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

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.