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

Unlabelled: STUDY DESIGN.: Biomechanical testing of vertebral body screw pullout resistance with relevance to top screw pullout in thoracoscopic anterior scoliosis constructs.

Objective: 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.

Summary Of Background Data: Top screw pullout is a significant clinical problem in thoracoscopic anterior scoliosis surgery, with rates of up to18% reported in the literature.

Methods: 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).

Results: 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).

Conclusion: The positioning of the top screw in anterior scoliosis constructs can significantly alter its pullout resistance.