Medial pterygoid muscle and stylomandibular ligament: the effects on postoperative stability

Abstract

The postoperative skeletal stability following surgical advancement of the mandible can be influenced by several factors. The effect of the medial pterygoid muscle and the stylomandibular ligament on the stability of results following surgical advancement has possibly been underestimated. In this retrospective study, the long-term postoperative skeletal stability following surgical advancement of the mandible in two groups of patients was studied and compared. In one group the medial pterygoid muscle and the stylomandibular ligament were stripped from the medial side of the angle of the mandible during the bilateral sagittal split osteotomy (BSSO) procedure while for the other group of patients these muscles and ligaments were left attached. The long term skeletal stability of the two groups was compared and the group that had the muscles and the ligaments stripped proved to be more stable than the other group.

In 1957 Trauner and Obwegeser described a surgical procedure for repositioning the mandible by splitting the mandibular ramus in a sagittal plane. This procedure enabled the surgeon to establish occlusal function and improved facial aesthetics by repositioning the mandible anteriorly or posteriorly. The technique was later modified by Dal Pont and further refined by Hunsuck in 1968 and Epker in 1977 .

Bilateral sagittal split osteotomy (BSSO) is currently the surgical procedure of choice for the correction of dentofacial deformities involving the mandible. The ingenuity of the surgical design, development of specialised instruments and the improvement of the surgeons’ experience and skills has made it possible to achieve surgical treatment goals relatively quickly and atraumatically with predictable results. Even though significant progress has been made, the procedure is still associated with complications. One well known and important complication is skeletal relapse following surgery. Studies of postoperative stability following mandibular advancement has reported post-surgical relapse varying from 5.4% to 36.5% ( Table 1 ). Factors that may influence the long term postoperative stability of results include: inadequate fixation, method of fixation, unfavourable post surgical growth, pre-existing internal derangement of the temporomandibular joints, the age of the patient at the time of operation, inadequate bony healing, technical factors such as bad splits, high mandibular and occlusal plane angles, condylar resorbtion, condylar sag, poor proximal segment control, surgeons experience and density of bone.

Table 1
A literature review of mandibular stability following surgical advancement.
Study No. of patients Follow-up period Advancement (mm) Relapse (%)
Van Sickels et al. 19 1 yr 5.0–6.0 9.0–8.3
Simmons et al. 35 5 yr 4.9 9.5
Gomes et al. 45 6 mo 2.9 7.5
Knaup et al. 41 14 mo 7.2–9.5 2.5–2.7
Scheerlink et al. 103 32 mo 5.9 8.9
Bouwman et al. 57 6.3 yr 4.4–4.7 5.4
Rebellato et al. 29 12.7 mo 6.2 22
Van Sickels et al. 127 2 yr 5.2 36.5
Mobarak et al. 61 3 yr 5.9 33
Eggensperger et al. 30 14 mo 4.4 30

It has also been reported that larger mandibular advancements will have a greater tendency to relapse than smaller advancements. Most studies of stability after mandibular surgery recognise the importance of the effects of the musculature on skeletal stability or lack thereof to adapt to its altered mandibular length, position and orientation. In this respect, when planning the repositioning of the jaws, designing the technique and performing the surgery, three factors should be kept in mind: excessive stretching of soft tissues; post surgical neuromuscular adaptation; and alteration of the muscle orientation.

Surgeons are continually striving towards eliminating factors that may contribute to skeletal relapse. The role of the medial pterygoid muscle and the stylomandibular ligament on long term stability following mandibular repositioning may not have been fully understood and may have been underestimated.

For patients with Class III malocclussions requiring mandibular setback procedures by means of BSSO it is standard surgical technique to strip the medial pterygoid muscle and the stylomandibular ligament off the medial side of the proximal segment. This is done to prevent interference by the muscle and ligament when the distal segment is setback and slid past the lingual aspect of the proximal segment. It also facilitates unhindered positioning of the proximal segment (and condyle) during placement of rigid fixation.

For mandibular advancement procedures, the medial pterygoid muscle and stylomandibular ligament were not traditionally stripped from the medial side of the proximal segment. It occurred to the authors that although part of the medial pterygoid muscle is attached to the medial side of the proximal segment and will not interfere with the advanced distal segment, part of the muscle remains attached to the medial side of the distal segment ( Fig. 1 a ). The fact that the muscle attachment is split with one part attached to the proximal segment and the other part attached to the advanced distal segment tends to influence proximal segment control and could cause clockwise rotation of the proximal segment. It is for this reason that the authors changed their surgical technique several years ago. The medial pterygoid muscle attachment and the stylomandibular were also stripped from the proximal segment for mandibular advancement procedures.

