Biomechanical comparison of anterior cervical spine locked and unlocked plate-fixation systems

Three different anterior plate-fixation systems are available for the stabilisation of the cervical spine: (1) the cervical spine locking plate (CSLP), (2) dynamic plates allowing vertical migration of the fixation screws, and (3) various types of plates that are secured with either monocortical or bicortical unlocked screws. Unicortical screw purchase does not involve the risk of posterior cortex penetration and possible injuries to the spinal cord. The development of locking plates with unicortical screw-fixation and intrinsic stability of the screw-plate interface, via an angle-stabilised connection, was an attempt to increase the stability of unicortical screw-fixation systems. The aim of the study was to compare the biomechanical properties of a non-locking, anterior-plate system with 4.5 mm screw fixation and a locking anterior-plate system, in a single destabilised cervical spine-motion segment. Using fresh cadaveric cervical spine specimen C3-C7, multidirectional flexibility was measured at the C4-C5 level in an unconstrained test system, before and after destabilisation and fixation with an anterior plate with either locked or unlocked screw purchase. Direct comparison of the fixed cervical spine segments with unlocked and locked anterior-plate fixation did not demonstrate significant differences. This in vitro study documented that neither locked nor unlocked anterior-plate fixation can increase stability in all modes of testing. H-plate spondylodesis with unlocked screws seems to provide sufficient mechanical integrity in most cases of monosegmental lesions.