Changes And Reattachment Of The Masseter Muscle Following Mandibular Angle Ostectomy

Background At present, treatment of madibular angle hypertrophy becomes one of the most important and hottest investigations. Frequently, mandibular angle hypertrophy always consists of both bony and muscular hypertrophy. Thus, whether the mandibular angle ostectomy should be combined with partial resection of masseter muscle simultaneously is still controversial. There are some problems remaining declaration, such as the morphological and functional changes of masseter muscle and the process of its reattachment after mandibular angle ostectomy. And an ideal imageological technique for three-dimentional reconstruction and dynamic observation of masseter muscle is still under research.Objective To investigate the changes and reattachment of the masseter muscle following osteotomy of the prominent mandibular angle and supply guidance for the resection of mandibular angle.Methods 1. Classifying ten goats into A and B group. In group A we performed unilateral curved osteotomy of the mandibular angle, in group B we performed unilateral dissection of the masseter muscle, the cross section area (CSA) and the sarcomere length of masseter muscle was measured at 1-week, 1-month, 2-month, 3-month, and 6-month post-operation. 2. Ultrasonography was used to assess the thickness changes of masseter muscle which took place after curved osteotomy in 10 patients with six months' following-up. The measurements were performed under both intercuspal position and maximal clenching positions through A, B and C three crossing sections of masseter muscle. Plane A contains the line from mouth angle to ipsilateral ear lobe. Plane B and C are two parallel planes respectively above and below plane A with the distance of 1cm between them. The contraction amplitude of masseter muscle under maximal clenching position was also measured to explore the functional changes of masseter muscle in different planes. 3. Real-time 3D ultrasonography was applied to reconstruct the the morphological changes of masseter under intercuspal position, maximal opening, maximal clenching and maximal protruding condition, assess the voluminal changes of masseter muscle and obtain the dynamic morphological changes of masseter during opening and closing mouth which took place after curved osteotomy in 10 patients with six months' following-up. 4. Classifying sixteen goats into A,B,C and D group. In group A we performed unilateral curved osteotomy of the mandibular angle, in group B we performed unilateral curved ostectomy with partially masseter resection, in group C we performed unilateral dissection of the masseter muscle, while in group D we performed unilateral angle splitting ostectomy. We examined the histologic changes in the interface and measured the thickness of periosteum at 1-month, 2-month, 3-month, and 6-month post-operation. 5. Classifying sixteen goats into A,B,C and D group. In group A we performed unilateral curved osteotomy of the mandibular angle, in group B we performed unilateral curved ostectomy with partially masseter resection, in group C we performed unilateral dissection of the masseter muscle, while in group D we performed unilateral angle splitting ostectomy. The constitution and ratio of collagen fiber in the interface were observed at 1-month, 2-month, 3-month, and 6-month post-operation.Results 1. The measurements of cross section area (CSA) and sarcomere length of masseter muscle: (1) Cross section area (CSA) of masseter muscle fiber in curved ostectomy group decreased at 1-month, 2-month, 3-month and 6-month post-operation in different extent. Comparing with the control group, the difference was statically significant (P＜0.01). CSA of masseter muscle fiber in dissection group decreased 1-month postoperatively, which had significant static difference with control group (P＜0.01). But, they had no significant difference with control group at 2-month, 3-month, and 6-month post-operation (P＞0.01). (2) Sarcomere length of masseter muscle in curved ostectomy group decreased in 1-week, 1-month and 2-month post-operation, which had significant static difference with control group (P＜0.01). At 3-month post-operation, sarcomere length recovered to normal. In dissection group, sarcomere length decreased in 1-week and 1-month post-operation, which had significant static difference with control group (P＜0.01). At 2-month post-operation, it recovered to normal. 2. The measurement of muscular thickness: (1)Whatever in relaxing or maximal clenching position, the preoperative and postoperative thickness of masseter muscle in plane A and B are of no static significance (p＞0.05), but there is significant static difference (p＜0.05) in plane C.(2) As the contraction amplitude of master muscle in concern, there were no static significance (p＞0.05) in all three crossing sections postoperatively. 3. The three-dimentional reconstruction and voluminal measurement of masseter muscle: (1) The reconstructured 3D images showed though the appearances of masseter muscle were of no obvious difference in variant postures 6-month postoperatively, but its longitudinal diameter decreased and its angle region turned to be arc-shape with significant thinning. (2) The mean volume of masseter muscle was 18.222±3.028cm~3 6-month postoperatively. Comparing with that of pre-operation 25.480±7.113cm~3, the static difference was significant (P＜0.01). (3) The dynamic images demonstrated that morphological changes of masseter muscle during the motions of opening-closing mouth were almost of no difference between pre- and post-operative status. Comparing with pre-operation, the transverse and longitudinal changes of the thickest section of masseter muscle 6-month postoperatively were of no static difference (P＞0.01). 4. The histological study on reattachment of masseter muscle: The histological examination showed that the reattachment of masseter muscle to the mandibular in each group were distinguishable before 3 months after operation, the recovery degree of the interface was dissection group, angle splitting ostectomy group, curved ostectomy group, curved ostectomy with partially masseter resection group by turns, after 3 months, their recovery process were indistinguishable, the interface between mandibular angle and masseter in each group was similar to the normal after 6 months, the periosteum thickness of masseter disscection group was similar to the normal after 2 months and other groups returned normal after 6 mouths. 5. The changes of collagen fibres in the process of muscular reattachment: At 1-month post-operation, the constitution of collagen fiber (typeⅠandⅢ) in groups of angle resection and angle-muscle resection were significantly different from that of control group (P＜0.05). On the contrary, those of muscle dissection and splitting group had no static difference with control group (P＞0.05). At 2-month, 3-month and 6-month post-operation, those of all experimental groups had no static difference with control group. And with the time went on postoperatively, the percentage of collagen fiber typeⅠincreased and the typeⅢdecreased gradually.Conclusion 1. Certain extent of atrophy dose happen to masseter muscle after mandibular angle ostectomy, simultaneously, it occurs mainly in angle region. Meanwhile, these changes do not significantly impair the function of masseter muscle. According to this, we suggest a simple mandibular angle ostectomy without partial resection of masseter muscle in case of mild to morderate mandibular angle hypertrophy. Doing so, we can not only achieve the cosmetic effect but also reduce the implications. 2. With its safety and convenience for patients' repeating examinations, real-time three-dimentional ultrosonography will be a brand new technique to reconstruct and observe the movements of masseter muscle. 3. The initial reattachment of masseter muscle establishes 2 months after osteotomy, the reattaching interface of mandible and masseter muscle recovers to normal statement 6 months after osteotomy. With normal constitution and arrangement of collagen fibers, a stable attachment of masseter muscle and mandible establishes. It means that patient will gradually regain normal mastication and other oral function since 2-month postoperation.