The need to optimize adhesion between the femoral stem and the surrounding tissue has led to the development of plasma sprayed HA-coated implants as attachment veichles for hip prostheses. The strength of attachment of bone to the HA surface cannot be fully explained by mechanical bonding at the macroscopic or microscopic level; thus, molecular interactions must contribute significantly to bone bonding, facilitating adhesion at the nanometer level. Observations of apatite crystallites and cell adhesion proteins in the direct vicinity of coated implants indicates that these play a role in modifying the surface for osteoblast attachment and subsequent collagen deposition.

An understanding of the mechanism of “bone bonding” could lead to improvements in the design of HA coatings, leading to enhanced performance of orthopedic implants. Transmission electron microscopy (TEM) has been used to elucidate a sequence of early bone bonding events at the interface of PSHA-coated titanium implants.

The deposition of biological apatite and subsequent formation of a bone/HA bond seem to depend on the dissolution of the coating and the reprecipitation of carbonated apatite. Previous investigations in vitro have shown that the degree of dissolution and reprecipitation decreases as the coating crystallinity increases. Thus, coatings which had undergone heat treatment were used to look at the effect of solubility of the coating on mechanism of bone bonding.