Biomechanical Fixation Properties of the Cortical Bone Trajectory in the Osteoporotic Lumbar Spine.

OBJECTIVE: Selecting optimal strategies for improving fixation in osteoporotic lumbar spine is an important issue in clinical research. Cortical bone trajectory (CBT) screws have been proven to enhance screw pullout strength, but biomechanical efficacy of these screws remains understudied. The aim of this study was to evaluate biomechanical efficacy of CBT screws in osteoporotic lumbar spine.

METHODS: Thirty-one vertebrae from 14 cadaveric lumbar spines were obtained. All specimens were measured by computed tomography; the diameter of pedicles, excluding those of vertebral bodies with very small pedicle developments, was calculated. After measuring bone mineral density, the CBT screw was randomly inserted into 1 side, and the traditional trajectory (TT) screw was inserted into the contralateral side. Maximum insertional torque was recorded after screw insertion. Of vertebrae, 21 were subjected to pullout testing at a rate of 5 mm/minute, and 10 were subjected to cyclic fatigue testing. Each construct was loaded until exceeding 5 mm.

RESULTS: Average bone mineral density was 0.567 ± 0.101 g/cm2 . CBT screws had higher maximum insertional torque (degrees of freedom = 30, t = 5.78, P < 0.001, 0.333 N-m vs. 0.188 N-m) and higher axial pullout strength (degrees of freedom = 20, t = 7.41, P < 0.001, 394 N vs. 241 N) than TT screws. Increased bone mineral density was not significantly associated with higher pullout load. Compared with TT screws, CBT screws showed better resistance to fatigue testing and required more cycles to exceed 5 mm (degrees of freedom = 9, t = 5.62, P < 0.001, 6161 cycles vs. 3639 cycles). Failure load for displacing screws was also significantly greater for CBT screws than for TT screws (degrees of freedom = 9, t = 5.75, P < 0.001, 443 N vs. 317 N).

CONCLUSIONS: CBT screws had better biomechanical fixation in osteoporotic lumbar spine compared with standard pedicle screws.