BACKGROUND:In clinical application, the structure of crista lambdoidalis of L5 was unclear. It needs to expose more tissue to define L5 entry point through transverse process or superior and inferior articular process. This increased the risk of trauma and iatrogenic superior intervertebral degeneration. Therefore, it is necessary to expose L5 entry point with a minimaly invasive way. OBJECTIVE:To investigate the accuracy of L5 pedicle screw insertion with the entry point of mastoid process slope by imaging. METHODS:Mastoid process was located on the base of L5 superior articular process. A cant was formed when the highest point of L5 mastoid process backward protuberance extended inwards and downwards. The cant was defined as mastoid process slope; it was lateral to pedicle medial superior side internaly, medial to transverse process root and superior to the top of crista lambdoidalis. The slope was first easily touched and exposed in lumbar posterior surgery through paraspinal muscle space approach. Fifty patients of lumbar spine disorders were treated by L5 pedicle screws fixation through the entry point of mastoid process slope. According to preoperative radiographic and CT images, pedicle screw insertion direction of the sagittal and transverse sections was calculated. The diameter of pedicle screw was 6.5 mm. The condition of intraoperative successful rate of screws placement at one time was analyzed. The accuracy of screw placement was evaluated by postoperative radiographic and CT images. RESULTS AND CONCLUSION:With the method of the mastoid process slope, the successful rate of screw placement at one time was 96% (96/100). Totaly 100 screws were inserted into L5. According to the criterion by Gertzbein, 95 screws (95%) totaly located in pedicles and 5 screws (5%) encroached on the pedicle from medial wal. Three (3%) out of 5 inaccurately placed screws cut out less than 2 mm of the inner wal, while 2 (2%) between 2 mm and 4 mm, without neurologic deficits. The method of mastoid process slope had a high successful rate of screw placement. Combined with preoperative X-ray films and CT images could obtain a high accuracy rate of screw insertion.