Post-surgical relapse is an essential concern in the correction of skeletal class II malocclusion via BSSO surgery . The use of supplemental MMF to decrease the relapse rate was the main hypothesis of this study. According to this study’s results, the MMF group had a lower rate of relapse in the vertical and horizontal dimensions at points A and B. The vertical relapse at point B was 14.02% of the initial movement in the MMF group and 25.63% in the non-MMF group. The horizontal relapse at point B was 20.31% of the initial movement in the MMF group and 29.23% in the non-MMF group.
According to the study results, the treatment group (MMF group) had a lower rate of relapse compared to the control group (no-MMF) in the mandible and maxilla in 1 year following osteotomies. Chen et al. demonstrated that age, the magnitude of mandibular advancement, preoperative MP, counterclockwise mandibular rotation, and bimaxillary surgery were independent risk factors for long-term sagittal skeletal relapse, whereas preoperative MP, counterclockwise rotation, and the magnitude of mandibular advancement were independent risk factors for vertical relapse . Tabrizi et al. reported that the magnitude of mandibular advancement was a more reliable surgical predictor for horizontal relapse at point B. The changes in the MP angle during surgery were responsible for vertical, but not horizontal, relapse at point B . According to the study results, the MMF group had a lower rate of relapse in the sagittal and vertical dimensions at points A and B. The study variables such as age, gender, changes in the MP and OP angles, and the magnitude of maxillomandibular movements were compared between the two groups. The statistical similarity was observed between the two groups for the aforementioned variables, and their effects could be ruled out.
Early relapse seems to be more related to surgical techniques and errors in rigid internal fixation or may even occur when the condyles are not positioned correctly in their locations. However, late relapse and long-term stability would be the result of the functional imbalance of forces that would lead to condylar resorption [1, 5, 11].
The sagittal relapse may occur due to the soft tissue tension and fixation site and has a correlation with the amount of initial movement . Condylar positioning, suprahyoid muscles, and the pterygomasseteric sling are responsible for the vertical relapse at point B [12, 13].
Immediate relapse would present itself with TMD, malocclusion, hardware failure, immediate overjet increase, and deviation. Since all surgical procedures were performed by the same oral and maxillofacial surgeon with the same osteotomy technique and none of the mentioned signs were observed, the operator error and its possible role in the surgical relapse rate were minimized.
Since in this study, subjects underwent bimaxillary osteotomies, the change in maxillary position has an impact on the mandibular position as well as mandibular change at the B point. It was demonstrated that 1-mm change in the maxillary position leads to 0.71-mm vertical and 0.21-mm horizontal movements of the mandible when the amount of maxillary impaction was 8 mm or less . The results of the study demonstrated a similar initial movement at the A point in the vertical and horizontal directions in the treatment and control groups. The initial maxillary movement in the treatment and control groups was 5.46±1.75 mm and 5.26±1.02 mm, respectively. Therefore, the effect of the superior maxillary repositioning on mandibular autorotation and the position of the B point was similar in the treatment and control groups. Regarding the similar maxillary superior positioning in the two groups, the change of the B point in the vertical and horizontal directions after 1 year demonstrated a reliable relapse rate in the A and B points. In view of the fact that the superior maxillary repositioning is a stable procedure in bimaxillary osteotomy , the significant relapse was related to the mandibular movement.
Hartlev et al. studied the use of intermaxillary fixation (IMF) in mandibular advancement to decrease the relapse rate. They found no difference in the relapse rate between the skeletal IMF group and the control group without skeletal IMF . On the other hand, Schwartz et al. demonstrated that BSSO in combination with skeletal IMF could be used as an alternative to distraction osteogenesis in large mandibular advancements (>10 mm) with equal stability .
MMF reduces the soft tissue tension and increases the stability of fixation devices to overcome muscle tension (suprahyoid and pterygomandibular sling) . The advantage of skeletal MMF over dental IMF is the minimal tooth movement, which decreases the tooth relapse after releasing the fixation . Paunonen et al.  reported a skeletal relapse rate as high as 25%, which was significant following BSSO advancement surgery despite insignificant dental alterations. This finding highlights the importance of dental relapse and its compensation by the postoperative skeletal relapse .
Internal rigid fixation methods have been used by several authors to achieve maximum short-term and long-term skeletal stability following BSSO. Different types of rigid internal fixation have been used following BSSO, such as miniplates with mono-cortical screws, bicortical screws, and a combination of both referred to as the hybrid technique. Most of the published studies have demonstrated minimal differences among these types of internal fixation methods in terms of skeletal relapse; hence, this type of fixation seems to be promising in providing long-term stability [1, 2, 4]. It could be expected that similar MMF’s effects would be achieved by other fixation modalities of mandibular advancement sagittal split-like positional screws.
The disadvantages of MMF include delay in return to normal function and difficulties maintaining proper oral hygiene . Generally, patients are not satisfied with MMF. The use of MMF screws is associated with increased risk of tooth root injury, soft tissue burying of screw heads in the anterior mandibular vestibule, and interference of wire loops with canine facettes or the upper incisor edges . Meanwhile, the clinical difference among groups was 1 mm in the vertical dimension (about 12% of the adsorption rate) in point B and 0.5 in the horizontal dimension (about 12% in the amount of change), so it could be recommended; however, the clinical judgment of the practitioner should justify the use of this method. This study was conducted in subjects who needed bimaxillary surgery. Consequently, it was difficult to estimate the relapse rate in the maxilla and mandible independently. For evaluation of MMF’s effect on the relapse of pure mandibular advancement, research on retrognathic patients needing a mandibular advancement is mandatory.