| The sagittal split ramus osteotomy (SSRO) has been used for 50 years after Trauner and Obwegeser first popularized this technique in 1957 for correction of prognathism and retrognathism. During the past half century, undergoing numerous modifications and improments,it has been one of the most popular surgical procedure for correcting various mandibular deforminties.SSRO involved soft tissue dissection and a horizontal corticotomy on the medial and lateral aspects in the vertical ramus to the posterior border. They found that this technique provided good contact of wide cancellous bone surfaces that resulted in quick bony union. However, there are a number of complications inherent in this technique, including excessive swelling and bleeding, intraoperative trauma to the temporomandibular joint, damage to the inferior alveolar nerve(IAN), and unfavorable fracture of the proximal and/or distal segments of the mandible.There have been numerous modifications to this technique, all attempting to optimize a variety of factors to aid in proper healing of osteotomy and decrease the associated complications with this procedure. Although anatomic sthdies of the mandibular have been performed, they generally involved human cadaver or dry mandibulles. Patients undergoing SSRO are generally young individuals with a mandibular deformity and it is likely that the mandibular ramus anatomy of these patients is quite different from those of typical cadaveric specimens or dry mandibles.The mediolateral width of the ramus is one of the important aspects during the surgery.Thus,one of the purposes of this study was to assess thickness of the mandibular ramus at lingular plane using spiral computed tomography,and to compare with the control patients, the finding of this investigation should provide the surgeon with meaningful informations for the SSRO.The purpose of the second parts of this study was to determine the location of cancellous bone and the changes of cortical plates width in the mandibular ramus of patients with mandibular prognathism using computed tomography(CT) scanning. The finding of this studies should provide the surgeons with meaningful informations about the best medial osteotomy site,depth and direction for the SSRO, and avoid unfavourable fracture.The aim of the third parts of this study was to investigate the position and course of the mandibular canal through the mandibular ramus using computed tomographic imaging and examined its relationship with postoperative nerosensory disturbance, and also to describe the anatomical variability of the mandibular canal within the rami in order to assist in finding the safest site for a verital corticotomy through the buccal plate when splitting the mandible,and to reduce injuries to the inferior alveolar nerve.Materials and methods1.Patients(1)Patients in the first part of this articalThe prognathism group was composed of 25 skeketal ClassⅢpatients(19 males and 16 females;age range,18-35 years;mean age 21.7 years;50 mandibular sides) who were scheduled to undergo SSRO from the Department of Oral and Maxillofacial Surgery,school of stomatology,China Medical University,from January 2005 to March 2007. The control group was composed of 20 individuals without dentofacial deforminties(9 males and 11 females;age range, 18-37 years;mean age 22.2 years;40 mandibular sides)who underwent a CT examination for other maxillomandibular diseases.(2) Patients in the second and third part of this artical45 patients(24 males and 21 females)who underwent SSRO between January 2005 and March 2007 at the Department of Oral and Maxillofacial Surgery, school of stomatology,China Medical University. The age of the patients ranged from 18 to 37 years with an everage of 22.1 years.2.Computed Tomography ExaminationThe CT scan machine was GE Lightspeed 16 (GE Medical System,American) with the following scan parameters for all patients: slice thickness of 1.25mm; slice interval of 1mm, tube voltage of 120 kVp, tube current of 320 mA, the image matrix size of 512×512. Patients were positioned in a supine position with cervical hyperextension and head support for stabilization.3.Methods and Area of Measurement(1) Method and Area of Measurement in the first part of this articalThe location of ramus thickness measurement was taken from the scans at the lowest point of the mandibular forman. A total of 110 CT scans from 55 patients were examined, The following parameters were measured: total thickness of the mandible through the center of the mandibular canal(D); diameter of the inner mandibular canal(I); the bone marrow space between the outer mandibular canal and the inner surface of the buccal cortical bone(P);The width of the buccal cortex(B) and lingual cortex(L).For each measurement, the same examiner performed two times.(2)Method and Area of Measurement in the second part of this artical45 patients, 90 mandibular sides. In each side,the plane which just superior to the lingular was used as the base plane.The region from the base plane to 20mm above it with a slice interval of 2.5mm was observed on spiral computed tomography, and a total of 9 planes were obtained. The fllowing parameters were measured at each plane: anteroposterior length of the ramus(AP), thickness of the anterior cortical plate(AC), thickness of the posterior cortical plate(PC), thickness of the anterior third of the ramus(AT), thickness of the lingular cortex and marrow space of the anterior third of the ramus(atc,a),thickness of the center of the ramus(MT), thickness of the lingular cortex and marrow space of the center of the ramus(mtc,m) and thickness of the posterior third of the ramus(PT), thickness of the lingular cortex and marrow space of the posterior third of the ramus(ptc,p) At each plane, it was decided whether the region of cancellous bone was separated, if it was separated, then measured the length of the anterior and posterior bone marrow space(aa,pp). At the base plane,we still measured the distances from lingula to the fusion of the posterior cortical plates(LP).