Plate fixation with hydroxyapatite-coated screws: a comparative loaded study.

The authors evaluated whether AO/ASIF screws coated with hydroxyapatite are better fixed than standard screws in a highly loaded plate fixation animal study. Twelve sheep were divided into two groups. The medial tibial middiaphysis was exposed and a 5-mm long bone cylinder was removed. The tibiae were fixed with six-hole dynamic compression plates. Six sheep received standard AO/ASIF stainless steel cortical screws (Group A), and six sheep received AO/ASIF stainless steel cortical screws coated with hydroxyapatite (Group B). Three months after surgery, the sheep were euthanized. The mean screw insertion torque was 4800 +/- 768 N/mm in Group A and 4847 +/- 450 N/mm in Group B. The mean screw extraction torque was 530 +/- 374 N/mm in Group A and 3733 +/- 849 N/mm in Group B. Extraction torque of Group A was significantly lower compared with the corresponding insertion torque. In Group B, there were no differences between extraction and insertion torque. Morphologic analyses showed marked fibrous tissue encapsulation in Group A and bone to screw direct contact in Group B. The results confirm that hydroxyapatite-coated AO/ASIF screws prevent deterioration of screw anchorage, even under highly loaded conditions. By using hydroxyapatite-coated screws, complications resulting from inadequate fixation could be avoided.