Biomechanical evaluation of a newly developed monocortical expansion screw for use in anterior internal fixation of the cervical spine. In vitro comparison with two established internal fixation systems.

Abstract

The primary biomechanical stability of anterior internal fixation of the cervical spine obtained with a new monocortical expansion screw in vitro was evaluated. To determine whether the anterior internal fixation of the spine obtained with the new monocortical expansion screw provides biomechanical stability comparable with that obtained with bicortical fixation. The anterior plate instrumentation used with bicortical screw fixation in the cervical spine provides a primary stability superior to that associated with monocortical screw fixation. However, bicortical screws have the potential to perforate the posterior cortex. Therefore, monocortical instrumentation systems were developed, but without the biomechanical stability associated with bicortical systems. A new expansion screw for monocortical fixation was developed to improve biomechanical stability of monocortical systems. Three different internal fixation systems were compared in this study: 1) H-plate with AO 3.5-mm bicortical screws, 2) cervical spine locking plate with monocortical screws, and 3) H-plate with the new monocortical expansion screws. Eight fresh human cadaver spine segments from C4 to C7 were tested in flexion-extension, axial rotation, and lateral bending using pure moments of +/- 2.5 Nm without axial preload. Five conditions were investigated consecutively: 1) intact spine; 2) uninstrumented spine with the segment C5-C6 destabilized; 3-5) instrumentation of the segment C5-C6 with the three implants mentioned above after removal of the disc and insertion of an interbody spacer. Between bicortical and monocortical expansion screw H-plate fixation, no significant differences were observed in all load cases concerning range of motion and neutral zone. The neutral zone and range of motion were significantly larger for the cervical spine locking plate than for bicortical and monocortical expansion screw fixation in all load cases, except neutral zone for axial rotation versus bicortical screw fixation. The instrumented cases only had a significantly lower range of motion and neutral zone than the intact cases in extension-flexion, whereas for lateral bending and axial rotation no significant differences could be observed. Because the experimental design precluded any cyclic testing, the data represent only the primary stability of the implants. In anterior instrumentation of the cervical spine using a H-plate, the new monocortical expansion screw provides the same biomechanical stability as the bicortical 3.5-mm AO screw and a significantly better biomechanical stability than the cervical spine locking plate. Therefore, the expansion screw may be an alternative to the bicortical fixation and does not involve the risk of penetration of the posterior vertebral body cortex.