Biomechanical pull-out strength of the cervical spine locking plate (CSLP) expansion screws

This study was designed to evaluate the biomechanical pull-out strength and mechanism of pull-out in cantilever bending of the Synthes cervical spine locking plate (CSLP) using a porcine spine model of four different length 4.35 mm diameter expansion screws and the 3.5 mm bicortical cancellous screw with and without end plates removed. Ten constructs using each screw length were tested in single cycle cantilever bending by loading the free end of the plate in both flexion and extension, with vertebral end plates both intact and violated. No discernable yield during loading in extension was observed while a yield was observed in flexion. Loads at specific displacements were greater once the end plates had been violated. The load experienced by unicortical screws of long constructs using cantilever bending was greater with the end plate violated. This was not expected, but is explained by the viscoelastic behavior of bone. This deformation mechanism accounts for the higher loads necessary for failure when the end plates are removed. In light of these results, the addition of a 16 mm or longer 4.35 mm expansion screw seems warranted, keeping in mind that the 18 mm and longer screws have some threat of penetrating the posterior cortex of the vertebral body.