MECHANICAL AND HISTOLOGICAL-EVALUATION OF HYDROXYAPATITE-COATED, TITANIUM-COATED AND GRIT-BLASTED SURFACES UNDER WEIGHT-BEARING CONDITIONS

Cylindric titanium rods with different surfaces were axially implanted into the femora of sheep. The three surfaces were grit-blasted titani um, plasma-sprayed titanium and plasma-sprayed hydroxyapatite (HA). Af ter 2 months, a 2-cm segment of the femoral shaft was completely resec ted to load the implant, and the animals were allowed full weight-bear ing for 9 months. Biomechanical and histological evaluation of the imp lants was undertaken 2 months after implantation and 9 months after th e segmental resection. The mechanical testings of well-fixed implants were performed 9 months after segmental resection. Loosening of 45% of the titanium-coated implants was observed in the first 3 weeks, but t hereafter, no further loosening occurred. The HA-coated implants remai ned entirely fixed for 3 weeks, but thereafter, a progressively increa sing incidence of loosening up to 55% after 9 months of loading was de tected as subsidence on X-radiographs. The maximum push-out strength o f the titanium-coated implants was 4.9 MPa compared with 2.3 MPa for H A-coated ones. No significant mechanical interlock between the grit-bl asted surface and bone was observed. The HA coating was found to be de laminated in all unstable implants, whereas the titanium coating remai ned completely intact. Morphometric analyses of well-fixed rods showed complete bony ingrowth onto the HA surface, whereas the contact area between the bone and the two titanium surfaces was less than 40%. Conc erning clinical significance bony ingrowth with long-term mechanical i nterlock between the implant surface and the bone cannot be achieved b y grit-blasting or HA-coating. The titanium plasma-coating, however, c an induce a bone-implant interface which resists the mechanical stress resulting from continuous cyclic loading in vivo.