SFA has become a popular method to treat various of dentofacial deformities because of its ability to early improve facial aesthetics and reduce treatment time. Postsurgical stability is one of the most concerned issuses for SFA which determines treatment success and avoids the occurrence of secondary surgery. To date, the study involving the skeletal three-dimensional postsurgical stability of patients undergone 2-jaw surgery and maxillary premolar extraction was limited. Especially, few reports were on the skeletal stability within three months after surgery. Our study can make up for the gap in the understanding of early postoperative skeletal relapse.
In this study, the postsurgical stability of maxilla and transverse position of mandible was clinical acceptable. Thus the prediction and simulation of dental alignment and decompensation should be based on the position of the maxilla after surgery. However, a considerable amount of mandibular sagittal and vertical relapse occurred in the first 3 month postsurgically, accounting for 76% and 69% of total relapse respectively(Table 2, T1T2/T1T4). After 3 month, mild skeletal changes occurred without clinical significance. Our results advanced previous studies’ time point when the major amount of mandibular postsurgical relapse occured within 6 month22.
Regard with the amount of mandibular sagittal relapse after the bimaxillary surgery, Ko EW et al6 reported a mean of 1.44mm in non-extraction patients after debonding, and the means ranging from 0.9mm to 2.3mm were reported in studies that might be mixed with premolar-extraction patients11,15,23,24. In this study, the mean amount of mandibular sagittal relapse within 12 month of observation was 3.02mm(showed in Table 2), indicating that the postsurgical stability of SFA might be worse in patients needed maxillary bilateral premolar extraction. Therefore, suitable patients selection, precise surgical design and standard operation, as well as postoperative orthodontic management are critical to the success of SFA combined with premolar extraction.
In this study, we showed that the more inferior positioned of B-point and PNS-point(indicating longer facial height) before surgery the better stability after SFA could be expect. Consistent with this result, Guglielmi et al suggested that the smaller gonial angle in patients was correlated with a greater risk of relapse after conventional bimaixllary surgery25. Previous studies has established that brachycephalic pattern with strong bite force and dolichocephalic with weak bite force26. Thus, the bite force might be one of the factors influencing the stability of the jaw after operation. In contrast, patients with Class III malocclusion with hypodivergent facial pattern showed high relapse after mandibular setback surgery with minimal presurgical orthodontics10. Therefore, more direct causal evidence is needed.
Ko EW et al reported that the initial overbite as an indicator to predict possible skeletal relapse of mandibular setback in SFA of non-extraction case6. In this study, there was no significant correlation between T0 and T1 OB and postoperative skeletal stability. Possible explanation is the patient characteristics difference. Our study enrolled patients with premolar extraction, the T1 OJ and OB are large enough to avoid the anterior teeth interference.
With regard to the surgical changes, condyle was relatively stable during the operation when the linear measurement was used to measure its position(showed in Table 4). However, the amount of anterior rotation of the condyle neck and condyle-coronoid connection during the surgery showed significant correlation with B point sagittal relapse. The results are consistent with those of previous studies26,27. The regression study by Lee J et al suggested that the vertical position of coronoid at immediated postsurgical staget was 47% reflected in the final mandibular setback extent in VD increased patients. Moreover our study also revealed that patients with longer coronoid process showed worse postsurgical stability. A range of studies have proved a association between the masticatory force and craniofacial morphology28.29. Sella-Tunis T et al implied that a large cross-sectional areas of masseter and temporalis muscle were associated with a more massive coronoid29. The masticatory muscle cross-sectional areas are approximately proportional to force generation. Thus, as depicted in Fig. 3, due to the existence of premature occlusal contact, the proximal segment retreated and descended at the same time during the operation, resulting in larger extent of sagittal surgical change at B point, though the initial difference between 2 groups were insignificant. And the distal segments descended as follow during rigid fixation. Subsequently, the stretched temporalis muscle would retract during the elimination of posterior premature occlusal contact, and strengthened muscle would aggravate the extent of relapse.
This study has also showed that the interoperative downward movement of anterior maxilla (A point) was unfavourable for mandibular postsurgical stability. It might be due to the secondary bone remodeling to the maxillary downward movement11 which further increase the mandibular pronation space. However, the mean intraoperative vertical change and vertical relapse from T1 to T4 of anterior maxilla were minimal(0.9mm and 0.48mm) in this study.
As for the postsurgical orthodontic treatment, Table 5 demonstrates our struggle in orthodontic treatment for clinically highly unstable group of patients. For instance, the most of the decompensation on incisor was completed within three months after surgery, and then due to the mandibular relapse, we increased the OJ through interproximal enamel reduction and intermaxillary Class III elastics, and resulting in proclined/retroclined reciprocating tipping of the incisors. Since the sagittal and vertical relapse of the mandible mainly occurred within three months after surgery, the orthodontic management during this period was vital to the stability of the whole treatment process and treatment success, ortherwise teeth position would be compensated. This study showed that the occlusal premature contact was mainly located in the maxillary second molar after operation, and the removal of it was unfavorable for the mandibular stability. Similarly, Han JJ et al showed that more than 50% of mandibular sagittal relapse came from the reduction of dental vertical dimension26. In order to stabilize the mandible position, some researchers have proposed to extend the usage the occlusal splint, apply chin pocket17, or use bone anchorage for intermaxillary Class III elastic, though foremost is not favorable for teeth alignment. Based on our clinical experience, the Class III elastic by bone anchorage was enough to maintain the postoperative stability of the mandible in clinical stable group of patients. For cases with posterior teeth occlusal interference, we recommend minimal orthodontics before surgery.
In conclusion, mandibular pronation occurred mainly in the first three months after surgery. The postsurgical stability of maxilla was clinically acceptable. The following are clinical treatment suggestions: 1) the prediction and simulation of dental alignment and decompensation could be based on the position of the maxilla after surgery. 2)attention should be paid on the masticatory muscle strength to the postsurgical sketetal stability. 3) cases with severe dental interference between arches, such as maxillary second molar premature contact might need to be corrected before surgery.