THE EFFECTS OF PEDICLE SCREW FIT - AN IN-VITRO STUDY

Study Design. The effects of pedicle screw size (major diameter and le ngth) on fixation stiffness in osteoporotic and nonosteoporotic verteb rae were evaluated in vitro. Methods. Lumbar vertebrae were obtained f rom two fresh frozen human spines. Bone mineral densities were determi ned using dual energy radiograph absorptiometry, followed by nondestru ctive mechanical testing of the specimens instrumented with pedicle sc rews. A loading technique was used that more closely mimics loading of pedicle screws in vivo. Results. Testing revealed that for good quali ty bone, screw size had a significant effect on fixation stiffness, bu t the effect of penetration depth depended on pedicle fill, and vice v ersa. In nonosteoporotic bone, the use of a longer screw increased fix ation stiffness if the screw filled up the pedicle by 70% or more. The use of wider screws increased the fixation stiffness if the penetrati on depth was 80% or more. Conclusion. Screw size had little or no effe ct on fixation stiffness in osteoporotic bone.