A Biomechanical Study on the Fixational Strength of the Trans-pedicular Screw: In vitro measurement / 대한정형외과학회잡지

ABSTRACT

With porcine vertebrae, the static and dynamic holding power of the pedicle screws under various conditions were measured to understand the biomechanical nature of the transpedicular screw fixation in spine. The objectives of the present study were; (1) to find the correlation between the insertion depth of the screw and the resulting holding power, (2) to determine the effect of bone cement augmentation on the screw fixation in a loosened hole, and (3) to assess the load-sharing mechanism between the cortical and the cancellous one surrounding the screw in this fixational system. The geomorphological characteristics of each porcine vertebra was measured directly with a micro-caliper. The bone mineral density of the specimens was also measured. Material with screw was holded in the fixed cross head of material testing system(Autograph E-10T). Testing force was applied and graph was obtained in the chart record. The results of the static pull-out tests in this study showed that there was a statistically-significant positive correlation between the screw diameter and the pull-out resistance(p < 0.05). The strength of the fixation did not actually increase as much as the insertion depth of the screw increased in these tests(r=0.457). In low-cycle fatigue tests, the increased number of cycles was required to clinical failure in the deeper-inserted crews. Considering the mechanical failure, a statistically-significant positive correlation between the failure cycle and the insertion depth was observed in both deep and shallow insertion groups(p < 0.05). Nonpressurized PMMA augmentation appeared to restore the ability of the screws to withstand pullout loading of the original value. On the effect of the cortical and cancellous bone, the most important factor was the cortical bone of the entrance near the screw.