Cervical transfacet versus lateral mass screws: a biomechanical comparison.

Abstract

The authors directly the compared biomechanical pullout strength of screws placed in the cervical lateral masses to that of screws placed across the facet joints. Posterior cervical fixation with lateral mass plates is an accepted adjunctive technique for cervical spine fusions. Altered anatomy resulting from congenital malformation, tumor, trauma, infection, or failed lateral mass fixation may limit traditional screw placement options. Transfacet screw placement, which has been studied extensively in the lumbar spine, may offer an alternative when posterior cervical fusion is required. Ten fresh human cadaveric cervical spines (postmortem age range, 69 to 91 years) were harvested. On one side, transfacet screws were placed at the C3-4, C5-6, and C7-T1 levels. On the other side, lateral mass screws were placed at the C3, C5, and C7 levels. The screw insertion technique at each level was randomized for right or left. After screw placement, each set of vertebral bodies were dissected and mounted in a custom jig for axial pullout testing using a servohydraulic testing machine. The load-displacement curves were obtained for each screw pullout. The mean pullout strength for the screws placed across the facets was 467 N (range, 192 to 1,176 N). This compares with 360 N (range, 194 to 750 N) for the lateral mass screws (p = 0.008). At each level, transfacet screws exhibited greater pullout resistance compared with the lateral mass placement, but the difference was most pronounced at the C7-T1 level (lateral mass = 373 N, transfacet = 539 N, p = 0.042). Cervical transfacet screw placement provides pullout resistance that is comparable to, if not greater than, lateral mass placement. This type of placement, although technically difficult, may be an alternative to lateral mass screws in cases with unusual anatomy, stripped screws, or when additional intermediate points of fixation are desired.