(3) Method and Area of Measurement in the third part of this articalThe region from a plane containing the lowest point of the mandibular foramen(base plane o) to 25 mm below it was mensured with 5mm distance every plane. The following parameters were measured: total thickness of the mandible through the center of the mandibular canal(LB), the thicknessof the buccal and lingular cortical plate(BC and LC), and the narrowest portion of the bone marrow space between the outer mandibular canal and both the buccal and lingular cortical(BP and LP).4.Evalution of Sensory DisturbanceAll patients were examined 1,6 and 12 months postoperation using questionnaire. We evaluated the prevalence of neurosensory disturbance according to the presence or absence of marrow space between the mandibular canal and the external cortical bone. The 90 sides were classified into 2 groups: a contact group, in which the mandibular canal came into contact with the external cortical bone(ie. no marrow space); and a noncontact group, in which no contact was observed. The incidence of neurosensory disturbance between the 2 groups was then compared. The vertical extent of contact between the mandibular canal and external cortical bone in the contact group was cauculated by multiplying the number of consecutive sliced planes. The relationship between the width of the bone marrow space and the presence or absence of postoperative neurosensory disturbance was then evaluated.5.Statistical AnalysisAll measurements were analyzed with SPSS 13.0. Differences between groups were examined for statistical significance based on the Student t test,differences between the measurements from different slices were examined for statistical significance based on the ANOVA, the relationship between the width of the bone marrow space and the presence or absence of the postoperative neurosensory disturbance was evaluated using the chi-square test, multiple comparisons using Dunnett.t,and a p value less than 0.05 was considered a statistically significant difference.Results1.The thickness of the mandible was 8.42mm in prognathic patients,and was 9. 65mm in control patients.The width of the bone marrow space at the buccal in prognathic patients was 1.58mm,and in control patients was 2.39mm.There was a significant statistic difference between two groups in the total thickness of the mandible(D)(t=-4.612,P=0.000).This trend was also found in the bone marrow space between the outer mandibular canal and the inner surface of the bucal cortical bone(P) (t=-5.434,P=0.000),and the correlation coefficient between D and P was 0.744 (P<0.01). 2.There were no statistical difference between prognathic and control groups with regards to the other 3 distances I, L and B. There were no statistical difference between two sides and for all measurements, and there were significant statistic difference between two genders in I and B (P=0.042,P=0.023).3.The anteroposterior length of the ramus (AP) increased in length from the lingular to the mandibular north, the anterior cortical plate(AC)increased too,the posterior cortical plate(PC)decreased.4.The anteroposterior length of the ramus (AP) was longer in the male than in the female (36.11mm vs 33.50mm),and there was a statistically significant difference (t=-8.502,P=0.000).We also found the statistically significant difference in LP(t=3.563,P=0.001) and d(t=2.488,P=0.013).5.The thickness of the lingular cortical plate decreased from the lingular to the mandibular north at the anterior of the ramus, remained relatively even at the center, and increased at the posterior of the ramus.At the plane 5mm above the lingular plane, the mean thickness of the lingular cortical plate was 1.68mm,1.68mmand 1.28mm separately at the anterior, center and the posterior.6.The distribution of the cancellous bone was classified into 2 categories: no separation of cancellous bone was 62.5%, separation of cancellous bone at any plane was 37.5%.7.There were no statistical difference between two sides with regards to the distances ID, LB, BC, LC, BP and LP, and there were significant statistic difference between two genders in ID and BP (BP:F=5.923,P=0.003, ID:F=32.058,P=0.000).8.From mandibular foramen to the mandibular body,the thickness of the mandibule(LB) increased, the thicknessof the buccal cortical plate(BC) and the bone marrow space between the outer mansubular canal and the lingular cortical(LP) were also increased.At every plane, the BP was thicker than LP.9.The width of the buccal side bone marrow space at each site could be classified into three types: (l)separated type with the bone marrow space visible, (2)contact type with the outer surface of the canal and inner surface of the buccal cortical bone in contact, and (3)fusion type with the outer cortical plate of the canal not evident. The separate type was most prevalent(n=473 of 540,87.59%), followed by the contact and fusion types(10.37% and 2.04%). 10.Neurosensory disturbance was observed on all 27 sides on which the mandibular canal came into contact with the external cortical bone( width of bone marrow space 0 mm).Neurosensory disturbance was observed on 1 of 4side with a bone marrow space of 0.3mm, on 1 of 4 side with a bone marrow space of 0.3mm, on 4 of 6 side with a bone marrow space of 0.6mm, on 4 of side with a bone marrow space of 0.9mm. Howerver, no neurosensory disturbance was observed on the 17 sides with a bone marrow spaces of 1.2 mm or more.Conclusion1.Compared with the control patients ,the mandibular ramus of the prognathism patients was thinner, and the width of the bone marrow space at the buccal was smaller. and the width of the bone marrow space at the buccal and the thickness of the mandible were positively significant correlated.2.The safest region in which to establish the medial osteotomy line was just 5mm superior to the lingular,and about 9.45mm posterior, with depth about 2mm,directing the line slightly inferior.3.On average, The mandibular were situated more lingulally at all sites,and the width of the bone marrow space at the buccal side was more narrow at 15-20mm bellow the mandibular foramen.4.Neurosensory disturbance was significantly more likely to be present When the width of the marrow space at buccal side was 0.9 mm or less. The increased risk of neurosensory disturbance associated when there was absent of the marrow space, especially the fusion type, the surgeon was suggested to select a procedure other than SSRO.
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