Abstract
Introduction: After more than 10 years of clinical experience, hydroxyapatite (HA) coated orthopaedic implants are now an established, viable alternative to porous coatings for achieving good implant fixation. However, developments are continuing to improve the nature and adhesion of the HA coating.
Aim: To investigate the biological attachment characteristics of titanium and highly crystalline HA implant coatings in the metaphysis of an animal model.
Method: Titanium alloy implants with a coating of commercially pure titanium (Ti) or highly crystalline HA were evaluated by light microscopy and pullout tests after one, two and four weeks of unloaded implantation in the tibial and femoral metaphyses of 18 sheep.
Results: The interface shear strength pullout increased from approximately 29N/cm2 at one week to approximately 326N/cm2 at two weeks. At four weeks the pull-out strength for Ti and HA coated implants was 1,004.87 ± 189.82N/cm2 and 1,043.26 ± 260.61N/cm2 respectively. The pullout strength increased significantly over time up to four weeks, but the difference between the coatings was not statistically significant at any time interval.
Histomorphometric analysis showed an increase of bone-implant contact between one and two weeks from 0 to 15% for Ti and 0 to 20% for HA coated implants. At four weeks Ti and HA implants showed 44% and 60% bone-implant contact respectively. There was a significant increase in bone-implant contact over time for both coatings. HA implants had significantly higher bone-implant contact at two and four weeks. Light microscopy revealed that bone grew into HA coated surfaces in the form of feet, spreading over the surface. Whereas for Ti the newly formed bone looked like a bridge linking the original bone with the implant surface.
Conclusion: The different growth patterns of bone into Ti and HA surfaces resulted in different bone-implant contact areas. Highly crystalline hydroxyapatite coatings enhanced the osseointegration in the early stages of bone healing. However there was a discrepancy between the mechanical and histological results. This may suggest that the mechanical failure does not occur at the implant-bone interface.
The abstracts were prepared by Professor A. J. Thurston. Correspondence should be addressed to him at the Department of Surgery, Wellington School of Medicine, PO Box 7343, Wellington South, New Zealand
