significant change of the inferior border in the control group.
Citation: The Angle Orthodontist 85, 3; 10.2319/030414-152.1
Citation: The Angle Orthodontist 85, 3; 10.2319/030414-152.1
Ranking the predictor variables confirmed that the greater the dentoalveolar growth postsurgery, the more complete the remodeling in both areas, but neither the amount of genial advancement nor the age at surgery were significant predictors. It is clear, therefore, that age at genioplasty, which affects the amount of incisor eruption afterward, does make a difference in the extent of both bone apposition and remodeling, with more apposition and remodeling in patients younger than 15 years, less in late adolescents, and still less in adults.
Stability of the Surgical Repositioning
It is important to keep in mind that postsurgical changes in the position of the chin were due to a combination of mandibular growth and surface remodeling at and near the chin. For the younger patients, this is best evaluated by comparing the change in group 1 with the control group. The mean A-P change at Pg after genioplasty (T2–T3) of group 1 was less than the control group (ie, the genioplasty patients were slightly more stable), but the difference was small and not statistically significant (see Figure 4). The vertical change at Me after surgery also was similar to the control group (see Figure 5).
The data show, therefore, that forward and downward growth at the chin in this sample was not significantly affected by genioplasty and that the changes in chin position produced by the genioplasty were maintained in growing patients.
DISCUSSION
The data from this study make it clear that both the amount of new bone formation after genioplasty and the extent of remodeling around the repositioned chin are greater in patients who are still in mid-adolescence than in late adolescents and adults. Our results both confirm and extend the earlier report by Martinez that showed better healing in patients younger than age 157 and support other findings6,14–18,21 that after genioplasty, bone remodeling occurs at the inferior border of the proximal segment between the distal point of the osteotomy cut and the advanced distal segment. Our groups 1 and 2 showed a statistically significant mean reduction of this notch (1.2 ± 1.3 mm and 0.6 ± 0.9 mm, respectively), while the adult group had a modest nonsignificant reduction of 0.3 ± 1.0 mm.
In this study, the control group had a slight but significant resorption at B point (0.4 ± 0.6 mm), which is consistent with the usual pattern of growth at the chin in adolescence. Following genioplasty, as in Park et al.,18 Shaughnessy et al.,19 and Precious et al.,11,17 we found that bone apposition occurred at B point, with a similar change in all three age groups (0.7 to 1.0 mm). Bony angles above the repositioned chin became rounded, and rough edges became smooth. Shaughnessy et al.19 suggested that the autogenous bone grafts from the iliac crest that they placed in this area were responsible for the improved contours. Since none of the patients of the present study received a graft and all had significant apposition at B point, we question the indication for grafting bone into that area, particularly with bone from a donor site such as the iliac crest that requires invasive surgery.
Would it have made a difference if we used skeletal age instead of chronologic age in separating the three groups? It would have been possible to do that without additional radiation by using maturation of the cervical vertebrae. The conclusion of a recent review of methods to establish peak growth at adolescence, however, concluded that chronologic age is better.10 It is possible that the group younger than 15 years had some relatively mature girls, while the age 15–19 group had some relatively immature males, but that would have minimized rather than augmented the differences we observed.
The increased remodeling of the facial alveolar bone above the osteotomy site is important in the context of bone support for the lower incisors, because lower incisors tend to be proclined in individuals with a deficient chin even without treatment. This often is increased during their orthodontic treatment as the lower arch is expanded to align crowded incisors and/or Class II elastics are used to correct the occlusion. The result can be bone dehiscence and stripping of gingival tissue. Our data show that as the bone remodels after genioplasty, there is formation of new alveolar bone facial to the teeth at a higher level in the younger patients, and this can be attributed to postgenioplasty eruption of the teeth as face height increases more in the younger group.
Our patients also had formation of new bone on the lingual side behind the prominence of the chin, with a greater increase in symphysis thickness in the younger patients that persisted during the first 2 years posttreatment. The symphysis is a highly stressed area during normal function. Is greater symphysis strength a long-term outcome of genioplasty via lower border osteotomy? We have no data to support that possibility, but there is nothing to indicate that genioplasty before the completion of mandibular growth weakens the chin.
Does repositioning the chin have a deleterious effect on mandibular growth? That is a valid concern and has been a major reason for delaying it until growth is essentially completed. Our control group of mandibular deficient young patients who did not accept genioplasty allows a comparison of mandibular growth in treated and untreated individuals with similar mandibular morphology. Growth at the chin is largely due to growth of the mandible, but in the normal growth pattern, the chin becomes more prominent, not by apposition in the pogonion area, but by resorption above pogonion that extends upward toward point B.14 In a growing individual with an indication for forward-upward genioplasty, data from our control group show that lip incompetency persists, facial convexity is maintained, bone resorption occurs at point B, and symphysis thickness has a tendency to decrease. Change or the lack of it in a typical control patient is shown in Figure 9.
