The removal torque and the loosening and
infection rates of AO/
ASIF 4.5 mm Schanz screws (MS) have been compared with those of
hydroxyapatite-coated (HA) and
hydroxyapatite/
silver phosphate-coated (HA/P) AO/
ASIF Schanz screws in vivo. In sheep, the left tibia was fixed after a midshaft
osteotomy by means of unilateral fixator with 6 Schanz screws in an antero-lateral position. The
osteotomy gap was filled with
bone wax to avoid bony overgrowth. After operation, the animals were allowed to walk around immediately. The loosening rate was 2.8% at the site of the two HA-coated screws and 36% at the site of the
metal screws. The 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. Histological and fluoroscopic evaluation revealed intimate bone-HA contact at the cortex near to the fixator, whereas at the site of the
metal screws a fibrous tissue layer was constantly found. Only in the cortex opposite to the
external fixator a direct bone-
metal interface was observed. Fluoroscopic analyses confirmed new bone formation next to the HA surface especially in the cortex adjacent to the fixator, whereas such phenomena were not detected adjacent to the surface of the
metal screws. Furthermore, bony resorption around the
metal screws (21%) was significantly greater than around the two coated screws (2%). These experimental data suggest that pin loosening can be reduced in
external fixators by coating the screws with
hydroxyapatite, especially if a long-term fixation is indicated.