Biomechanical and morphometric analysis of hydroxyapatite-coated implants with varying crystallinity

Purpose: The level of crystallinity in hydroxyapatite (HA) is thought to be responsible for its degradation in the physiologic milieu. The purpose of this study was to compare the in vivo bony response to HA coatings of varyi ng levels of crystallinity and determine the optimum composition for promot ing osseointegration. Materials and Methods: Cylindrical implants of sand-blasted CP titanium and HA-coated titanium of 50% (low), 70% (medium), and 90% (high) crystallinit y were inserted into the canine femur for 1, 4, 12, and 26 weeks. Morphomet ric analysis of undecalcified Sections determined the percentage of bone co ntact with the implant surface. A pullout test was used to measure the inte rfacial attachment strength of the bone-implant interface. Scanning electro n microscope (SEM) examination of the implant surface aided in identifying the failure mode. Coating thickness was measured under light microscopy to determine whether degradation occurred. Results: No significant differences could be found in the percentage of bon e contact and interfacial attachment strength between the three types of HA -coated implants throughout the four implantation periods. A significantly higher percentage of bone contact on HA-coated implants than on uncoated ti tanium implants was noted at 4 weeks. (ANOVA, P < .05). HA-coated implants were also found to have significantly higher interfacial attachment strengt h than titanium implants at 4, 12, and 26 weeks. Coating thickness decrease d gradually with time. The most noticeable reduction was found on the low-c rystallinity coatings during the first 4 weeks. Failure of the bone-coating -implant complex occurred mostly within the coating or near the coating-imp lant interface. Conclusions: HA coatings on metal implants enhance osseointegration in the early stage of bone healing and provide strong bone-bonding capability, alt hough titanium implants had about the same level of bone contact in the lat er stage of healing. Crystallinity of HA,coatings has no: significant influ ence over the bone formation capacity and the bone bonding strength. Howeve r, an HA coating of higher crystallinity is more desirable in providing dur ability and maintaining osteoconductive properties.