Morphologic evaluation of the thoracic vertebrae for safe free-hand pedicle screw placement in adolescent idiopathic scoliosis: a CT-based anatomical study.

OBJECTIVE: To evaluate the morphologic characteristics of the thoracic pedicle with regard to safe free-hand thoracic pedicle screw placement, based on multi-planar reconstruction CT images.
METHODS: Thirty adolescent idiopathic scoliosis (AIS) patients who had undergone posterior corrective surgery for major thoracic curve were included in this study. Reconstructed CT axial images at each thoracic vertebra were used to measure: (1) the shortest distance from an entry point to the ventral cortex of the lamina (critical distance), and (2) the distance from the entry point to the tangent of the spinal canal at the medial wall of the pedicle (safe distance). The critical length was defined as the distance between the critical distance and the safe distance. The distance from the entry point to the medial wall breach site (breach distance) was measured on post-operative CT images.
RESULTS: The mean critical distance was 9.3 ± 1.1 mm. The critical distance of vertebrae from different levels was relatively constant, between 8.1 and 10.1 mm. The mean safe distance was 15.2 ± 1.3 mm. The safe distance of vertebrae from different levels was also relatively constant, between 14.5 and 16 mm. The mean critical length was 5.9 ± 1.0 mm. The critical length of vertebrae between T3 and T12 was relatively constant, ranging from 5 to 6.5 mm. The mean breach distance was 12.3 ± 1.3 mm and the each breach always recognized between the critical distance and the safe distance.
CONCLUSIONS: The risk of pedicle medial wall perforation increases as the pedicle probe advances beyond the critical distance of 8-10 mm from the entry point, while it decreases entering into the safe distance at 14.5-16 mm. These parameters were relatively constant even in the most rotated vertebrae at T9 or those with the narrowest pedicle at T7 or T4.