Abstract
Introduction
Surgical correction of the patients with mandibular asymmetry by bilateral sagittal split ramus osteotomy (BSSRO) creats remarkable results in the immediate post operative period, but it carries a high risk of relapse by powerful muscle forces and the osteotomized segments. To minimize this risk, literatures highlight the procedure of an intentional osteotomy and stripping of muscles. A prospective study was conducted in Government Dental College, Kottayam, Kerala, to assess the effect of intentional osteotomy of posterior part of distal segment and stripping of medial pterygoid muscle on the proximal segment on affected side in mandibular asymmetry patients treated with BSSRO.
Materials and Methods
20 patients above 18 years with mandibular asymmetry underwent surgical correction by BSSRO setback and rotation. Clinical evaluation and postero anterior cephalogram (PA Ceph) were used to assess relapse. PA cephalogram taken post operatively at 2 weeks, 6 months and 1year were compared using cephalometric parameters by Grummons analysis to assess relapse. Paired t test and Chi-square test was used to analyse quantitative and qualitative parameters respectively with statistical significance of p < 0.05.
Results
Barring Methodological errors which is inevitable, the amount of relapse was not statistically significant with respect to dental midline, chin midline and frontal photograph.
Conclusion
Intentional osteotomy of posterior part of distal segment and stripping of medial pterygoid muscle on the affected side can prevent relapse in patients who underwent surgical correction of mandibular asymmetry by BSSRO set back and rotation.
Electronic supplementary material
The online version of this article (doi:10.1007/s12663-016-0884-8) contains supplementary material, which is available to authorized users.
Keywords: Asymmetry, Intentional osteotomy, Stripping of medial pterygoid muscle, BSSRO
Introduction
Human faces exhibit some degree of asymmetry. The ancient Greeks were probably the first to recognise these variations between the two halves of the face. Facial asymmetry, when obvious has an enormous psychosocial impact on affected individual [1]. The level at which asymmetry becomes unacceptable to patient and public is variable and depends on many factors [2]. Generally patients who present with significant facial asymmetry are not only concerned with restoring functional occlusion but are concerned with improving aesthetics and appearance.
Lower facial symmetry is constituted by mandible with its unique horseshoe shaped body, ascending ramus, condyle articulating with skull by synovial joints, and supported by muscles of mastication. Asymmetry of lower face results in, deviation of chin, dental midline, malocclusion and articular dysfunction. The possible impact of this will be elongated condylar neck, outward bowing and downward growth of mandibular body and ramus on unaffected side, resulting in fullness of face on the unaffected side and flattening of face on affected side.
The correction of mandibular asymmetry needs complex skeletal movements in all three planes. This is accomplished mainly by BSSRO, Bilateral Vertical Ramus Osteotomy (BVRO), and Genioplasty. BSSRO which is a versatile procedure allows corrections of mandibular asymmetry in all three planes, and has become the most commonly performed surgical technique for correction of mandibular asymmetry.
Patients with skeletal Class III malocclusion and mandibular asymmetry require mandibular setback and rotation procedures by means of BSSRO. It is a standard surgical procedure to strip medial pterygoid muscle from the medial side of the proximal segment [3].This is done to prevent interference by the muscle when the distal segment is set back to slid pass the lingual aspect of proximal segment. In correction of mandibular asymmetry by BSSRO, rotation of distal segment results in deflection of proximal end over the affected side which in long term results in relapse (Fig. 1A, B). Intentional osteotomy of posterior end of the distal segment of affected side and stripping of medial pterygoid muscle prevents relapse.
Fig. 1.
A Distal and proximal segments in position after surgical split non asymmetric case. B Setback and rotation of distal fragment of affected side to move laterally causing a “flare’’ of proximal segment during fixation
Aim and Obectives
To assess relapse in mandibular asymmetry patients treated with bilateral sagittal split osteotomy using post cephalometric parameters (Grummons analysis).
To evaluate the post surgical stability in patients who had undergone BSSRO with Intentional osteotomy of distal segment and medial pterygoid muscle stripping for the correction of mandibular asymmetry.
