Static and dynamic loading of mandibular condyles and their positional changes after bilateral sagittal split advancement osteotomies

Abstract

This study analysed the effects of change of direction of masseter (MAS) and medial pterygoid muscles (MPM) and changes of moment arms of MAS, MPM and bite force on static and dynamic loading of the condyles after surgical mandibular advancement. Rotations of the condyles were assessed on axial MRIs. 16 adult patients with mandibular hypoplasia were studied. The mandibular plane angle (MPA) was <39° in Group I ( n = 8) and >39° in Group II ( n = 8). All mandibles were advanced with a bilateral sagittal split osteotomy (BSSO). In Group II, BSSO was combined with Le Fort I osteotomy. Pre and postoperative moment arms of MAS, MPM and bite force were used in a two-dimensional model to assess static loading of the condyles. Pre and postoperative data on muscle cross-sectional area, volume and direction were introduced in three-dimensional dynamic models of the masticatory system to assess the loading of the condyles during opening and closing. Postsurgically, small increases of static condylar loading were calculated. Dynamic loading decreased slightly. Minor rotations of the condyles were observed. The results do not support the idea that increased postoperative condylar loading is a serious cause for condylar resorption or relapse.

Surgical mandibular advancement procedures may give rise to condylar remodelling and condylar resorption, with subsequent skeletal relapse. Patients with a long face, in particular females, are more prone to this than those with a short face. A postoperative change in the loading of the temporomandibular joint (TMJ) may be a reason for these degenerative changes of the condyle. TMJ loading depends largely on the magnitude, moment arm and direction of jaw muscle force and bite force. In this context, it is remarkable that, in studies on the possible causes of relapse, postoperative changes of the masticatory muscles are hardly mentioned. Two previous studies have demonstrated that, in long face patients, significant changes occur in cross-sectional area and volume, and in the main directions of the masseter (MAS) and medial pterygoid muscles (MPM) after surgical mandibular advancement procedures.

It is also likely that the positional changes of the condyle, which occur after mandibular advancement surgery, cause previously unloaded parts of the condyle to become loaded as suggested by Hwang et al. This could make the condyle more prone to resorption.

The aims of this study were to analyse the effects of change of direction of MAS and MPM and the changes of moment arms of MAS, MPM and bite force on static and dynamic forces on the TMJ and to quantify the positional changes of the condyle.

Materials and methods

16 patients (8 males; 8 females) with mandibular hypoplasia were included in this study. The average age of the patients at the time of surgery was 27 years (range 16–45 years.). None of the patients showed marked facial asymmetry or had signs or symptoms of TMJ dysfunction. Eight patients had a mandibular plane angle (MPA) (sella–nasion (S–N) to gonion–menton (Go–Me)) of less than 39° (Group I). The other 8 patients had an MPA of 39° or more (Group II). All patients had orthodontic treatment before and after surgery. To advance the mandible, a bilateral sagittal split osteotomy (BSSO) with the modification according to Hunsuck and Dal Pont, was carried out in all patients. Three bicortical positioning screws on each side were used for fixation and intermaxillary fixation was not used. All patients were allowed to open and close the jaw immediately after surgery. In Group II patients a Le Fort osteotomy with posterior intrusion was carried out, to avoid counter clockwise rotation of the mandible. In 5 of these patients an additional advancement genioplasty was done. All surgical procedures were carried out in one hospital by the same team of surgeons. Informed consent for participation in this study was given by all patients. Patients in this study participated in previous studies on size and direction of jaw opening and jaw closing muscles before and after surgical mandibular advancement.

