Transverse stability of the proximal segment after bilateral sagittal split ramus osteotomy for mandibular setback surgery

Abstract

This study aimed to evaluate the correlation between the transverse displacement of the proximal segment after bilateral sagittal osteotomy for mandibular setback and the amount and design of the mandibular setback. Patients who underwent either bilateral sagittal split ramus osteotomy (BSSRO) alone or two-jaw surgery were selected, and cephalographic postero-anterior (PA) measurements were taken pre-operatively (T1), immediately post-operatively (T2), and at follow-up (T3). The inter-gonal (IG) and inter-ramal (IR) width increased immediately after surgery, but decreased to the initial value during follow-up ( P = 0.002; IR, P = 0.046). Only the immediate IG changes after surgery correlated with the amount of mandibular setback ( P = 0.009). The IG changes were significant in the symmetric group, but not in the asymmetric group. There was no difference in the IG and IR changes between the symmetric group and the asymmetric group. The immediate IG change in two-jaw patients with symmetric setback showed correlation with the setback amount. The gonial width of the deviated group showed more significant changes than that of the non-deviated group. There was no difference in the unilateral gonial width between the deviated and the non-deviated group, but the difference was significant for the unilateral ramal angle between the two groups. These correlations will be helpful in predicting post-surgical results for patients.

Bilateral sagittal split ramus osteotomy (BSSRO) is an effective and widely used method to correct a prognathic or retrognathic mandible. In Asian countries, the majority of patients treated with BSSRO are skeletal Class III. Despite the popularity of this surgical method, many unpredictable results have been described. In particular, changes in the gonial area after mandibular setback BSSRO are not well understood.

Several studies have reported the horizontal expansion of the gonial area following BSSRO or internal vertical ramus osteotomy (IVRO). Despite the various methods to maintain the condyle in its natural position, the setback of the distal segment appears to cause flaring of the proximal segment, leading to discomfort in the temporomandibular joint (TMJ) and a wider gonial width in the frontal view. A better understanding of gonial widening would contribute to improved surgical planning, and the surgeon would be able to consider mandibular angle osteotomy or shaving pre-operatively.

Many factors affect the transverse displacement of the proximal segment, requiring different surgical considerations in each case. The condyle must be seated in its fossa with minimal change in the antero-posterior and mesio-distal ramal inclination. Also, the surgeon must decide on the location of the vertical osteotomy at the body of the mandible with prudence, because it determines the size and thickness of the proximal segment as well as affecting the contact area between the proximal and distal segments. Finally, the amount and design of the mandibular setback presents diversely for each individual as the treatment plan differs accordingly. This decides the amount of overlap of the two segments and the kicking out direction and amount of the distal segment. Among the above factors, the last factor mainly requires modifications of the surgical considerations for BSSRO.

The purpose of the present study was to evaluate the correlation between transverse changes in the proximal segment after BSSRO for mandibular setback surgery and the amount and design of the mandibular setback from four perspectives: (1) the inter-gonial distance (IG) and inter-ramal distance (IR) changes in association with the setback amount and difference; (2) the difference in IG and IR related to symmetric or asymmetric setback; (3) IG and IR changes in association with symmetric setback, also in regard to one-jaw and two-jaw surgery; and (4) the unilateral gonial width and the unilateral ramal angle changes related to the deviated or non-deviated side of the mandible.

Materials and methods

This study was conducted as a single-centre, retrospective investigation of 94 skeletal Class III patients who underwent orthognathic surgery; all the information from the patients was anonymized. Patients were selected using the following criteria: (1) patients who received BSSRO and Le Fort I + BSSRO surgery without any combinations; (2) patients fully able to complete the follow-up in accordance with the study protocol and who had complete records including frontal and lateral cephalograms; (3) patients for whom the BSSRO was internally fixated with monocortical miniplates and screws; (4) no craniofacial syndrome, trauma, or prior maxillofacial surgery; (5) no active periodontal disease judged severe by the surgeon; and (6) no severely missing teeth that may affect post-operative occlusion.

