Evaluation The Split Patterns Of The Mandible In A Bilateral Sagittal Split Ramus Osteotomy

Purpose:To evaluate the split patterns of the mandibular ramus in sagittal split ramus osteotomy (BSSRO) using cone-beam computed tomography (CBCT), and examine the related anatomical features that influence these split patterns.Patients and methods:Research1:A total of130consecutive patients (62males and68females) in Stomatological Hospital of Nankai University who underwent a BSSRO between July2011and October2012were included in this study. The age range was17-44years (mean age,23.26±4.85years). The patients were excluded if they were treated with the mandibular angloplasty simultaneously, the unfavorable splits occur during the operation, or fracture story of the mandible. All the patients underwent a CBCT(Implagraphy, vatech, Korea) examination within3days before and1month after BSSRO. A BSSRO with a Hunsuck modification was performed in all patients. To categorize the split patterns of the mandibular ramus in a BSSRO, the gonion (Go) was defined as the boundary. And the cortical bone thickness of the ramus, the degree of the mandibular angle and the axial shapes of the ramus in different split types were observed at the same time. Satistical tests were performed using SPSS17.0software package.Research2:A total of304patients (126males and168females) in Stomatological Hospital of Nankai University with different maxillofacial deformities who underwent a BSSRO between May2010and August2013were included. The age range was17-44years (mean22.06±4.15years). The CBCT exiamination and operation were the same as the experiment1.Result:Research1:The BSSRO split patterns were artificially classified into two types:(1) Type Ⅰ, split at the lingual side nearby the mylohyoid sulcus (75.38%);(2) Type Ⅱ, split at the posterior border of the ramus (24.62%). The thickness of the cortical bone is uneven, and the thickness of the lingual cortical between the mandibular canal and the posterior border of the ramus is the thinnest. The axial shapes of the ramus can be artificially into three types:(1)half-crescent shape(61.54%);(2) sim-triangle shape (28.46%);(3)well-distributed shape(10.00%). The thickness of the cortical bone in the position c and d, the degree of the mandible, and the axial shapes of the ramus is significant different between type I and type Ⅱ pattern (P<0.05) through the single factor statistical analysis. The above four factors were significantly associated with the split pattern (P<0.05) through the Logistic regression analysis.Research2:The incidence of bad splits is3.78%(single bad splits,20sides,3.29%; multiple bad splits,3sides,0.49%). The single bad splits were classified into three types subsequently:(1) the ramus split at the buccal side (2.63%);(2) the superior corner of the proximal bone fracture (0.49%);(3) the lingual cortical bone fracture (0.16%). The multiple bad splits were classified into two types:(1) the ramus split at the buccal side, and the sigmoid notch happened fracture meantime (0.33%);(2) the ramus split at the buccal side, and the middle point of the proximal bone happened bad splits meantime (0.16%). The pathway of the axial buccal cortical bone of the ramus ("the buccal cortical bone travels posterior-lateral for a distance then directly posterior for the remainder" or "the buccal cortical bone travels posterior-lateral for a distance then posterior-medial for the remainder") and the pathway of the posterior border of the ramus (medial-posterior direction) can increase the risk of bad split accident. And lesser Angle formed by the middle bone cut and the posterior border of the ramus can also increased the risk of bad split accident.Conclusion:1. The normal BSSRO split patterns was affected by some anatomy factors that including the cortical bone thickness of the ramus, the degree of the mandibular angle and the axial shapes of the ramus.2. It may play an important role in predicting the BSSRO split patterns, if we examined the anatomy of the ramus with CBCT before the operation.3. The bad split of is difficult to avoid in a BSSRO. The bad split is significantly associated with the anatomy of the ramus and the operating factors.