Thoracoscopic Anterior Spinal Instrumentation and Fusion for Idiopathic Scoliosis: A CT Analysis of Screw Placement and Completeness of Discectomy

Texas Scottish Rite Hospital, Dallas, TX

INTRODUCTION: Thoracoscopic anterior instrumentation and fusion is gaining more widespread use in the treatment of idiopathic scoliosis. The adequacy of disc and endplate excision and the accurate positioning of instrumentation has not been previously studied.

PURPOSE: To evaluate the completeness of discectomy at each operated motion segment and to evaluate the position of the screws relative to the spinal canal and surrounding vital structures.

METHODS: Computed tomography (CT) examinations were performed following thoracoscopic anterior spinal fusion (ASF) and instrumentation in 12 patients with idiopathic scoliosis. The images were analyzed to determine the area of disc excision expressed as a percentage of the total surface area of the vertebral endplate. The position of each screw with respect to the thoracic aorta was evaluated. The distance from the tip of the screw to the aorta and the distance from the posterior aspect of the screw, in the mid-body, to the spinal canal was measured.

RESULTS: All 12 patients were female with an average age of 13.3 years (12.4 to 15.1) and a single right thoracic curve. The average preoperative coronal Cobb measurement was 55.9° (bend: 26.4°) with correction to 9.4° postoperatively. The average number of levels fused was 6.6 (5 to 8). The average area of disc excision was 73.3% (56% to 82%). There was no significant difference in the amount of disc excised at each motion segment, however, a trend toward less discectomy was seen at the two most distal levels. When each disc level was analyzed, a relative deficiency of discectomy was noted on the contralateral posterolateral region. 88 screws were used (ave 7.3/patient). For those screws placed at or above T9: 50% (22/44) of the tips of the screws were positioned posterior to the aorta compared to 93.2% (41/44) of those screws placed below T9. (P ‹ 0.05) The average distance from the screw to the aorta was 3.92 mm, however, 27.3% (24/88) of screws were adjacent to the aorta. The ave. distance from the spinal canal to the posterior edge of the screw was 4.6 mm and 3/88 broached the spinal canal.

CONCLUSIONS: Thoracoscopic instrumentation and fusion is technically demanding and relies on adequate visualization for discectomy and accurate screw placement. A thorough and safe discectomy can be obtained allowing for excellent correction and potential fusion. Safe screw placement was achieved with adequate distance from the spinal canal, however, screw proximity to the aorta was seen. Closer attention to appropriate screw length and orientation is necessary.