Fracture risk during pedicle screw insertion in osteoporotic spine.

Although using a larger screw can enhance the stability of screw-bone interface in pedicular screw instrumentation, it may cause pedicle fracture during screw insertion in osteoporotic spine. We investigated structural changes of the pedicle with the advance of osteoporosis and its implication for the risk of pedicle fracture. Bone mineral density of trabecular, subcortical, and cortical bone of pedicle in an osteoporotic group, assessed by peripheral quantitative computed tomography, was significantly lower than that of those in the normal group. Cortical thickness also declined. There were no pedicle fractures in normal vertebrae, but seven (41.2%) fractures were observed among 17 pedicles in osteoporotic vertebrae in which bone mineral density measured with dual-energy x-ray absorptiometry was <0.7 g/cm2 and screw diameter >70% of the outer diameter of the pedicle. Our results suggest that screw diameter should not exceed 70% of the outer diameter of the pedicle in the case of osteoporosis in which vertebral bone mineral density is <0.7 g/cm2.