We present the 5.6- to 7.6-year results of our first 118 hydroxyapatite (HA)-coated total hip replacements in patients under 66 years of age. The titanium femoral component has proximal HA coating and was usually articulated with an HA-coated threaded cup. The HA coating of 50 microns thickness has a porosity of below 3%, 97% HA purity and 65% crystallinity. The survival rate at a mean of six years was 100% for the HA-coated stems and 99% for the HA-threaded cups. The average Harris hip score at one year was 96, and at three years and thereafter 98. There was a very low incidence of early pain after surgery. Serial radiographs showed rapid bony integration of implants with evidence of bone apposition on the coating within six months. By Engh's criteria, all the femoral components had confirmed bone ongrowth after three years. We found no deterioration of results with time, and consider that our clinical and radiological results show that HA coatings can provide early pain relief and durable implant fixation.

Implants of solid sintered hydroxyapatite form very tight bonds with living bone, but are susceptible to fatigue failure. This problem can be overcome by using plasma-sprayed apatite coatings on titanium implants. A very strong bond is formed between bone and this composite material; this was studied in canine bone with plug implants, avoiding any mechanical retention. Mechanical testing showed an interface shear strength at six weeks of 49 MPa with a maximum of 64 MPa after six months. There was histological evidence of direct bonding between the apatite coating and living bone while uncoated control plugs were easily extracted. The results indicate that apatite-coated implants can form a chemical fixation with a strength comparable to that of cortical bone itself. This fixation is far stronger than that provided by current cemented or uncemented fixation techniques.