Clinical results of rigid osseous fixation in sagittal split ramus osteotomy using a three dimensional simulation method for locating the mandibular canal after movement of the distal fragment

A few disadvantages of the rigid osseous fixation method of osteomized fragments in sagittal split ramus osteotomy are still being pointed out by some surgeons. One of the major problems is a disturbance of the inferior alveolar nerve caused by injury at the time of screw tightening and another is the displacement of the condyle.We have developed some techniques to alleviate these problems, for example, el repositioning technique of the proximal fragment of the mandibular ramus after splitting. This technique can maintain the pre-operative temporomandibular joint relationship after surgery. Another technique is the three dimensional simulation method to locate the position of the mandibular canal after movement of the distal fragment. This method can indicate the safety zone for screw tightening in the overlapping area between the distal and the proximal fragment before surgery by employing a simplified three dimensional model of the maxilla and the mandible.In this paper, the post-operative results of 64 cases (127 rami) using both techniques and the relationship between the mandibular canal and screws applied for fixation of osteomized fragments are investigated. The success rate of avoiding contact with the mandibular canal was 87.4%(111/127 rami). In order to improve the success rate, an indicator for the screw tightening position of the upper side of the mandibular canal was devised. In 51 of the cases this indicator was used and the success rate was raised to 92. 1%. Totally 379 screws were used. Only 22 screws contacted or crossed the mandibular canal. Most of them were applied to the space on the lower part of the mandibular canal, because the screw tightening position of the upper part of the mandibular canal was pinpointed by our indicator. However, the screw tightening position of the lower part of the mandibular canal was only determined visually.