An experimental study on transpedicular screw fixation in relation to osteoporosis of the lumbar spine

In order to elucidate the relationship between the severity of osteoporosis and the fixation strength of a pedicle screw, screw pull-out tests were performed using cadaveric lumbar vertebrae. The severity of osteoporosis was evaluated by the Jikei osteoporosis grading scale (Jikei method), bone mineral density, and microdensitometry. When a 7.0-mm screw was used, the pull-out force of the screw was 1,056.4 N in the normal group (as determined by the Jikei method), while it was 495.6 N in the Grade I osteoporosis and 269.5 N in the Grade II osteoporosis groups, respectively. There were also positive correlations between the pull-out force and bone mineral density and each parameter of the microdensitometry method. When bone cement was used in an osteoporotic vertebra, twofold stronger pull-out force was obtained in comparison to that obtained without bone cement.