Hydroxyapatite dip coated and uncoated titanium poly-axial pedicle screws: an in vivo bovine model.

STUDY DESIGN: A 1-year-old calf was implanted with titanium pedicle screws either uncoated (n = 7) or coated with hydroxyapatite (n = 7) by the dipping method on the pedicles of vertebrae (t10-l3). OBJECTIVE: To evaluate biomechanical and histomorphologic responses to titanium pedicle screws uncoated and coated with hydroxyapatite. SUMMARY OF BACKGROUND DATA: Failure of fixation caused by loosening of pedicle screws is a problem in spinal surgery. Enhancement of the fixation ability of screws by coating with biocompatible materials may improve prognosis of surgery. METHODS: The calf was euthanized 4 months after implantation for determination of insertion and extraction torques for screws, and histologic and scanning electron microscopic examinations of areas screw embedded. RESULTS: Insertion torques did not differ by the kind of screws (99 +/- 5.7 Ncm). However, extraction torques for screws coated with hydroxyapatite were higher than for screws uncoated (249 vs. 133 Ncm, P < 0.01). As compared with uncoated screws, implanted areas for hydroxyapatite-coated screws were denser, had more cracks, and provided better bonding. Fibrous tissue and new bone formation were observed around the areas of uncoated and hydroxyapatite-coated screws embedded, respectively. CONCLUSIONS: Hydroxyapatite coating of titanium pedicle screws by the dipping method improved fixation and vertebral bone-implant interface, suggesting a decreased risk of a screw-loosening problem.