Image analysis of pedicle screw misplacement in axis

Objective:To investigate the causes of pedicle screw misplacement in axis and the way to ensure the accuracy.Method:From January 2002 to December 2006,50 patients suffering from upper cervical trauma and tumor and undergoing posterior surgery were included in this study.A total of 100 axis pedicle screws were placed.Based on preoperative slice CT scan results,the vertebral artery grooves were divided into 4 types(Ⅰ,Ⅱ,Ⅲ,Ⅳ).After operation,the pedicle trajectory's medial-inclination angle and superior-inclination angle were measured respectively on lateral radiograph and CT scan image.The pedicle screw misplacement were divided into two groups:type A,the vertebral artery groove violation;type B:the vertebral canal violation.Result:The 100 axis vertebral artery grooves included 76 type Ⅰ,10 type Ⅲ and 14 type Ⅳ.Type A malposition accounted for 8(8%),including 5 type Ⅰ,1 type Ⅲ,and 2 type Ⅳ;while type B malposition accounted for 2(2%),including 1 type Ⅲ and 1 type Ⅳ.With the malposition rate for type Ⅰ,Ⅲ and Ⅳ was 6.6%,20% and 21.4% respectively.The superior inclination angle for type A was 14.8°±8.5°,which was less than no malposition group(18.3°±9.7°);while that for type B was 17.5°± 6.8°,which showed no statistical difference compared with no malposition group(P0.05).The medial inclination angle for type A was 21.4°±7.1°,which was less than no malposition group(33.7°±7.3°)(P0.05);and that for type B was 44.9°± 8.8°,which was higher than no malposition group(P0.05).Conclusion:The anatomic abnormality for vertebral artery foramen is an objective factor to affect the accuracy of pedicle screw placement;less medial inclination angle and superior inclination angle are more prevalenting attention to vertebral artery foramen in axis based on CT scan and placement principle "upper better than down and introversion better than extroversion" can improve screw placement accuracy.