Pedicle Screw Safety: How Much Anterior Breach Is Safe?: A Cadaveric and CT-Based Study.

Abstract

Clinical retrospective chart review and basic science study. To determine the safety limits of an anterior/anterorolateral misplaced pedicle screw on computed tomography (CT) scan in spinal deformity. Although the limits of medial breaches (<4 mm) are known, the safe limits for anterior/anterolateral breaches in spine deformity are not yet defined. The present study had two parts. In part I, postoperative CT scans of 165 patients operated on for spine deformity were reviewed for screw misplacement (2800 screws). The amount of anterior/anterolateral breach was measured. Protrusions were also evaluated for proximity to vital structures. All scans were reviewed by musculoskeletal radiologist. In part II, eight cadavers were instrumented with 6 × 30 and 6 × 40 mm bilaterally from T1-S1. Screws were randomly inserted under navigation guidance either "IN" or "OUT-anterior/lateral." CT scan was performed, followed by gross dissection to determine screw position. Part I: 116(4.2%) screws were misplaced anterior/anterolaterally. Thirty-one (26.7%) were adjacent to vital structures. Fisher exact test showed 4 mm or less breach has significantly lower likelihood of impingement (P < 0.001). Screws adjacent/impinging the aorta protruded an average 5.7 ± 0.6 mm, whereas screws not involving the aorta breached an average 3.9 ± 0.2 mm, (P < 0.001). Part II: 285 screws were inserted. On CT scan, 125 were misplaced anterior/anterolaterally. On gross dissection, 89 were visibly misplaced; 23 were covered entirely by soft tissue but were palpable; and 13 were contained in bone. All 23 screws did not endanger any structures and protruded less than 4 mm on CT scan. Anterior/anterolateral breaches of 4 mm or less on CT poses no significant risk of impingement and therefore can be considered safe. 3.