This study evaluated whether surgical mandibular advancement procedures induced a change in the direction and the moment arms of the masseter (MAS) and medial pterygoid (MPM) muscles. Sixteen adults participated in this study. The sample was divided in two groups: Group I ( n = 8) with a mandibular plane angle (mpa) <39° and Group II ( n = 8) with an mpa >39°. Group I patients were treated with a bilateral sagittal split osteotomy (BSSO). Those in Group II were treated with a BSSO combined with a Le Fort I osteotomy. Pre- and postoperative direction and moment arms of MAS and MPM were compared in these groups. Postsurgically, MAS and MPM in Group II showed a significantly more vertical direction in the sagittal plane. Changes of direction in the frontal plane and changes of moment arms were insignificant in both groups. This study demonstrated that bimaxillary surgery in patients with an mpa >39° leads to a significant change of direction of MAS and MPM in the sagittal plane.
Research in orthognathic surgery has largely been focused on skeletal changes and their stability and to a lesser degree on soft tissue changes. Little attention has been paid to the effect of changes that occur in the mastication muscles, as a result of these skeletal changes. These changes may be significant as shown in two previous studies. The biomechanical performance of these muscles depends on the overall muscle size, the intrinsic muscle strength, the direction and the moment arm of the muscles and the innervations responsible for motor programming and coactivation strategies.
One previous study showed that there was a significant reduction in the cross-sectional area and volume of the masseter (MAS) and medial pterygoid (MPM) muscles after advancement bilateral sagittal split osteotomies (BSSOs), both in patients with a short and long face. This decrease in cross-sectional area and volume was still present 18 (10–48) months after surgery and is likely to be permanent. One would expect a weakening of masticatory performance unless a change in direction of these muscles compensates for this loss.
In theory, relocation of the proximal segments of the mandible after a BSSO changes the position of the insertions of both the MAS and MPM, as mentioned by Finn et al. and Throckmorton et al. This is particularly true in cases of mandibular advancement used to treat mandibular hypoplasia.
It may be that the direction and moment arms of MAS and MPM change to such an extent that the muscles can be more effective when chewing. It is the aim of this study to analyse and quantify the postoperative changes of the direction and moment arms of MAS and MPM.
Material and methods
Sixteen patients (8 males and 8 females) with mandibular hypoplasia were included in this study. Their age ranged from 16 to 45 years (average 27 years). All patients were orthodontically treated pre- and postoperatively. Eight patients had a mandibular plane angle (mpa) (sella-nasion (S-N) to gonion-menton (Go-Me)) of <39° (Group I), while in eight this angle was >39° (Group II). All patients underwent a BSSO to advance the mandible; in eight of them a Le Fort I type osteotomy was performed with posterior intrusion to avoid counterclockwise rotation of the mandible. In five of these patients an additional genioplasty was carried out. Fixation was achieved by three bicortical position screws per side. Patients were allowed to open and close their mouth immediately after surgery. Each surgery was performed by the same team of surgeons at one hospital.
MRI examination was performed prior to orthodontic treatment and after completion of treatment (i.e. after removal of the orthodontic appliances). The mean time after surgery for the post treatment examination was 28 months (range 10–63 months). MRI examinations were performed with a 1.0 T MR system (Siemens, Erlangen, Germany). The patients were scanned in a supine position and with the Frankfurt Horizontal plane oriented perpendicular to the scan table. The patients were instructed to close the teeth without clenching. Axial images were obtained with a multislice T1 spin-echo sequence (TR/TE = 700 ms/15 ms) with 5 mm thickness. Sagittal images were made with the same sequence, with 4 mm slice thickness. Images parallel to the right and left ascending ramus of the mandible were made with oblique sagittal multislice T1 spin-echo sequences (TR/TE 400 ms/15 ms) with 3 mm thickness.
When the patient’s head in the pre- and postoperative MRI examinations showed a different position, the angle between the S-N line and the axial scan plane was compared in the pre- and postoperative midsagittal image. The difference between the pre- and postoperative values of this angle was used to align the axial scan plane in the preoperative and the postoperative midsagittal image to correctly superimpose both images on S-N.
Craniofacial morphology was assessed on preoperative lateral cephalographs. These were digitized with Viewbox ® software (dHal software, Kifissia, Greece). Apart from assisting in the diagnosis of the skeletal deformity, the cephalometric analysis was mainly used to calculate the mpa (i.e. S-N to Go-Me).
Calculating advancement at surgery
The amount of mandibular advancement achieved at surgery was determined on lateral cephalographs taken preoperatively and 1 day postoperatively. For this purpose, a Cartesian coordinate system with S-N as the horizontal axis and a perpendicular line through S as the vertical axis was applied. The projection of the centroid of the mandibular symphysis on both axes was used to calculate the horizontal and vertical components of the changes achieved.
Calculating sagittal rotation of proximal segments
Sagittal rotation of the proximal segments was assessed by measuring the difference of the angle between the axial scan plane and the tangent to the dorsal side of the ascending ramus on pre- and postoperative oblique sagittal MRI series ( Fig. 1 ). The use of MRI for this purpose allowed the measurement of rotation of left and right proximal segments separately.
Calculating muscle direction and moment arms
The contours of the cross-sections of MAS and MPM were outlined in the axial MRI series with customized computer software (VISIAN, VU University, Dept. of Clinical Physics), allowing semi automatic segmentation of structures on pixel level. The centroids of the segmentations of the MPM and MAS were used to estimate the muscle direction as described by Koolstra et al. The muscle direction was assumed to coincide with the principal direction of its reconstructed shape ( Fig. 2 a and b ). This calculation was performed in a Cartesian coordinate system with its origin located in the right condyle (i.e. the centre of the uppermost MRI slice in which the condyle was visible). The sagittal axis ( X ) of the system was directed anteriorly and parallel to the axial scan plane, the transversal axis ( Y ) was directed to the left of the condyle and the vertical axis ( Z ) was directed cranially ( Fig. 3 ). The direction of the muscles in the sagittal plane was quantified by the angle between the projection of the centroid line and the vertical axis on the sagittal plane.
The direction in the frontal plane was calculated using the angle between the projection of the centroid line and the vertical axis on the frontal plane. The sagittal and frontal moment arms were defined as the perpendicular distance from the projection of the origin in the right condyle to the projection of the centroid lines on the sagittal and frontal planes, respectively ( Fig. 4 a and b ).
The data were analysed statistically with SPSS 16.0 for windows (SPSS Inc., Chicago, IL, USA). Analysis of variance was used to determine if the changes of orientation and moment arms of MAS and MPM were significant in the two groups.
In almost all patients the proximal segments had been rotated anteriorly after surgery. An example of this anterior rotation is shown in Fig. 1 . The extent of this anterior rotation differed between the two groups. This is illustrated in Fig. 5 which also shows the change of direction of MAS and MPM in the two groups. The more vertical postoperative direction of MAS and MPM in the sagittal plane as seen in Group II appeared to correspond to the anterior rotation of the proximal segments. The almost identical declination of the sagittal angle of both the ramus and the muscles ( Fig. 5 ) illustrates this.