Vertebral rotation and pedicle length asymmetry in the normal adult spine.

Rotation in the horizontal plane of vertebra T8, T9 or T10 was determined on CT scans of 25 male and 25 female patients with normal spines. The pedicle length was measured using a new method, and the right/left pedicle length index was calculated. In 38 (76%) of the patients there was vertebral rotation to the right with a mean Cobb angle of 3.0 degrees, and in 4 (8%) rotation to the left, mean Cobb angle 2.2 degrees (P < 0.01). In 8 (16%) there was no measurable rotation. The pedicle length index was greater than 1.05 in 9 subjects, between 0.95 and 1.05 in 16 and less than 0.95 in 25, indicating a predominance of longer pedicles on the left side. In 21 out of the 38 patients with vertebral rotation to the right, the left pedicle was longer than the right one (P < 0.01). The results indicate that the normal spine is afflicted with a vertebral rotation to the right in association with a longer pedicle on the left. The significance of these observations for the pathogenesis of idiopathic scoliosis remain uncertain.