A study of low elastic modulus expandable pedicle screws in osteoporotic sheep.

Study Design: Low elastic modulus expandable pedicle screw (L-EPS) was inserted into osteoporotic sheep. Biomechanical tests, micro-CT analysis and histologic observation were performed. Objective: To find out whether the L-EPS can further improve fixation strength compared to the expandable pedicle screws (EPS) in osteoporotic sheep spine. The screw-bone interface is also detected by micro-computed tomography (CT) and histologic techniques. Summary of Background Data: There are some studies on EPS in vivo; however, no earlier study has focused on the elastic modulus of EPS, especially in osteoporosis. Methods: Twelve months after ovariectomy, bilateral pedicles of lumbar vertebrae (L1 to L5) of 4 female sheep were fixed with EPSs. The L-EPS and EPS were randomly placed into each pedicle and then were expanded. Six months later, the sheep were sacrificed and biomechanical tests, micro-CT analysis, and histologic observation were conducted on the isolated specimen vertebrae. Results: Twelve months after ovariectomy, animal model of osteoporosis was established successfully. The axial pull-out strength in L-EPS group was significantly enhanced compared with that in the EPS group (P < 0.05). Micro-CT reconstruction and analysis showed that there was more bone around the L-EPS group compared with those in EPS group (P < 0.05), meanwhile the more homogeneous bone formation distribution around the screws was found in the L-EPS group. Histologic observation showed that newly formed bone extended along the expandable fissures and grew into the center of EPS; meanwhile, the more direct contact and the less fibrous tissue on the screwbone interface were observed in the L-EPS group. Conclusions: The L-EPS can further improve the biomechanical fixation strength of EPS in the osteoporotic sheep. The screw elastic matching with surrounding bone is helpful to distribute stress uniformly, relieve the stress shielding effect, and strengthen the screw-bone interface. Although the experience with the L-EPS is very limited and preliminary, results to date indicate that it is of value in treating patients with osteoporosis and warrants further study.