Magnetic resonance imaging protocol

Magnetic resonance images (MRIs) were taken prior to orthodontic treatment and after completion of treatment (i.e. after removal of the orthodontic appliances). The mean time lapse, after surgery, for the post treatment scans 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 with the Frankfurt Horizontal (FH) plane oriented perpendicular to the scan table. The patients were instructed to close the teeth but to avoid clenching. Axial images were made with a multislice T1 spin-echo sequence (TR/TE = 700 ms/15 ms) with 5 mm thickness and 1.25 mm interslice gap. Sagittal and coronal imaging was performed in the same sequence with 4 mm thickness and 1 mm interslice gap. 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 and 1 mm interslice gap.

Cephalometrics

Craniofacial morphology was assessed on preoperative lateral cephalographs digitized with Viewbox ® software (dHal software, Kifissia, Greece). The cephalometric analysis was used to calculate the MPA (S–N to Go–Me) and assisted in the diagnosis of the skeletal deformity.

Mandibular advancement and sagittal rotation of the proximal segments

The advancement achieved at surgery and the sagittal rotation of the proximal segments were presented in a previous paper. In short, the advancement achieved at surgery was determined on preoperative (T1) and 1 day postoperative (T2) lateral radiographs in a carthesian coordinate system with S–N as the X axis and a perpendicular line through S as the Y axis. The projection of the centroid of the symphysis of the mandible on both axes was used for the calculation of the horizontal and vertical components of the advancement.

The sagittal rotation of the proximal segments was defined as the difference between the pre- and postoperative angle between the tangent to the dorsal side of the ramus and the axial scan plane ( Fig. 1 ).

Fig. 1
Sagittal rotation of the proximal segment. The tangent to the ascending ramus is drawn in the T1 weighted oblique sagittal images through the condyle parallel to the mandibular ramus. The horizontal line at the top of the image represents the axial scan plane.

Calculation of the angle of the condyle to the sagittal plane

The angle of the long axis of the condyle and the angle of the tangent to the ramus to the sagittal plane were measured in pre- and postoperative axial images ( Fig. 2 ).

Fig. 2
Axial view of the mandible. β is the angle between the tangent to the inferior border of the ascending ramus (proximal segment). γ is the angle between the condylar axis and the sagittal plane. ICA, intercondylar angle.

Calculation of the mechanical advantage

The moment arm of the bite force was measured in the oblique sagittal images parallel to the ramus ( Fig. 3 ). To estimate the direction (vector) of the bite force, a line perpendicular to the occlusal plane was drawn at the level of the right and left first lower molar. The moment arm of the bite force was calculated as the distance from the top of the ipsilateral condyle to this bite force line (Figs 3 and 4 ).

Fig. 3
Moment arm of the bite force. The moment arm of the bite force was drawn in the T1 weighted oblique sagittal images through the condyle parallel to the mandibular ramus at the level of the first lower molar.

Fig. 4
Two-dimensional static model with the vectors of the masseter (Fmas), medial pterygoid muscle (Fmpm), their resultant (Fres), bite force (Fb) and joint reaction force (Fj). α is the angle between the force vectors (direction) of masseter and medial pterygoid muscle. H, height of the parallellogram of forces.

The results for the calculations of moment arms of MAS and MPM were reported in a previous study on the same patient sample. In short, in a projection on the sagittal plane, the distance from the top of the right condyle to a line representing the main direction of the muscle was calculated ( Fig. 5 ).

Fig. 5
Sagittal main direction (yellow) and sagittal moment arm of the right masseter. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)

The mechanical advantage was defined as the ratio between the moment arm of the muscle and the moment arm of the bite force.

Forces on condyle articulating surface during mouth opening

The pre- and postoperative coordinates determining the direction of MAS, MPM, the lateral pterygoid (LPM) and anterior digastric muscles (DIG) were imported into biomechanical models of the human masticatory system. Two models were used, one representing Group I and one representing Group II. Together with the imported data on muscle cross-sectional area and volume this allowed an assessment of the forces on the condyle during jaw opening and closing movement to be made ( Fig. 6 ). The assessment was based on pre- and postoperative models. The imported data on the main direction of MAS and MPM were derived from a previous study on the same patient sample.

