Bilateral sagittal split osteotomy of the mandible with counterclockwise rotation of the occlusal plane alone has traditionally been considered the least stable treatment method. Two miniplates on each side of the osteotomy may resolve this problem. The authors compared early vertical and transverse stability of a simple mandibular advancement (group A), mandibular advancement with counterclockwise rotation (CCW) stabilized with one miniplate (group B), and two miniplates (group C) on mini-pig mandibles mounted on a custom-made loading unit. Two miniplates markedly increased the resistance to vertical bite forces. On a 100-N load, a median of dislocation of 0.53 mm, 0.46 mm, and 0.23 mm was achieved in groups A, B, and C, respectively. The difference was statistically significant between groups A and B in comparison with group C. The results of transverse displacement were not statistically significant. The use of two miniplates in larger shifts, as well as in CCW cases, increases stability in the vertical direction.
Skeletal anterior open bite in adults can be treated successfully using a combined orthodontic and surgical approach. Different methods can be used after orthodontic decompensation. Aesthetic and functional results can be achieved by osteotomy in the Le Fort I line with the maxilla downfractured in one piece or more, sometimes with a pre-maxilla clockwise rotation , as well as bimaxillary osteotomy . Some authors have described good long-term results with a bilateral sagittal split osteotomy (BSSO) of the mandible with counterclockwise rotation (CCW) of the occlusal plane, which is advocated in patients with an aesthetic position of the maxilla . Other authors describe the use of bone anchorage techniques to intrude lateral segments with a normal orthodontic approach alone or after local corticotomies .
BSSO of the mandible with CCW rotation of the occlusal plane alone was traditionally considered to be the least stable treatment method. Some authors suggest that a long-term higher tension of the suprahyoid and infrahyoid muscles tends to rotate the mandibular teeth-bearing fragment . If the fixation material is insufficiently strong, early relapse may occur. Two miniplates on each side of the osteotomy were suggested to resolve the problem . Although this treatment is accepted by many surgeons for mandibular angle fractures , the present authors have not found scientific proof of the benefits of this fixation in BSSO advancement with CCW.
The authors designed an experimental study to compare the early stability of simple mandibular advancement and mandibular advancement with CCW stabilized with one or two miniplates. There were two goals. First, the authors determined whether a simple change in the angle of the distal segment, and therefore a change in the vector of the force transmitted on the plate would cause lower stability. Second, they determined whether two miniplates significantly increased stability. The influence of two miniplates on transverse extrusion of the condyle of the temporomandibular joint (TMJ) was also examined.
Materials and methods
A prospective experimental study using fresh mini-pig mandibles cut in the midline was designed. Fifty-four hemi-mandibles were used and placed in three experimental groups, 20 in group A and 17 each in groups B and C. The vertical and horizontal displacements of a joint-bearing fragment were measured.
A sagittal split osteotomy was performed on each hemi-mandible in the manner described by H unsuck and E pker . Mandibular advancement of 10 mm and fixation with one miniplate was simulated in group A. Group B had an advancement of 10 mm with a 20° CCW rotation of teeth-bearing fragments (which corresponded to an anterior open bite of 15 mm). In group C, the movement was the same as in group B, but the fragments were fixed with two parallel miniplates with a distance of 10 mm between them. 2.0 Le Forte ® miniplates and 8 mm long screws (Jeil Medical Corporation, Korea) were used in all three groups ( Fig. 1 ).
A custom-made loading unit was developed in cooperation with Prospon ® Ltd. (Kladno, Czech Republic; Fig. 2 ). The loading unit consists of a steel horizontal plate, which served as the base for four vertical carrier bars. The two dorsal bars hold a pulley representing the forces of chewing muscles (pterygomasseteric sling) and fixed a horizontal bar running through the mandibular condyle, simulating a TMJ. The anterior bars were used to fix the teeth-bearing fragment of a mandible, as well as an occlusal stop in the position of the first molar (the centre of occlusion forces vectors) . Thirty orthopantomograms of patients without skeletal abnormalities treated in the authors’ department for wisdom teeth extraction were used to obtain the mean distance between the TMJ, pterygomasseteric sling, and occlusal centre. The magnification of 29% of the radiograph machine (Orthopantomograph OP100 ® ; Instrumentarium Imaging, Tuusula, Finland) used in the authors’ department was taken into account when calculating the distances.
The mandible was fixed at four points. A horizontal 5 mm bar running through a hole in the condyle represented the TMJ (point A). The hole had a diameter of 5 mm in a centre diverging to the side. This design allowed rotational movements as well as transverse displacements of the condyle-bearing fragment, which corresponded to the situation in a human TMJ. The condyle-bearing fragment was supported by a blade located 4 cm from a line running through the centre of the hole in the condyle and perpendicular to the horizontal plane. This point B represented the pterygomasseteric loop.
The teeth-bearing fragment was propped against the occlusal stop (point C, representing a first molar) 7.6 cm from that perpendicular line. Anchorage of the teeth-bearing fragment was achieved by pins at point D. These pins also prevented lateral displacement of the fragment (which is provided by the other half of the mandible in humans).