Fig. 1
(a) (A) The attachments of the medial pterygoid muscle and (B) the stylomandibular ligament to the medial surface of the mandibular angle. (b) Repositioning of the distal mandibular segment and control of the proximal segment during positioning of the condyle will tend to be limited by the medial pterygoid muscle (arrow A) and stylomandibular ligament (arrow B).

Materials and methods

50 patients (34 females and 16 males) requiring surgical advancement of the mandible for the correction of mandibular anteroposterior deficiency were included in the study. The average age of the patients was 25.3 years (15–47 years) at the time of surgery.

Group 1 included 25 patients (16 females and 9 males) with an average age of 23 years (15–43 years) at the time of surgery. In this group the medial pterygoid muscles and stylomandibular ligaments were left attached to the medial surface of the mandible. This group formed part of the patients who had undergone mandibular advancement procedures before the surgical technique was changed. Group 2 included 25 patients (18 females and 7 males) with an average age of 26.6 years (15–47 years) at the time of surgery. In this group the medial pterygoid muscles and stylomandibular ligaments were stripped off the medial aspect of the proximal segment while the anterior part remained attached to the medial aspect of the distal segment ( Fig. 1 b). The mean follow up time for both groups was 7.75 months (6–16 months). For Group 1 the mean follow up time was 7.8 months (6–12 months) and for Group 2 it was 7.7 months (6–16 months). Patients who had undergone concurrent orthognathic procedures or had open bites were excluded from the study. The mandibular occlusal planes of the patients with Class II deep bite malocclusions were changed due to clockwise rotation of the distal segment. In these cases the lower incisors were advanced more than the chin (Pog) ( Fig. 2 a and b ). The bone segments of all the patients were fixated by means of three 2 mm bicortical screws (Biomet microfixation) on each side and positioned in a triangular or straight-line fashion. All surgeries were performed by the same surgeon using the same technique according to the technique originally described by Trauner and Obwegeser and subsequently modified by Epker, except for the stripping of the muscles and ligaments. The attachment of the pterygomasseteric sling was stripped off the lower border of the distal (repositioned) segment in all cases. Immediately after surgery, occlusal guiding elastics were placed in the canine region (3.5 Oz, ¼ in.) and a soft diet was recommended for a period of 4 weeks postoperatively.

Fig. 2
(a) A sagittal section of the anterior mandibular: note the relationship of the dental and cephalometric landmarks in a Class II deep bite case. (b) Rotation of the anterior mandible as a result of mandibular advancement in a Class II deep bite case showing a larger advancement at B-point (A) in comparison to menton (B).

Cephalometric radiographs were obtained 3–4 days before surgery (T0), 1 week after surgery (T1), and for the longest postoperative time (a minimum of 6 months) after surgery (T2). All radiographs were taken by the same radiographer on the same X-ray machine (Planmeca 2002 CC Prolive).

As a basis for the measurements, an X Y coordinate system was constructed on the radiographs. A horizontal plane was constructed through sella (S) at 7° from the anterior cranial base (sella-nasion plane) and a line was drawn perpendicular to the horizontal plane through S. Various reference points on the anterior part of the mandible were used to correlate horizontal and vertical dimensional changes at the anterior mandible ( Fig. 3 ).

Fig. 3
Cephalometric measurements: 1, condylion to gnathion (Co to Gn); 2, vertical line to B-point; 3, vertical line to pogonion (Pog); 4, vertical line to menton (Me); 5, horizontal line to B-point; 6, horizontal line to pogonion; 7, horizontal line to menton.

To monitor the skeletal changes the following measurements were performed ( Fig. 3 ). Changes in the mandibular length were evaluated by measuring the distance in mm from condylion (Co) to gnathion (Gn) (measurement 1). To monitor vertical skeletal changes perpendicular lines were drawn from the constructed horizontal plane to the mandibular B-point, pogonion (Pog) and menton (Me) and the distances measured (measurements 2, 3 and 4). To measure the horizontal skeletal changes, lines were drawn perpendicular from the vertical line to the mandibular B-point, Pog and Me and the distances measured (measurements 5, 6 and 7).

To establish the surgical changes the preoperative cephalometric measurements, (T0) were compared to the immediate postoperative cephalometric measurements (T1), while long term skeletal changes were recorded by comparing T1 (immediate postoperative position) to T2 (long term postoperative position). Any long term postoperative skeletal changes (T2–T1) in the opposite direction of the surgical repositioning of the mandible were recorded as negative change, while a postoperative movement in the same direction as the surgical repositioning was recorded as positive change.

