[The effect of hydroxylapatite coating of AO/ASIF screws on bonding strength in bones]

Bonding strength of hydroxyapatite-coated (HA) 4.5 mm AO/ASIF screws and of conventional AO/ASIF screws to bone has been compared in an animal experiment. A 5-hole AO/ASIF compression plate was fixed to the tibia in ten beagle dogs. The cortex beyond the central hole of the plate was resected up to the half of the diameter of the tibial shaft and over a length of 10 mm. In the holes immediately proximal and distal to the central hole either HA-coated or conventional screws were inserted with standardized force. Two conventional screws were inserted into the two holes at the ends of the plate. These screws were removed after 2 weeks. Four animals were sacrificed after 4 and 8 weeks and one animal after 16 and 26 weeks. The removal torque of HA-coated screws was 385.50 +/- 49.1 Ncm, compared with 29.50 +/- 7.62 Ncm for the conventional screws (P < 0.0001; T-test). The fluorescence and morphometric data confirmed new bone formation immediately under the HA surface without any bone resorption or intervening fibrous tissue. At the bone-implant interface of conventional screws, however, extensive bone resorption was observed and a thick intervening layer of fibrous tissue. Bone resorption of 13.6% was observed in the cortical areas under the thread of the AO/ASIF screws, in contrast to 1.9% under the surface of the HA-coated screws. These histological and biochemical data support the clinical use of HA-coated screws if early loosening of the screws is otherwise expected or removal of implants is not intended.