Introduction: Optimal strategies for fixation in the osteoporotic lumbar spine are not well established. Cortical pedicle screws have been reported to provide increased pullout strength. We hypothesize that cortical screws demonstrate superior kinematic properties when compared to pedicle screws in cadaveric specimens with osteoporosis.
Methods: Eight fresh frozen human spino-pelvic specimens with low average bone mineral densities (T score <= -2.5) were utilized. To effectively evaluate the bone/screw interface no interbody cages were used. Multidirectional flexibility testing was performed using a custom designed six degree of freedom spine simulator. Testing sequence consisted of:
1) intact spine
2) destabilized spine with laminectomy and bilateral facetectomies at L2-3 and L4-5
3) static testing #1 with cortical screw rod construct at L2-3 and pedicle screw construct at L4-5
4) Fluoroscopy of reconstructions
5) Fatigue testing with 50,000 cycles
6) Static testing #2 with cortical screws at L4-5 and pedicle screws at L2-3
7) Fluoroscopy and microradiography
8) Pull out testing of individual screws
Results: Under flexion-extension loading, the cortical screw fixation group (pre-fatigue) exhibited reduced segmental motion when compared to pedicle screw fixation (p<0.05). Additionally, post-fatigue comparisons of the segmental neutral zones
exhibited significantly less motion for cortical screws than transpedicular fixation (p<0.05).
When stratifying the pullout data with fixation technique and operative vertebral level, cortical screw fixation exhibited a marked increase in average load at failure in the lower vertebral segments, while transpedicular screw fixation demonstrated a reverse trend of lower failure loads in the inferior vertebral
elements and higher failure loads in the superior vertebral elements. (See figure).
Conclusions: Results from this study suggest that contact of screws with higher density bone within the pedicle illicit greater segmental fixation/stability. The cortical trajectory fixation constructs demonstrate this effect at lower lumbar levels due to the increased implant surface area exposure to higher density bone compared to standard pedicle screws.
Patient Care: providing techniques to improve fixation in patients with osteoporosis
Learning Objectives: To document the use and efficacy of cortical screw fixation as a viable alternative to traditional pedicle screw fixation in osteoporotic spines.