Improvement of the bone-screw interface strength with hydroxyapatite-coated and titanium-coated AO/ASIF cortical screws

Objective: To evaluate whether coating AO/ASIF screws with osteoconductive materials can improve bone-screw fixation. Design: Ninety-six AO/ASIF 4.5-millimeter cortical screws were divided into four paired groups and implanted in the femurs and tibiae of six sheep: Group A = standard stainless steel screws; Group B = stainless steel screws coated with highly crystalline hydroxyapatite; Group C = stainless steel screws coated with low crystalline hydroxyapatite; Group D = titanium screws coated with titanium. The screws were implanted according to the standard AO technique to an insertion torque of 2,000 Newton-millimeters. Sheep were killed at one, three, and twelve months after surgery. Main Outcome Measures: Extraction torque was measured on six screws from each group selected at random at time of each euthanization. Morphologic analysis of the bone-screw interface was performed on the remaining screws. Results: At each euthanization the extraction torque of Group A was lower than that of the other groups (p <0.0001). At three and twelve months the extraction torque of Group B was higher than that of Group D (p = 0.002). Morphologic results showed extensive bone-screw gap in Group A. Optimal osteointegration was observed in Groups B and C. Osteointegration of Group D was higher than that of Group A and lower than that of Groups B and C. Conclusions: It was demonstrated that AO/ASIF screws coated with osteoconductive materials achieve optimal fixation strength, even in the early phase. This fixation strength was significantly higher than that of the standard screws.