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.%背景：在临床应用中，第5腰椎人字嵴结构不清，往往需扩大显露范围，通过横突、上下关节突等结构明确进钉点，增加了创伤和医源性上位椎间退变的风险。因此有必明确一个微创的L5椎弓根钉进钉点。目的：通过影像学评价以L5乳突“斜坡”作为进钉点的L5椎弓根螺钉置入准确性。方法：第5腰椎上关节突基底部的乳突向后突起的最高点向内下方延伸过程中形成一斜面，将其定义为乳突“斜坡”；其内侧为椎弓根部内上缘，向外比邻横突根部，位于人字嵴结构顶点的略上方，“斜坡”是椎旁肌间隙入路时可最先触及且显露的骨性部位。以乳突“斜坡”为进钉点，对50例腰椎退行性疾病患者行 L5椎弓根螺钉置入内固定，根据术前CT和X射线透视确定椎弓根钉矢状面和横断面的进钉方向。椎弓根钉直径均为6.5 mm。修复过程中评价其一次性置钉成功率，置钉后行腰椎正侧位X射线片和CT检查，评价置钉准确率。结果与结论：应用乳突“斜坡”定位法的一次性置钉成功率为96%(96/100)；于L5椎体内共置入100枚椎弓根螺钉，根据Gertzbein等推荐的标准，95枚(95%)螺钉完全位于椎弓根内，5枚(5%)螺钉均向内侧穿透椎弓根皮质，其中3枚(3%)螺钉穿透距离<2 mm，2枚(2%)螺钉穿透距离为2-4 mm，均未出现神经根损伤等并发症。提示L5乳突“斜坡”定位法的一次性置钉成功率高，结合置钉前的X射线片和CT评价可获得较高的置钉准确率。
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