Materials and Methods
Study was carried out in 20 patients reported to Government Dental College Kottayam, Kerala during a period of March 2012 to November 2013. Approval from Institutional Ethical Committee was obtained. Patients of age groups above 18 years with lower facial asymmetry who have completed mandibular growth were included. Mandibular asymmetry resulting from maxillofacial pathology, trauma and those with uncontrolled systemic disease were excluded from the study. Clinical evaluation, facial photographs, PA cephalogram and lateral cephalogram were done. All patients received pre operative orthodontic treatment and underwent mandibular setback and rotation procedures by means of BSSRO. Intentional osteotomy of distal segment was done on the affected side. Medial pterygoid muscle stripping was also done around the proximal segment of the intentional osteotomy side. Pre operative PA cephalogram was taken immediately before surgery and post operative PA cephalogram was taken at interval of 2 weeks, 6 months and 1 year. The change in linear measurement in midline was measured by tracing the base of triangles (SR and SL, Fig. 4) using Grummons analysis [4, 5] pre-operatively as well as post-operatively at 2 weeks, 6 months and 1 year.
Fig. 4.
Mandibular triangle
Evaluation of dental midline, chin midline, and occlusion achieved post operatively were also compared with the preoperative to assess the achievement of symmetry (Figs. 2, 3). PA cephalogram taken at 2 weeks, 6 months and 1 year were compared with preoperative using Grummons analysis to predict relapse following BSSRO and rotation clinical and radiographic corelation of dental midline and chin midline in PA cephalogram were also compared to predict relapse.
Fig. 2.
Pre op. A frontal photograph, B PA cephalogram, C tracing
Fig. 3.
Post op. A frontal photograph, B PA cephalogram, C tracing
Cephalometric Evaluation
Assessment of severity of facial asymmetry carried out by Grummons Analysis, which is a comparative and quantitative analysis, presented in two forms: (1) the comprehensive frontal asymmetry analysis and (2) the summary of frontal analysis. The analysis consists of different components including horizontal planes, mandibular morphology, and volumetric comparison of asymmetry assessment, maxillomandibular relation and frontal vertical proportions. Mandibular morphology of one side with other side can be compared by constructing two lines perpendicular to MSR from Ag, lines from Cg to Ag also drawn (Tables 1, 2). Two pairs of triangle are formed in this way on each side of Co- MSR line (Fig. 4). Measurement of both bases of triangle on each side of MSR will give amount of mandibular asymmetry before surgical correction.
Table 1.
Land marks and abbreviations in Grummons analysis
| Ag | Antegonial Notch |
| ANS | Anterior nasal spine |
| Cg | Crista Galli |
| Go | Condylon (most superior aspect) |
| J | Jugular process |
| Me | Menton |
| MSR | Mid Sagittal Reference Line at Crista Galli |
| GM | Geometrically constructed vertical axis |
| A1 | Upper central incisor edge |
| B1 | Lower central incisor edge |
| Go | Most inferior point at angle of mandible |
Table 2.
Linear measurements in Grummons analysis
| SR GoGM | Measurement of base of mandibular triangle from right side |
| SL GoGM | Measurement of base of mandibular triangle from left side |
| CHR GM(GR) | Measurement of base of chin triangle from right side |
| CHL GM(GL) | Measurement of base of chin triangle from left side |
| PR | Measurement of long leg of mandibular triangle gives cant in mandible right side |
| PL | Measurement of long leg of mandibular triangle gives cant in mandible left side |
| KR | Measurement of long leg of chin triangle gives cant in mandible right side |
| KL | Measurement of long leg of chin triangle gives cant in mandible left side |
The mandibular triangle is constructed by connecting the C-point with Go bilaterally using lines PR and PL. The base of mandibular triangle is halfed into SR and SL by GM. The chin triangle is constructed by connecting C-point with CHR and CHL; the long legs of the triangle are called KR and KL (Fig. 5). A line is drawn from B point perpendicular to CHP to evaluate the mandibular incisor midline in relation to midline of chin. The base of this triangle is divided into two halves GR and GL by GM. By comparing the left and right sides of the bases of the triangles, transverse discrepancies on rotations can be assessed. Midline asymmetries of the nasal spine, menton, and dental midline can be evaluated. Any discrepancy between mandibular dental midline and the midpoint of the chin can be evaluated by the vertical line perpendicular to CHP.
Fig. 5.
