Study design: The effects of pedicle screw size (major diameter and length) on fixation stiffness in osteoporotic and nonosteoporotic vertebrae were evaluated in vitro.
Methods: Lumbar vertebrae were obtained from two fresh frozen human spines. Bone mineral densities were determined using dual energy radiograph absorptiometry, followed by nondestructive mechanical testing of the specimens instrumented with pedicle screws. A loading technique was used that more closely mimics loading of pedicle screws in vivo.
Results: Testing revealed that for good quality bone, screw size had a significant effect on fixation stiffness, but the effect of penetration depth depended on pedicle fill, and vice versa. In nonosteoporotic bone, the use of a longer screw increased fixation stiffness if the screw filled up the pedicle by 70% or more. The use of wider screws increased the fixation stiffness if the penetration depth was 80% or more.
Conclusion: Screw size had little or no effect on fixation stiffness in osteoporotic bone.