Cementless arthroplasty has progressed substantially in the recent decades from pressfit implantation to porous-coated and later HA-coated implant fixation as its ultimate current state-of-the-art incarnation. As a consequence ever younger and older patients have received the benefits of hip and other arthroplasty although attention to age-related factors is key to success. Key factors for success, from the implant perspective, are adequate primary stability of the device in the bone supported by design and surface structure variables that together with optimal implant biocompatibility result in durable osseo-integration of the device. The high activity levels of younger patients but similarly the generally inferior muscular condition of elderly patients require special attention for the stability of the hip joint with avoidance of impingement. Also bone quality may be a topic for consideration to avoid problems. Excellent survival rates past twenty years are documented in both literature and registries with quantitative studies confirming the excellent implant stability and bone quality. With an optimal consideration for patient related factors as well as anatomic reconstruction of the arthroplasty, modern cementless arthroplasty provides every patient an outlook on both excellent long term functionality and survival.

Recent improvements in HA-coating technique make it possible to apply HA by different methods. The most promising is the precipitation technique. A thin Ca-P of 10 – 30 microns thickness can be applied using precipitation in fluid. The process takes place at room temperature in stead of at temperatures of many thousends of degrees centigrade. Still providing an HA-coating of similar type as the plasma-spray technique, there are several advantages associated with its use. To start with, the coating is of a micro-crystalline type in stead of macro-crystalline. This provides enhanced bioactivity because the exposed surface area of a micro-crystralline surface is much greater. At second it is possible to coat irregular surfaces, including porous materials that can not be coated using conventional techniques because the pores would be closed down. At third it is possible to include biological factors in the coating such as growth factors, bone-morphogenetic proteins or antibiotics at the time of manufacturing because the process takes place at room temperature. The high temperatures used with plasma-sprayed HA-coatings would preclude this. Experiments confirm these improvements. They show much enhanced bone apposition. Also the combination with antibiotics, growth factors or BMP’s is attractive because it may lead to a reduction in the need for bone graft materials or infection especially in revision surgery. This will greatly influence the possibilities of bone repair especially in revision surgery. Several experimental and clinical studies are currently ongoing and it is expected that these new enhanced coatings will expand the use of HA-coatings on more implant surface structures.

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.