Evaluation of a more ventral starting point for thoracic pedicle screws: higher maximal insertional arc and more medial and safer screw angulation

OBJECTIVE To demonstrate that a more ventral starting point for thoracic pedicle screw insertion, produced by aggressively removing the dorsal transverse process bone down to the superior articular facet (SAF), results in a larger margin for error and more medial screw angulation compared to the traditional dorsal starting point (DSP). The margin for error will be quantified by the maximal insertional arc (MIA). METHODS The study population included 10 consecutive operative patients with adult idiopathic scoliosis who underwent primary surgery. All measurements were performed using 3D visualization software by an attending spine surgeon. The screw starting points were 2 mm lateral to the midline of the SAF in the mediolateral direction and in the center of the pedicle in the cephalocaudal direction. The DSP was on the dorsal cortex. The ventral starting point (VSP) was at the depth of the SAF. Measurements included distance to the pedicle isthmus, MIA, and screw trajectories. RESULTS Ten patients and 110 vertebral levels (T1-11) were measured. The patients' average age was 41.4 years (range 18-64 years). The pedicle isthmus was largest at T1 (4.04 +/- 1.09 mm), and smallest at T5 (1.05 +/- 0.93 mm). The distance to the pedicle isthmus was 7.47 mm for the VSP and 11.92 mm for the DSP (p < 0.001). The MIA was 15.3 degrees for the VSP and 10.1 degrees for the DSP (p < 0.001). Screw angulation was 21.7 degrees for the VSP and 16.8 degrees for the DSP (p < 0.001). CONCLUSIONS A more ventral starting point for thoracic pedicle screws results in increased MIA and more medial screw angulation. The increased MIA represents an increased tolerance for error that should improve the safety of pedicle screw placement. More medial screw angulation allows improved triangulation of pedicle screws.