A biomechanical study of top screw pullout in anterior scoliosis correction constructs.

Abstract

STUDY DESIGN.: Biomechanical testing of vertebral body screw pullout resistance with relevance to top screw pullout in thoracoscopic anterior scoliosis constructs. To analyze the effect of screw positioning and angulation on pullout resistance of vertebral body screws, where the pullout takes place along a curved path as occurs in anterior scoliosis constructs. Top screw pullout is a significant clinical problem in thoracoscopic anterior scoliosis surgery, with rates of up to18% reported in the literature. A custom-designed biomechanical test rig was used to perform pullout tests of Medtronic anterior vertebral screws where the pullout occurred along an arc of known radius. Using synthetic bone blocks, a range of pullout radiuses and screw angulations were tested, in order to determine an "optimal" configuration. The optimal configuration was then compared with standard screw positioning using a series of tests on ovine vertebrae (n=29). Screw angulation has a small but significant effect on pullout resistance, with maximum strength being achieved at 10-degree cephalad angulation. Combining 10-degree cephalad angulation with maximal spacing between the top 2 screws (maximum pullout radius) increased the pullout resistance by 88% compared with "standard" screw positioning (screws inserted perpendicular to rod at midbody height). The positioning of the top screw in anterior scoliosis constructs can significantly alter its pullout resistance.