A contemporary snapshot of the use of hydroxyapatite coating in orthopaedic surgery.

©2005 British Editorial Society of Bone and Joint Surgery doi:10.1302/0301-620X.87B8. 16692 $2.00 J Bone Joint Surg [Br] 2005;87-B:1046-9. The capacity for biological materials to combine with inorganic salts is well established. Bio-mineralisation was an evolutionary milestone, and a key contributor to the ‘cladogenesis of life’ which occurred in the early Cambrian period, 570 million years ago. 1 A protracted ice age in the late Precambrian era may have been the catalyst. This phenomenom is also the bedrock upon which both the art and science of bone and joint surgery is founded. The role of calcium salts in osteogenesis has long been a focus of interest. Clinical and experimental evidence was first published in the late 19th century. 2 In the formation of bone, calcium phosphate mainly in the form of apatite, combines with collagen by both epitaxic and chemical bonding. This achieves an integrated composite material which is ‘living’ in that it contains both osteoblastic and osteolytic cells which are responsible for grooming the structure. Mechanical loading and strain are the fundamental influence for remodelling, the “functional adaptation” described by Wolff in 1890. 3 In order to achieve a sound interface between bone and an implant, normal mechanical continuity appears essential. Any fibrous interlayer serves to alter the normal loading, producing a sclerotic surface when seen on radiographs. Such material is relatively avascular and may be susceptible to opportunist infection. In addition, it may not be able to staunch the flow of fluids and material debris which is thought to account for bone granulomata and aseptic loosening. Calcium hydroxyapatite can be produced artificially. Maxillofacial research has indicated that it functions well as a bone filler, 4 although understanding of the mechanism for this effect remains incomplete. It does not result in bone formation when placed in other tissues. Biocompatability depends on acceptance by a host cell, with no adverse immune or toxic reaction. There is evidence that the rate of degradation of the calcium salt is an important factor in osteoconduction. There may be advantages in a biphasic composition with tricalcium phosphate to achieve an optimum supply of calcium ions. 5 Radiographs do not accurately reflect the histological situation in radiopaque material which can only be achieved by histological and mechanical analysis. 6