THE ROLE OF IMAGING AND IN-SITU BIOMECHANICAL TESTING IN ASSESSING PEDICLE SCREW PULL-OUT STRENGTH

Study Design. This study determined the predictive ability of quantita tive computed tomography, dual energy x-ray absorptiometry, pedicular geometry, and mechanical testing in assessing the strength of pedicle screw fixation in an in vitro mechanical test of intrapedicular screw fixation in the human cadaveric lumbar spine. Objective. To test sever al hypotheses regarding the relative predictive value of densitometry, pedicular geometry, and mechanical testing in describing pedicle scre w pull-out. Summary of Background Data. Previous investigations have s uggested that mechanical testing, geometry, and densitometry, determin ed by quantitative computed tomography or dual energy x-ray absorptiom etry, predict the strength of the screw-bone system. However, no study has compared the relative predictive value of these techniques. Metho ds. Forty-nine pedicle screw cyclic-combined flexion-extension moment- axial pull-out tests were performed on human cadaveric lumbar vertebra e. The predictive ability of quantitative computed tomography, dual en ergy x-ray absorptiometry, insertional torque, in situ stiffness, and pedicular geometry was assessed using multiple regression. Results. Se veral variables correlated to force at failure. However, multiple regr ession analysis showed that bone mineral density of the pedicle determ ined by quantitative computed tomography, insertional torque, and in s itu stiffness when used in combination resulted in the strongest predi ction of pull-out force. No other measures provided additional predict ive ability in the presence of these measures. Conclusions. Pedicle de nsity determined by quantitative computed tomography when used with in sertional torque and in situ stiffness provides the strongest predicti ve ability of screw pull-out. Geometric measures of the pedicle and de nsity determined by dual energy x-ray absorptiometry do not provide ad ditional predictive ability in the presence of these measures.