Citation: The Angle Orthodontist 85, 3; 10.2319/030414-152.1
Although our data show no evidence to support a negative effect on mandibular growth from a lower border osteotomy, whether it is done in early adolescence or later, it will be important to follow the younger patients until the end of the normal growth period to be sure that there is no residual effect, and this is planned. The mandibular plane angle decreases slightly during normal adolescent growth, and this is what we observed in both the younger genioplasty patients and the controls. It can be difficult to avoid unerupted permanent teeth during a lower border osteotomy in a child, and this is a contraindication for early genioplasty. Eruption of mandibular canines, usually around age 12–13 years, removes that limitation for most individuals.
Most previous studies of stability after genioplasty have reported that it is the most stable of the orthognathic surgery procedures and that significant relapse is almost never observed.15, 19, 20 Tulasne,16 using a different surgical procedure than the one in this study, reported greater relapse (about a 40% change) for young patients. Martinez et al.7 also noted greater relapse in their younger group (a 16% change), but it was neither clinically nor statistically significant. Our findings do not support a greater relapse at Pg for younger growing patients. Since almost all of our patients (91%) had wire fixation, better postsurgical stability with more costly bone screws may not be a consideration for this type of genioplasty.17
CONCLUSIONS
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Benefits of genioplasty via a lower border osteotomy that moves the chin forward and upward (a functional genioplasty) include increased symphysis thickness, bone apposition at B point, and remodeling at the inferior border. Better bone apposition and remodeling is observed in younger patient compared with adults.
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When indications for such a genioplasty are recognized, early surgical correction (before age 15) produces a better outcome in terms of bone remodeling. This is related primarily to greater vertical growth of the dentoalveolar process in the younger patients.
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There is no difference in postsurgical stability in younger and older genioplasty patients.
Cephalometric landmarks and dimensional measurements. Symphysis thickness was evaluated by measurement of the distance between the anterior and posterior borders 4 mm below the apex of the lower incisors (ACP-PCP). Vertical chin height was evaluated by the perpendicular distance from the mandibular plane to the lower incisor tip (ADH). Remodeling of the area above the repositioned chin was evaluated by change at B point and symphysis thickness increase; remodeling of the area on the inferior border was evaluated by the change of the depth of the notch at the posterior limit of the osteotomy cut, by measuring the perpendicular distance from PGP to the mandibular plane (MP).
Facial changes before and after functional genioplasty for a typical patient. Note the improvement in facial proportions, improved lip closure at repose, and improved display of the incisors on smile. Moving the chin up also moves the lower lip upward and decreases the display of lower incisors.
The percentages of patients with >2- and >4-mm changes in symphysis thickness after genioplasty. Note the differences in the genioplasty age groups and the contrast to the control patients.
Horizontal change at Pg. The younger genioplasty patients and the controls showed significant forward growth at Pg; the change at Pg for groups 2 and 3 was nonsignificant. The growth change of group 1 was not statistically different from the controls (ie, there was no evidence of decreased forward growth in the young genioplasty patients).
Vertical change at Me. Vertical growth change at Me after surgery (T2–T3) was significant for groups 1 and 2 and was similar to the control group, showing that vertical growth at Me was not affected by genioplasty.
Vertical alveolar dental change. All changes were statistically significant for each group. Note that the mean 6-mm difference between the young genioplasty patients and controls created by surgery was maintained at 2-year recall.
Remodeling at the inferior border. The notch at the inferior border of the proximal segment between the distal point of the osteotomy cut and the advanced distal segment was significantly reduced for groups 1 and 2 (1.2 ± 1.3 mm and 0.6 ± 0.9 mm, respectively), while the adult group had a modest nonsignificant reduction of 0.3 ± 1.0 mm. The net outcome at T3 showed a significant decrease of the depth of this notch when comparing group 1 to adults (P = .018).
The typical pattern of bone remodeling in young patients as seen in superimposed cephalometric tracings. Note that as growth occurred, remodeling added bone above the repositioned chin segment and decreased the depth of the notch on the inferior border. These changes are greater in the younger patients.
Facial changes in a typical untreated control patient, who was age 15 years 10 months at the end of orthodontic treatment and age 17 years 9 months on follow-up. Note that the mentalis and lip strain and A-P chin deficiency were not improved at follow-up.
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