Defining the Differences in Transverse Plane Trajectories for Thoracic Pedicle Screw Insertion: Anatomic Versus Medial.

Study Design: Comparing thoracic pedicle screw trajectories, screw lengths, and starting points by examining osteologic specimens.

Objective: Describe a medial screw trajectory (MST) compared to a screw trajectory along the anatomic pedicle angle (APA) in terms of trajectory, screw length, and starting point.

Summary Of Background Data: Although thoracic pedicle screw insertion is commonly used for posterior fusion and instrumentation, there is little data to quantify an MST that avoids the great vessels and allows for greater screw purchase.

Methods: Thirty adult female skeleton thoracic vertebral columns from the University of New Mexico Maxwell Museum of Anthropology Osteology Collection were photographed from axial and right and left lateral views from T1 to T12. Axial plane measurements included APA and MST (both measured from the midline), screw lengths, and APA/MST intersection on the superior articular facet (SAF). The MST was defined as an insertion angle through the midpoint of the pedicle isthmus intersecting the anterior midpoint of the vertebral body. The intersection of each trajectory with the SAF was measured in relation to the lateral base of the SAF, reported as a percentage of the SAF base width from the lateral SAF border.

Results: At every vertebral level, the APA was different from the MST for angle, screw length, and SAF intersection (p < .0001), with the largest difference at T12. The T12 differences were APA versus MST angles (-25.5°, 95% CI -22.7° to -28.4°), screw lengths (11.0 mm, 95% CI 9.2 mm to 12.9 mm), and percentage of SAF width from the lateral border of the SAF base (38.6%, 95% CI 29.1% to 48.1%).

Conclusions: The MST was approximately 8° to 10° greater at T1-T10 (19° at T11 and 25° at T12) than the traditional APA insertion angle. This resulted in a much more lateral starting point on the SAF and longer screw length, greatest at T12.