After the osteotomy was completed, both segments were mounted into the loading unit. Shifts and fixation with one or two miniplates, according to the protocol for each group, were carried out. The distances between the chosen points on the medial and lateral side of the teeth-bearing fragment, and the plane determined by the inner part of the vertical bars were measured and maintained during shifts to avoid undesired rotation of the fragment along its longitudinal axis. This problem never occurs in a patient with a whole mandible. Pins at points E and F for the distal segment, and point I for the proximal segment, were used on the medial and lateral sides of the mandible to prevent displacement during drilling and fixing of miniplates, and were released before measurement started.
Two horizontal bars in the upper part of the loading unit were used to place a measuring device and to reinforce the entire loading unit. Bending of the vertical bars during maximum loading was calculated on a computer model and did not exceed 0.009 mm; this could not influence the results. A bite-like force of 10–100 N (N defined as kg m s −2 ) was simulated by adding weights on a bar hanging on a wire connected via a pulley with the blade representing the pterygomasseteric sling.
Two screws were fixed in the condyle-bearing fragment and were used to register the movements of the condyle-bearing fragment. Point G, which was placed on the highest portion of the body of the mandible as near as possible to the osteotomy line, was used to register vertical displacement. Point H, which was located approximately 1 cm under the upper border of the mandibular body on the lateral part of the mandible as near as possible to the osteotomy line, allowed registration of mediolateral displacement. Both points were chosen in places in which the biggest magnitude of displacement was expected.
A measuring device (Somet; a clock-type dial indicator with a scale division 0.01 mm with technical parameter given by the norm EN ISO 463, International Organization for Standardization) was used to register displacement. Measuring pins remained in contact with screws during the entire experiment to reduce measurement errors. Displacement was registered in 10 N increments 1 min after weight placement. Displacement of 0.01 mm corresponded to ‘number 1’ in a table. Only cranial movements were observed in the vertical direction; this compared with the transverse measurement, in which movements in both directions occurred. Lateral displacement was marked as ‘positive’ and medial displacement as ‘negative’.
All measurements were analysed using the Kolmogorov–Smirnov test and showed a non-normal distribution of results. The Kruskal–Wallis test was used to analyse the results and compare the results between all groups. The level of statistical significance was established at a P < 0.05.
The median of vertical displacement in group A with simple mandibular advancement reached a value of 0.25 mm when 50 N was used and a value of 0.41 mm on a 100 N load.
The median of transverse displacement reached −0.02 mm on a 50 N load and −0.04 mm when a 100 N force was applied. Lateral movements were noted in 7 cases, medial movements in 11 cases, and no mediolateral movement in 2 cases ( Table 1 ).
|Force (N)||Vertical displacement (Q3 − Q1)||Transversal displacement (Q3 − Q1)|
|10||0 (0–0)||0 (0–0)|
|20||5.5 (9.25–4)||0 (3–(−3.5))|
|30||15 (21.5–9)||0 (8.75–(−16))|
|40||19 (37.5–13.75)||−0.5 (12.25–(−19.75))|
|50||24.5 (43.75–17.75)||−1.5 (13.25–(−23.75))|
|60||30.5 (50.5–22.5)||−1.5 (18.25–(−26.5))|
|70||33 (56.5–25)||−2.5 (25–(−29.25))|
|80||36 (60.5–26.75)||−3.5 (29–(−30.5))|
|90||38.5 (64.5–29.5)||−3.5 (29.75–(−31))|
|100||40.5 (69.25–30.5)||−4 (39.5–(−30.5))|
The vertical displacement in group B with mandibular advancement and CCW rotation of the teeth-bearing segment fixed with one miniplate reached a median value of 0.28 mm when a 50 N force was used and 0.43 mm on a 100 N load.
The median of transverse displacement was 0.0 mm on a 50 N load and reached 0.04 mm when a 100 N force was applied. Lateral movements were noted in seven cases, medial movements in eight cases, and no mediolateral movement in two cases ( Table 2 ).
|Force (N)||Vertical displacement (Q3 − Q1)||Transversal displacement (Q3 − Q1)|
|10||0 (0–0)||0 (0–0)|
|20||6 (11–5)||0 (0–(−1))|
|30||18 (19–13)||0 (1–(−1))|
|40||25 (29–20)||0 (2–(−3))|
|50||28 (37–22)||0 (3–(−14))|
|60||33 (41–22)||1 (5–(−16))|
|70||36 (44–24)||2 (7–(−18))|
|80||38 (47–27)||4 (10–(−19))|
|90||40 (49–28)||3 (11–(−20))|
|100||43 (51–30)||4 (14–(−19))|
Vertical displacement in group C with mandibular advancement and CCW rotation of the teeth-bearing segment fixed with two miniplates reached a median value of 0.12 mm when a 50 N force was used and 0.18 mm on a 100 N load.
A median transverse displacement of −0.01 mm on a 50 N load was achieved and a median transverse displacement of −0.02 mm when a 100 N force was applied. Lateral movements were noted in six cases, medial movements in nine cases, and no mediolateral movement in two cases ( Table 3 ).