BIOMECHANICAL EVALUATION OF CASPAR CERVICAL SCREWS - COMPARATIVE STABILITY UNDER CYCLICAL LOADING

ANTERIOR CERVICAL INSTRUMENTATION is used as an adjunct to bone fusion ; however, definitive biomechanical data to support some applications and techniques are lacking. In the absence of supportive experimental data, posterior cortical penetration has been recommended with the Cas par system. Previously, we compared the axial pull-out strength of Cas par screws with and without posterior cortical penetration. This study compares the stability of unicortical versus bicortical screw penetra tion groups under cyclical loading simulating physiological flexion-ex tension. Caspar screws were placed in human cadaveric vertebrae with o r without posterior cortical purchase. Each screw was separately teste d, simulating flexion-extension to 200 cycles. Deformation time data a llowed a direct comparison of screw ''wobble' with and without posteri or cortical purchase. The mean deformation differences between subcort ical and bicortical groups were statistically significant and increase d over time within both groups. Enhanced stability was noted with bico rtical purchase throughout most of the examined range, becoming more p ronounced over longer periods of cyclical loading. Significant (P < 0. 05) increases in deformation over time were noted for both groups, sug gesting potentially significant deterioration at the screw-bone interf ace, despite bicortical purchase. Such deterioration with repeated fle xion-extension loading may be of concern in the use of Caspar plates i n the presence of multicolumn instability.