Chin triangle
Surgical Technique
Extended third molar incision is given on the desired site and then mucoperiosteum reflected. Lingula and inferior alveolar nerve is located and protected, horizontal osteotomy cut given 2 mm above lingula and cut extended vertically along anterior border of ramus to external oblique line up to distal surface of lower second molar. The cut is then dropped vertically downward up to the lower border of mandible to osteotomize both cortex. Osteotomy cut completed with osteotome and mallet. Mandible is then separated into proximal (condyle) and distal segment using Smith spreader. Intentional osteotomy of distal segment is done by lateral traction of the proximal segment to expose the posterior part of distal segment with a channel retractor placed on the medial side of the distal segment above the mandibular foramen. This manoeuvre provides protection of inferior alveolar neurovascular bundle. Using a bur No 703 kept at posterior part of the distal segment a vertical cut along a vertical line posterior to the last tooth was carried out and posterior part of distal segment fractured and kept as such (Fig. 6). Medial pterygoid muscle stripping was done around the proximal segment. Bilateral fixation of proximal and distal segment was carried out using 2 mm four hole stainless steel plate and 8 mm stainless steel screws after achieving satisfactory midline and desired occlusion. Haemostasis was achieved and suturing done using 3-0 vicryl.
Fig. 6.
Intra op photograph of osteotomised distal segment
Results
In this study 20 patients had undergone surgical correction for mandibular asymmetry with chin deviation, with mean age of 21.8 years, of which 11 were males and nine females. Five patients had deviation towards the right side and remaining 15 patients had deviation towards left side. The mean length of normal side of mandibular triangle (Fig. 4) based on measurement (Grummons Analysis) from the midline (Fig. 4; SR-GoGM, SL-GoGM) was 3.83 with a SD of 0.365 and SE of 0.0817. The mean length on the asymmetrical side was 4.31 with a SD of 0.381 and SE of 0.0725 (Graph 1 in ESM).
Post-operative PA cephalogram of 6 months revealed a mean distance of the tracings of 3.875 with a SD of 0.320 and SE of 0.071 on SR-GM and SL-GM which when compared to 2 weeks post-operative results has a p value of 0.330 which is statistically significant implying that no relapse has occurred within a period of 6 months. Comparison of 6 months post-op evaluation with 1 year follow up post-op, indicates that the mean distance after 1 year of follow up had a mean of 3.88 with SD 0.31 and SE 0.07 on SR-GM and SL-GM. The p value of 6 months versus 1 year is 0.16 which is statistically insignificant implying that there has been no relapse (Graph 7 in ESM).
Pre-operative Versus 2 Weeks Post-operative
The mean distance on the deviated side 2 weeks post-operatively was measured 3.87 with a SD of 0.324 and SE 0.0725 which when compared with pre-operative value has a p value of 0.001 which is statistically significant as would be expected (Tables 3, 4; Graph 2 in ESM).
Table 3.
Pre-operative versus 2 weeks post-operative paired samples statistics
| Mean | N | Standard deviation | Standard error of mean | |
|---|---|---|---|---|
| Pair 1 | ||||
| Pre op | 4.3100 | 20 | 0.38100 | 0.08519 |
| 2 weeks post op | 3.8700 | 20 | 0.32461 | 0.07258 |
Table 4.
Pre-operative versus 2 weeks post-operative paired samples test
| Paired differences | t | df | Sig.(2 tailed) | |||||
|---|---|---|---|---|---|---|---|---|
| Mean | Std. deviation | Std. error of mean | 95 % Confidence interval of the difference | |||||
| Lower | Upper | |||||||
| Pre op versus 2 weeks post op | 0.44000 | 0.18750 | 0.04193 | 0.35225 | 0.52775 | 10.494 | 19 | 0.000 |
Six months Post-op Versus 1 Year Post-op
The mean distance after 1 year of follow up had a mean of 3.88 with SD 0.31 and SE 0.07. The p value of 6 months versus 1 year is 0.16 which is not statistically significant implying that there has been no/minimal relapse after 6 months and those that occurred can be attributed to other factor like cuspal guidance (Table 5; Graph 3 in ESM).
Table 5.
Six months post-op versus 1 year post-op paired samples test
| Paired differences | t | df | Sig.(2 tailed) | |||||
|---|---|---|---|---|---|---|---|---|
| Mean | Std. deviation | Std. error of mean | 95 % Confidence interval of the difference | |||||
| Lower | Upper | |||||||
| 6 months versus 1 year post op | 0.01000 | 0.03078 | 0.00688 | 0.02441 | 0.00441 | −1.453 | 19 | 0.163 |
Frontal Photographs
All patients had deviated chin pre-operatively which were corrected surgically. Four patients out of 20 developed statistically insignificant minimal deviation within 6 months which remained constant even after 1 year (Graph 4 in ESM).