Fig. 6
Dynamic loading of the condyle. The course of the joint reaction forces during opening and closing in the pre- and postoperative models of Groups I and II. Note that the largest joint reaction force is built up during opening at the moment of initial contact between the condyle–disc complex and the articular eminence. 1, closed mouth; 2, initial contact with the articular eminence; 3, maximum mouth opening.

Results

The pre- and postoperative cephalometric characteristics are shown in Table 1 . The sagittal direction, the mean sagittal moment arms of MAS and MPM and the mechanical advantage of MAS and MPM are given in Table 2 . This table also shows the anterior rotation of the ramus, the axial rotation of the tangent to the ramus and the axial rotation of the condyle.

Table 1
Mean cephalometric characteristics on preoperative lateral radiographs.
Group I Group II
SNA (°) 80.65 (4.16) 77.98 (4.91)
SNB (°) 74.54 (4.20) 69.10 (2.76)
ANB (°) 6.11 (1.25) 8.88 (3.00)
ATFH (mm) 120.63 (8.28) 131.38 (6.32)
ALFH (mm) 65.84 (9.71) 80.18 (6.18)
PTFH (mm) 86.43 (11.67) 74.45 (3.80)
ALFH/ATFH × 100 (%) 54.45 (6.00) 61.00 (2.96)
PTFH/ATFH × 100 (%) 71.83 (10.35) 57.17 (1.50)
Ar–Go–Me (°) 112.96 (8.46) 129.24 (9.43)
Sp Go–Me (°) 15.73 (7.53) 38.00 (5.04)
S–N Go–Me (°) 23.09 (10.63) 45.78 (3.31)
Horizontal advancement (mm) 2.29 (2.20) 9.08 (2.85)
Vertical advancement (mm) 6.63 (4.43) −0.01 (1.93)
Mean horizontal and vertical advancement on 1 day postoperative lateral radiographs (standard deviations between parentheses). ATFH, anterior total face height (nasion–menton); ALFH, anterior lower face height (anterior nasal spine–menton); PTFH, posterior total face height (sella point–gonion); Ar, articulare; Go, gonion; Me, menton; Sp, spinal plane (anterior nasal spine-posterior nasal spine).

Table 2
Descriptive statistics (mean and SD) of: main direction and moment arms of MAS and MPM; moment arm of bite force; mechanical advantage; sagittal and axial rotations of the condyle/ramus.
Group I, MPA < 39° ( n = 8) Group II, MPA > 39° ( n = 8)
Pre Post Pre Post
Sagittal direction (°)
Right and left sides averaged
MAS 12.0 (11.2) 14.6 (8.6) 20.8 (5.6) 11.8 (5.9)
MPM 17.1 (11.4) 17.5 (10.2) 27.7 (9.3) 18.1 (8.4)
α 5.1 2.9 6.9 6.3
Sagittal moment arm (mm)
Right and left sides averaged
MAS 32.2 (2.3) 32.9 (2.5) 29.1 (2.3) 29.7 (2.8)
MPM 23.8 (2.9) 24.1 (2.5) 21.9 (4.1) 23.2 (3.6)
Bite force 57.1 (4.5) 61.3 (4.5) 59.4 (5.1) 65.0 (6.0)
Group I, MPA < 39° ( n = 8) Group II, MPA > 39° ( n = 8)
Pre Post Diff Pre Post Diff
Mechanical advantage
Right and left sides averaged
MAS 0.57 (0.07) 0.54 (0.07) −0.03 (0.04) 0.51 (0.05) 0.48 (0.07) −0.03 (0.04)
MPM 0.42 (0.06) 0.40 (0.06) −0.02 (0.04) 0.36 (0.08) 0.36 (0.08) 0.00 (0.04)
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Jan 26, 2018 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Static and dynamic loading of mandibular condyles and their positional changes after bilateral sagittal split advancement osteotomies
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