Vertebral pedicle diameter as determined by computed tomography: inaccuracies observed by direct measurement of cadaveric lumbar spine.

The isthmic diameter of the vertebral pedicle is an important dimension affecting the stability of transpedicular screwing. The isthmic diameter of the pedicle was measured in the lumbar spines of 18 Japanese cadavers (10 male, 8 female) by axial computed tomographic (CT) scan and directly with calipers. In L1 and L2, there was no significant difference between the average value of the isthmic diameter as determined by the two methods (Student's t-test). However, in L3-5, the average value of the isthmic diameter as determined by axial CT scan was significantly greater than that given by caliper measurement. In view of the fact that caudally the longitudinal axis of the pedicle diverges towards the mid-line of the vertebral body in the coronal plane, the plane of the CT scan cannot be parallel to the real isthmic diameter of the pedicle. Therefore, it is suggested that below L2 the diameter as determined by axial CT scanning is greater than the real isthmic diameter.