Pull-out strength of the suprapedicle claw construct: a biomechanical study

Abstract

The pull-out of the superior screw is a well recognized problem in anterior instrumentation of the spine for scoliosis. A biomechanical pull-out study of anterior vertebral body screw in cadaveric thoracic spine was therefore designed to investigate and compare the pull-out strength of three different anterior vertebral body fixations using the AO Universal Spine System: simple bicortical screw, bicortical screw with an opposite washer (sometimes called pull-out resistant nut), and a new construct made of a bicortical screw with the addition of a suprapedicular hook on the same vertebra (or claw construct). The T4 to T9 vertebral bodies from six human cadavers (total of 36 specimens) were instrumented with three different instrumentation constructs after measuring the bone mineral density of each individual vertebra. After stabilization of the vertebral bodies, the screws were extracted employing a material testing system using axial pull-out. The maximum axial forces were recorded at the time of the construct failure. The mean ultimate fixation strength (UFS) values after being adjusted for bone mineral density and vertebral body diameter were 631, 711, and 1244 N for the three different constructs, respectively (screw alone, screw with an opposite washer, and screw with a suprapedicle claw). The difference in UFS was not significant for the first two constructs tested (screw alone and screw with an opposite washer). However, the difference in ultimate fixation strength between the claw and the other constructs was highly significant (P<0.0001). Specimens with low BMD did not benefit as much from claw construct as the ones did with a normal BMD. The failure mode of each construct was described, but was in neither case judged dangerous for the spinal cord. This study shows that the suprapedicle claw construct improves the pull-out strength of an anterior vertebral body screw by 80%, and changes the mode of failure so as not to rely only on the screw characteristics or solely on the vertebral body. By adding a suprapedicle hook in a claw configuration, one may prevent superior screw pull-out in anterior spine surgery for scoliosis.

Fig. 1

AP X-ray and saw bone representation of the superior claw construct. The claw construct consists of an anterior vertebral body screw attached to a rod, with a hook inserted just proximal to the screw over the pedicle. Compression between the screw and the hook forms a claw construct

Fig. 2

Each instrumented vertebra was gripped with pneumatic holding clamps. The constructs were mounted on a rod in order to apply axial pull-out forces at the two ends of the rod (see arrow a). The pull-out test was performed with a material testing machine. VB vertebral body

Fig. 3

Ultimate fixation strength (UFS) plotted for each implant group, and adjusted for bone mineral density (BMD) and coronal vertebral body dimension

Fig. 4

The effect of BMD on the pull-out forces of the three constructs. Significant first order interaction term exists between BMD and the three constucts group. With low bone mineral density the axial pull-out forces tend to be the same