The preoperative mandibular position (T0), the surgical changes (T1) and long term skeletal changes (T2) were recorded digitally on a computer using the Viewbox version 3.1.1 Software System. All radiographs were traced and analysed by the same individual. Intra-operator accuracy was correlated by re-capturing and re-measuring the data from 10 randomly selected patients 1 week after initial data capturing by the same examiner. An independent examiner did the same for 10 other randomly selected patients to correlate inter-operator accuracy.

Statistical methods

The two treatment groups were compared with respect to long term postoperative relapse (T2–T1) using an analysis of covariance (ANCOVA) with covariate the intra-operative advancement (T1–T0). Testing was done at the 0.05 level of significance.

Results

Measurement 1: mandibular length

The average increase in mandibular length for Group 1 was 5.95 mm (9.4 mm to 0.1 mm) while for Group 2 the mandibular length increased by 4.86 mm (9 mm to 1 mm) ( Table 2 ). The difference in the increase was found to be significant ( t test; p = 0.083) between the two groups. The average relapse for Group 1 was 1.16 mm (−5.2 mm to +3.5 mm) and 0.26 mm (−2.5 mm to +1.8 mm) for Group 2 which was statistically significant ( t test; p = 0.034), but the greater forward movement in Group 1 was not taken into consideration. To adjust for the difference in advancements between the two groups an ANCOVA was performed with a covariate set to a mean of 5.4 mm. The relapse was then predicted as 1.03 mm for Group 1 and 0.39 mm for Group 2. Thus, if the mandible was advanced 5.4 mm in both groups, Group 1 would have relapsed 1.03 mm and Group 2 0.39 mm. This difference in relapse was not statistically significant (ANCOVA p = 0.1165).

Table 2
Mandibular length in mm: measurement 1.
T0 (SD) T1 (SD) T2 (SD) T1–T0 (SD) Mean relapse (SD) % Relapse
Group 1
Condylion to gnathion (mm) 126.55 (6.39) 132.5 (6.62) 131.34 (6.41) 5.95 (2.16) −1.16 (1.75) 19
Group 2
Condylion to gnathion (mm) 112.52 (6.76) 117.38 (7.74) 117.12 (7.64) 4.86 (2.19) −0.26 (1.06) 5

Measurement 2: horizontal line to B-point

The height of the mandible as measured at B-point increased by 3.06 mm (−1.3 mm to 8.3 mm) in Group 1 and by 3.86 mm (−0.8 mm to 5.3 mm) in Group 2 ( Tables 3 and 4 ). The difference in the vertical change between the two groups was not statistically significant ( t test; p = 0.54). In the long term, B-point moved superiorly by a mean of 0.74 mm (−3.5 mm to 2.7 mm) in Group 1 and by 0.39 mm (−2.4 mm to 1.6 mm) in Group 2. The difference in change between the two groups was not statistically significant ( t test; p = 0.4).

Table 3
Vertical changes in mm for Group 1: measurements 5, 6 and 7.
Group 1 T0 (SD) T1 (SD) T2 (SD) T1–T0 (SD) Mean relapse (SD) % Relapse
5. Horizontal line to B-point (mm) 95.4 (6.12) 98.46 (7.39) 97.72 (7.28) 3.06 (2.50) −0.74 (1.57) 24
6. Horizontal line to pogonion (mm) 109.51 (7.75) 112.17 (7.97) 111.32 (8.21) 2.65 (2.25) −0.85 (1.53) 32
7. Horizontal line to menton (mm) 117.42 (7.97) 120.66 (8.80) 119.58 (8.72) 3.23 (2.07) −1.1 (1.36) 34

Table 4
Vertical changes in mm for Group 2: measurements 5, 6 and 7.
Group 2 T0 (SD) T1 (SD) T2 (SD) T1–T0 (SD) Mean relapse (SD) % Relapse
5. Horizontal line to B-point (mm) 83.59 (9.56) 87.45 (6.96) 87.05 (6.74) 3.86 (6.02) −0.39 (1.31) 10
6. Horizontal line to pogonion (mm) 97 (7.98) 99.83 (8.32) 99.5 (8.48) 2.84 (1.90) −0.34 (1.27) 12
7. Horizontal line to menton (mm) 104.8 (8.01) 107.67 (8.25) 107.35 (8.11) 2.87 −0.32 (1.09) 11
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Jan 24, 2018 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Medial pterygoid muscle and stylomandibular ligament: the effects on postoperative stability
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