MECHANICAL AND HISTOLOGICAL-EVALUATION OF HYDROXYAPATITE-COATED AO ASIF SCHANZ SCREWS FOR EXTERNAL FIXATION - AN ANIMAL-MODEL

The removal torque and the loosening and infection rates of AO/ASIF 4. 5 mm Schanz screws (MS) have been compared with those of hydroxyapatit e-coated (HA) and hydroxyapatite/silver phosphate-coated (HA/P) AO/ASI F Schanz screws in vivo. In sheep, the left tibia was fixed after a mi dshaft osteotomy by means of unilateral fixator with 6 Schanz screws i n an antero-lateral position. The osteotomy gap was filled with bone w ax to avoid bony overgrowth. After operation, the animals were allowed to walk around immediately. The loosening rate was 2.8% at the site o f the two HA-coated screws and 36% at the site of the metal screws. Th e removal torque after 2, 4 and 6 months was 6.2 +/- 2.8 MPa for HA/P, 4.7 +/- 2.7 MPa for HA screws and 1.2 +/- 1.1 MPa for metal screws. H istological and fluoroscopic evaluation revealed intimate bone-HA cont act at the cortex near to the fixator, whereas at the site of the meta l screws a fibrous tissue layer was constantly found. Only in the cort ex opposite to the external fixator a direct bone-metal interface was observed. Fluoroscopic analyses confirmed new bone formation next to t he HA surface especially in the cortex adjacent to the fixator, wherea s such phenomena were not detected adjacent to the surface of the meta l screws. Furthermore, bony resorption around the metal screws (21%) w as significantly greater than around the two coated screws (2%). These experimental data suggest that pin loosening can be reduced in extern al fixators by coating the screws with hydroxyapatite, especially if a long-term fixation is indicated.