ABSTRACT
A major problem associated with the use of internal skeletal prostheses is the fixation of the device to bone 1 . Deficiencies in current methods of mechanical fixation (“bone cement” and bone ingrowth into porous coatings) limit the function and longevity of total joint replacement prostheses. Recent studies have indicated that bone will bond chemically as well as mechanically to coatings of a calcium phosphate substance, hydroxyapatite 2,3,4 Plasma-spray (PS) techniques 5 are currently being employed to apply hydroxyapatite (HA) coatings to certain titanium (Ti) alloy orthopedic and dental implants. Histological evaluation of specimens retrieved from animal models and human subjects has revealed that bone forms directly on the surface of these coatings within a few weeks of implantation. The strength of attachment of the bone to the coating is much greater than that achieved with uncoated Ti alloy control specimens, providing evidence that the bonding is chemical as well as mechanical. However, investigations indicate that the strength of attachment of the PS coating to the metallic substrate degrades with time, leading to detachment of the coating 6 . Detachment of the PS coating can result in loosening of the prosthesis and abrasion of the fragments of the coating against the Ti alloy device. This wear process can lead to large amounts of metallic particulate debris that is capable of eliciting an inflammatory response with the production of agents that are known to stimulate bone resorption. This loss of bone stock, that can occur relatively rapidly, greatly complicates revision arthroplasty. A recent histological investigation of PS HA coated femoral stems 7 , retrieved at autopsy, revealed that after only 5 months the coating was no longer present along as much as 20% of the perimeter of the device. These findings indicate that there is a need for coatings or surface treatments that allow for bone bonding, but that are more durable and have a higher strength and longer-lasting bond to the metallic substrate than current PS coatings.