Inferior Alveolar Nerve Paraesthesia
Post-operatively after 2 weeks, 13 patients out of 20 were having paresthesia of the mental region which reduced to six patients in 6 months; p value 0.044 was statistically significant [attributable to reversible nerve damage (neuropraxia)] which further reduced to only one patient at the end of 1 year validating the assumption that temporary paresthesia was due to reversible nerve damage (Graph 5, 6 in ESM).
Discussion
Correction of mandibular asymmetry is a complex procedure which needs movement of distal segment in all three dimensions with greater movements towards the deficient side. BSSRO is a well established surgical method for the correction of mandibular prognathism, retrognathism and mandibular asymmetry. The main concern in mandibular asymmetry correction is relapse occurring post operatively due to pull of medial pterygoid muscles (Fig. 7) and distraction of condylar segment by the distal bony extention of distal fragment (Fig. 8). In patients with mandibular asymmetry,when the distal segment of deficient side is moved laterally to the right or left side, the bony interference between the proximal and distal segments is inevitable to some extent [8, 9].
Fig. 7.
Medial pterygoid muscle interference in set back
Fig. 8.
Osteotomised distal segment
This interference leads to the displacement of the proximal segment resulting in relapse and temperomandibular joint disorders in long term [10]. It has been known that this bony interference is less in IVRO [intra oral vertical ramus osteotomy] than in BSSRO, because the length of the distal segment in IVRO is shorter than in BSSRO; the larger the asymmetry, the greater the expected displacement of the proximal segment of the short side. To overcome these BSSRO shortcomings resulting from the long length of the distal segment at the ascending ramus, several modified techniques of conventional BSSRO, such as the distal cutting technique or the short lingual osteotomy (SLO) technique,have been introduced by Young et al. [6] and concluded that SLO is the most favorable osteotomy method for reducing displacement of the proximal segment in mandibular prognathism with asymmetry.
In order to avoid relapse following surgical correction of mandibular asymmetry due to displacement of proximal segment by distal segment, an intentional osteotomy cut is made in the posterior part of distal segment on its medial surface of deficient side and then the fractured segment is kept as such (Fig. 8).This procedure reduces or prevents the push of the proximal segment of condylar segment in lateral direction. Kim et al. [7] tried a technique i.e., intentional posterior ostectomy of distal segment without medial pterygoid stripping for correction of mandibular prognathism and suggested that this method should be considered to maximize stability.
Comparison of preoperative value with 2 weeks post operative shows an average value of 4.3 mm in the asymmetric side preoperatively and 3.8 mm post operatively after correction. The muscular factor is regarded as being most important in post operative relapse following mandibular set back [11]. In the present study modified intentional osteotomy of the posterior part of the distal segment and stripping of medial pterygoid muscle around the proximal segment minimized the severity of relapse by eliminating the distal segment interference and effect of muscular forces acting from medial direction. This allows a functional stable adaptation of the sagittaly spilt ramal segments and improves facial esthetics.
Conclusion
Relapse following surgical correction of mandibular asymmetry is inevitable and severity of relapse can be minimized by intentional osteotomy of the posterior and stripping of medial pterygoid muscle around the proximal segment [3]. In accordance with the aim of the study 20 patients were evaluated to assess relapse following surgical correction of mandibular asymmetry by BSSRO and rotation. The parameters used for evaluation were achievement of dental midline coinciding with chin midline and post operative achievement of normal occlusion. Each patient was followed up for 2 weeks, 6 months and 1 year. There was statistically significant improvement in esthetics and functional parameters.
Hence it can be concluded that while performing BSSRO setback in mandibular asymmetry patients; rotation with intentional osteotomy of posterior part of distal segment and medial pterygoid stripping on affected side should be performed in all patients in order to prevent linear relapse and to maintain stable occlusion and midline.
Electronic supplementary material
Below is the link to the electronic supplementary material.
Compliance with Ethical Standards
Conflict of interest
None declared.
Ethical Standard
This study was approved by communication of decision of institutional ethics committee (IEC)/institutional review board (IRB), Government Dental College and Hospital, Kottayam, Kerala, India.
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