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

Study Design. The primary biomechanical stability of anterior internal fixa tion of the cervical spine obtained with a new monocortical expansion screw in vitro was evaluated. Objectives. To determine whether the anterior internal fixation of the spin e obtained with the new monocortical expansion screw provides biomechanical stability comparable with that obtained with bicortical fixation. Summary of Background Data. The anterior plate instrumentation used with bi cortical screw fixation in the cervical spine provides a primary stability superior to that associated with monocortical screw fixation. However, bico rtical screws have the potential to perforate the posterior cortex. Therefo re, monocortical instrumentation systems were developed, but without the bi omechanical stability associated with bicortical systems. A new expansion s crew for monocortical fixation was developed to improve biomechanical stabi lity of monocortical systems Methods. Three different internal fixation systems were compared in this st udy: 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 e xpansion screws. Eight fresh human cadaver spine segments from C4 to C7 wer e tested in flexion-extension, axial rotation, and lateral bending using pu re moments of +/-2.5 Nm without axial preload. Five conditions were investi gated consecutively: 1) intact spine; 2) uninstrumented spine with the segm ent 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 a n interbody spacer. Results. Between bicortical and monocortical expansion screw H-plate fixati on, no significant differences were observed in all load cases concerning r ange of motion and neutral zone. The neutral zone and range of motion were significantly larger for the cervical spine locking plate than for bicortic al and monocortical expansion screw fixation in all load cases, except neut ral zone for axial rotation versus bicortical screw fixation. The instrumen ted cases only had a significantly lower range of motion and neutral zone t han the intact cases in extension-flexion, whereas for lateral bending and axial rotation no significant differences could be observed. Because the ex perimental design precluded any cyclic testing, the data represent only the primary stability of the implants. Conclusions. In anterior instrumentation of the cervical spine using a H-pl ate, the new monocortical expansion screw provides the same biomechanicals stability as the bicortical 3.5-mm AO screw and a significantly better biom echanical stability than the cervical spine locking plate. Therefore, the e xpansion screw may be an alternative to the bicortical fixation and does no t involve the risk of penetration of the posterior vertebral body cortex.