MECHANICAL AND HISTOLOGICAL-EVALUATION OF AMORPHOUS CALCIUM-PHOSPHATEAND POORLY CRYSTALLIZED HYDROXYAPATITE COATINGS ON TITANIUM IMPLANTS

The effect of amorphous calcium phosphate (Ca/P) and poorly crystalliz ed (60% crystalline) hydroxyapatite (HA) coatings on bone fixation to 'smooth'' and 'rough'' (Ti-6A1-4V powder sprayed) titanium-6A1-4V (Ti) implants was investigated. Implants were evaluated histologically, me chanically, and by scanning electron microscopy (SEM) after 4 and 12 w eeks of implantation in a rabbit transcortical femoral model. Histolog ical evaluation of amorphous vs. poorly crystallized HA coatings showe d significant differences in bone apposition (for rough-coated implant s only) and coating resorption (for smooth- and rough-coated implants) that were increased within cortical compared to cancellous bone. The poorly crystallized HA coatings showed most degradation and least bone apposition. Mechanical evaluation, however, showed no significant dif ferences in push-out shear strengths between the two types of coatings evaluated. Differences between 4 and 12 weeks were significant for co ating resorption and push-out shear strength but not for bone appositi on. Significant enhancement in interfacial shear strengths for biocera mic coated as compared to uncoated implants were seen for smooth-surfa ced implants (3.5-5 times greater) but not for rough-surfaced implants at 4 and 12 weeks. Rough implants showed greater mean interfacial str engths than uncoated smooth implants at 4 and 12 weeks (seven times gr eater) and to coated smooth implants at 12 weeks only (two times great er). Mechanical failure of the bone/coating/implant interface consiste ntly occurred within the bone, even in the case of the poorly crystall ized HA coatings, which had almost completely resorbed on rough implan ts. These results suggest that once early osteointegration is achieved biodegradation of a bioactive coating should not be detrimental to th e bone/coating/implant fixation.