A total of 94 patients (49 females and 45 males) were selected. The mean age of the patients was 24 years (standard deviation 4.6 years). Forty-two patients received BSSRO and 52 patients received Le Fort I and BSSRO surgery. All patients received BSSRO for mandibular setback as described by Obwegeser and modified by Dal Ponte. The desired occlusion was designed into a prefabricated splint. The proximal segment was manually guided into its original position by measuring the distance of a certain point in the ascending ramus to two or more points in the maxillary archwire. The cephalometric radiographs were all taken using a CX 90SP instrument (Asahi, Japan). The same equipment, same film, and same focus distance were used to examine the patients. All radiographs were taken at optimal exposure; the anatomic landmarks were clearly visualized and were hand-traced for examination. The cephalograms were taken after the pre-surgical orthodontic treatment (T1), within 1 week after orthognathic surgery with intermaxillary fixation in the desired occlusion (T2), and at least 2 months post-operatively (T3), to analyse the skeletal movements.

Postero-anterior (PA) cephalometric radiographs were used to evaluate the angulation of the proximal segment and the mandibular width at the same time-points ( Fig. 1 ). The reference points and planes were as follows: (1) ramus point (RP): the intersection of the mastoid process and the lateral border of the ramus (i.e., the most superior visible part on the lateral border of the ramus); (2) gonion (GO): most infero-lateral visible part of the ramal outline at the mandibular angle; (3) upper orbital margin (UOM): the most superior visible margin of the orbit.

Fig. 1
Cephalometric analysis: upper orbital margin (UOM), ramus point (RP), gonion (GO), inter-ramal width (IR), inter-gonial width (IG), unilateral ramal angle (RA), and unilateral gonial width (GW).

Reference lines were designed to pass through GO and RP, and a tangent line to the UOMs was used as a horizontal reference plane. A true vertical line passing through the crista galli was used. Points GO and RP were identified on both sides and were then marked on the T1 to the T2 and T3 radiographs so that the anatomic landmark was best superimposed on the proximal segment cortical outline.

The measurements recorded were as follows: (1) medial ramus angles between UOMs and GO–RP lines (right and left); (2) distance between the left GO and the right GO (inter-gonial width); (3) distance between the left RP and the right RP (inter-ramal width); (4) perpendicular distance from GO to the true vertical line (right and left).

This study had four aims and patients were grouped accordingly: (1) To determine the association between the IG and IR changes and the setback amount and difference ( n = 94). (2) The difference of IG and IR related to symmetric or asymmetric setback was analysed by grouping the patients into a symmetric setback group ( n = 60) and an asymmetric setback group ( n = 34). Asymmetric setback was defined as a setback difference of 2 mm or greater. (3) To determine the IG and IR changes in association with symmetric setback, symmetric setback was defined as a setback difference of less than 2 mm, giving a total of 60 patients. To examine the association in more detail, the patients were divided into a one-jaw symmetric setback group ( n = 24) and a two-jaw symmetric setback group ( n = 36). (4) To analyse the unilateral gonial width and the unilateral ramal angle changes related to the deviated or non-deviated side of the mandible, the deviated side of the mandible was identified on the cephalometric PA film by the position of the menton point to the true vertical line. Patients who had the menton point along the true vertical line were excluded from this analysis ( n = 86).

The differences in IG and IR values at T1, T2, and T3 were analysed by one-way analysis of variance (ANOVA), and the difference of IG and IR values at T1, T2, and T3 according to the particular study groups (symmetric/asymmetric setback, deviated/non-deviated side) was analysed by repeated measures ANOVA. The correlations between the setback amount and the respective groups were analysed by Spearman’s correlation method.

To test method error, 10 cases were selected at random, and all tracings and measurements were re-done by the same investigator (JY). Measurement errors were determined using the Dahlberg formula:

<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='∑D22N’>D22N∑D22N
∑ D 2 2 N

where D is the difference between re-measured values and N is the number of double measurements ( n = 10). Measurement errors for IG, IR, gonial width, and ramal angle were obtained, and the values were considered clinically insignificant.

Results

Table 1 shows the values of IG and IR before surgery (T1), within 2 days after surgery (T2), and at least 2 months post-surgery (T3). The transverse change in the proximal segment was clearly observed in the significant changes of IG and IR values. The IG and IR values increased from T1 to T2, with a mean of 2.72 mm and 2.49 mm, respectively ( P = 0.005 and P = 0.050), but decreased in the follow-up radiograph (T3) ( P = 0.002 and P = 0.046, Table 2 ), almost corresponding to the pre-surgical values ( Table 1 ).

