Biomechanical Investigation of a Novel Revision Device in an Osteoporotic Model: Pullout Strength of Pedicle Screw Anchor Versus Larger Screw Diameter

Abstract

Study Design: In vitro cadaveric biomechanical study. Objective: To assess revision pullout strength of novel anchored screws (AS) versus conventional larger diameter traditional pedicle screws (TPS) in an osteoporotic model. Summary of Background Data: Pedicle screws are the most ubiquitous method of treating spinal pathologies requiring lumbar fusion. Although these screws are effective in providing 3-column stabilization of the spine, revision surgeries are occasionally necessary, particularly for geriatric and osteoporotic populations. Innovative technologies should be tested to ensure continued improvement in revision techniques. Methods: For 4 specimens at L2-L5 (T-score=-3.6±0.54), 6.5-mm-diameter TPS were inserted into left and right pedicles and were pulled out; revision screws were then inserted. Polyether-ether-ketone anchors, designed to expand around a 6.5-mm screw, were inserted into all left pedicles. On the contralateral side, 7.5-mm-diameter TPS were inserted at L2-L3, and 8.5-mm-diameter TPS at L4-L5. Pullout testing was performed at 10 mm/min. The maximum pullout strength and insertion forces were recorded. Results: The initial average pullout force (6.5-mm screw) was 837 N (±329 N) and 642 N (±318 N) in L2-L3 and L4-L5 left pedicles, and 705 N (±451 N) and 779 N (±378 N) in L2-L3 and L4-L5 right pedicles, respectively. Comparison of revision pullout forces versus initial pullout forces revealed the following: 87% and 63% for AS in L2-L3 and L4-L5 left pedicles, respectively; 56% for 7.5-mm and 93% for 8.5-mm TPS in L2-L3 and L4-L5 right pedicles, respectively. Conclusions: Anchor sleeves with 6.5-mm-diameter pedicle screws provided markedly higher resistance to screw pullout than 7.5-mm-diameter revision screws and fixation statistically equivalent to 8.5-mm-diameter screws, possibly because of medial-lateral expansion within the vertebral space and/or convex filling of the pedicle. AS results had the lowest SD, indicating minimal variability in bone-screw purchase.