The role of imaging and in situ biomechanical testing in assessing pedicle screw pull-out strength.
METHODS
This study determined the predictive ability of quantitative 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 intra-pedicular screw fixation in the human cadaveric lumbar spine.


OBJECTIVE
To test several hypotheses regarding the relative predictive value of densitometry, pedicular geometry, and mechanical testing in describing pedicle screw pull-out.


BACKGROUND
Previous investigations have suggested that mechanical testing, geometry, and densitometry, determined by quantitative computed tomography or dual energy x-ray absorptiometry, predict the strength of the screw-bone system. However, no study has compared the relative predictive value of these techniques.


METHODS
Forty-nine pedicle screw cyclic-combined flexion-extension moment-axial pull-out tests were performed on human cadaveric lumbar vertebrae. The predictive ability of quantitative computed tomography, dual energy x-ray absorptiometry, insertional torque, in situ stiffness, and pedicular geometry was assessed using multiple regression.


RESULTS
Several variables correlated to force at failure. However, multiple regression analysis showed that bone mineral density of the pedicle determined by quantitative computed tomography, insertional torque, and in situ stiffness when used in combination resulted in the strongest prediction of pull-out force. No other measures provided additional predictive ability in the presence of these measures.


CONCLUSIONS
Pedicle density determined by quantitative computed tomography when used with insertional torque and in situ stiffness provides the strongest predictive ability of screw pull-out. Geometric measures of the pedicle and density determined by dual energy x-ray absorptiometry do not provide additional predictive ability in the presence of these measures.
Myers
B S
BS
Department of Biomedical Engineering, Duke University, Durham, North Carolina, USA.
Belmont
P J
PJ
Richardson
W J
WJ
Yu
J R
JR
Harper
K D
KD
Nightingale
R W
RW
D015502:Absorptiometry, Photon; D001696:Biomechanical Phenomena; D015519:Bone Density; D001863:Bone Screws; D002102:Cadaver; D006801:Humans; D008422:Materials Testing; D011237:Predictive Value of Tests; D013131:Spine / Q000000981:diagnostic imaging / Q000601:surgery; D014057:Tomography, X-Ray Computed; D019415:Torque