Table 1
Inter-gonial (IG) and inter-ramal (IR) measurements, before surgery (T1), within 2 days after surgery (T2), and at least 2 months post-surgery (T3) ( n = 94).
T1 T2 T3 T1–T2 T2–T3 T1–T3
Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD
IG 105.8 6.81 108.5 6.55 105.5 6.54 2.72 2.84 −3.05 2.62 −0.33 3.42
IR 116.3 12.37 118.7 6.29 116.2 5.72 2.49 10.82 −2.54 1.92 0.47 10.74
SD, standard deviation.

Table 2
Significance of inter-gonial (IG) and inter-ramal (IR) value changes between T1, T2, and T3.
T1–T2 T2–T3 T1–T3
IG P = 0.005 P = 0.002 P = 0.733
IR P = 0.050 P = 0.046 P = 0.970
T1, before surgery; T2, within 2 days after surgery; T3, at least 2 months post-surgery.

IG and IR changes in association with the setback amount and difference

Tables 3 and 4 show the significance in the correlation of the IG and IR changes according to setback amount and difference. Only the IG value change from T1 to T2 was significantly associated with the setback amount ( P = 0.009). There was no significant correlation with either the setback amount or the setback difference in IG and IR values at other T1, T2, and T3.

Table 3
Correlation of inter-gonial (IG) and inter-ramal (IR) changes in association with the setback amount.
Setback amount T1–T2 T2–T3 T1–T3
P -value r -value P -value r -value P -value r -value
IG 0.009 −0.267 0.116 0.163 0.243 −0.122
IR 0.225 −0.126 0.536 0.065 0.231 −0.125
T1, before surgery; T2, within 2 days after surgery; T3, at least 2 months post-surgery.

Table 4
Correlation of inter-gonial (IG) and inter-ramal (IR) changes in association with the setback difference.
Setback difference T1–T2 T2–T3 T1–T3
P -value r -value P -value r -value P -value r -value
IG 0.917 −0.011 0.702 −0.040 0.502 −0.070
IR 0.068 0.189 0.867 0.018 0.066 0.190
T1, before surgery; T2, within 2 days after surgery; T3, at least 2 months post-surgery.

Differences in IG and IR related to symmetric or asymmetric setback

Table 5 shows the IG and IR values in symmetric ( n = 60) and asymmetric ( n = 34) setback groups at T1, T2, and T3. Table 6 shows the significance of IG and IR value changes at T1, T2, and T3. The IG values for the symmetric group were significant, following the trend of immediate increase after surgery and reduction at follow-up ( P = 0.024). The asymmetric group displayed the same pattern but without much significance ( P = 0.122). The IR values showed no significant difference in either the symmetric or asymmetric group ( P = 0.089 and P = 0.248, respectively). There was no statistically significant difference in the symmetric group and the asymmetric group for the changes of IG ( P = 0.270) or IR ( P = 0.461) values.

Table 5
Inter-gonial (IG) and inter-ramal (IR) changes in symmetric a and asymmetric b setback patients.
T1 T2 T3 T1–T2 T2–T3 T1–T3
Mean SD Mean SD Mean SD Mean SD Mean SD Mean SD
IG
Symmetric ( n = 60) 106.14 6.80 108.83 6.41 105.87 6.29 2.69 3.01 −2.96 2.78 −0.27 3.48
Asymmetric ( n = 34) 105.17 6.90 107.95 6.86 104.75 6.99 2.79 2.53 −3.20 2.32 −0.42 3.37
IR
Symmetric ( n = 60) 117.53 6.26 118.71 6.59 116.18 5.93 1.18 2.29 −2.53 1.77 −1.35 1.92
Asymmetric ( n = 34) 114.01 18.80 118.81 5.83 116.25 5.41 4.80 17.66 −2.56 2.18 2.24 17.61
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Jan 24, 2018 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Transverse stability of the proximal segment after bilateral sagittal split ramus osteotomy for mandibular setback surgery
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