Biomechanical Study of Top Screw Pullout in Anterior Scoliosis Instrumentation

ABSTRACT ID 139\ud \ud BIOMECHANICAL STUDY OF TOP SCREW PULLOUT IN ANTERIOR SCOLIOSIS CORRECTION\ud \ud INTRODUCTION Failure of the top screw in anterior scoliosis correction is a common problem with rates of 5-15% reported in the literature. Previous biomechanical studies of anterior vertebral body screws have evaluated their resistance to either straight pullout or cephalo-caudad compression forces with different definitions of failure. The aim of this study was to assess screw resistance to more realistic loading conditions, namely cephalo-caudad compression between adjacent screw heads, followed by a pullout force directed in an arc centred on the screw below with the construct’s rod as it's radius. The effects of degree of compression, screw angulation, and pullout arc radius on vertebral body screw pullout strength were investigated.\ud \ud METHOD Four tests were performed using sawbone and one with sheep vertebrae. Firstly, straight pullout tests of screws inserted at different angles were performed to define optimum angle. Secondly, a test rig that allowed the screws to be pulled out in an arc of four different radii tested screws inserted straight into the sawbone. Thirdly, the rig was used to test screws inserted at different angles (from 20 caudad to 20 cephalad in steps of 10). Fourthly, a surgeon performed 3 compressions of screws inserted straight into a strain gauged model to assess maximum compression force. Screws inserted into sawbone with or without a staple then had the maximum force achieved by the spinal surgeon applied to them prior to pullout testing. Finally an "ideal" configuration for the screws was decided, and 29 sheep vertebral bodies were instrumented either in the traditional or new "ideal" way to compare pullout strength between the two groups. Results were analysed using pooled t-tests.\ud \ud RESULTS Sawbone testing revealed that increasing the radius of pullout increased the pullout strength. With a radius of 23mm (the average distance between a T5 and T6 vertebral body screw) a mean maximum pullout force of 392 N was required. For a radius of 33mm (the maximum achievable distance between a T5 and T6 vertebral body screw in an average sized person) the mean maximum pullout force increased to 578N (n=3, P<0.01).\ud An angle of 10 in a cephalad direction was the strongest (at a radius of 33mm the mean maximum pullout of a 10 cephalad screw was 615N compared with 531N at 20 cephalad (n=3, P<0.01) and 578N at 0 (n=3, P<0.05).\ud \ud The maximum compression force achieved by the spinal surgeon was just less than 800N. Non-compressed screws were strongest and those compressed without staples weakest.\ud Sheep vertebrae instrumented in a traditional manner (23mm between screws inserted parallel to the endplates n=15) achieved pullout = 993N compared with our "ideal" configuration (33mm between screws inserted at 10 cephalad angle to the endplate n=14) 1864N (P