Biocompatibility: a biomechanical and biological concept in total hip replacement.

The insertion of any implant or prosthesis into bone usually changes the biomechanical environment and, thus, alters the stresses and strains applied to the bone. Both bone overload and excessive stress protection can result in bone resorption. The material and geometry of any implant should be designed to avoid excessive flexural mismatch. Incompatibility of materials may result in interface and mechanical failure, with the consequent generation of particulate debris, subsequent osteolysis and implant failure. Particulate debris can be generated from articulating surfaces and from any other modular or fixation interface. Larger particles are associated with a foreign body giant cell reaction. Polyethylene particles in the size range of 0.5 to 10 microm excite a cytochemical reaction that culminates in osteolysis. The precise pathogenesis of osteolysis has not been characterised, but it is probable that different pathogenetic mechanisms are involved in the different radiological types of osteolysis. A large number of very small metallic particles are released from metal-on-metal couples. These may cause mutagenic damage (chromatid breaks, chromosome translocations, aneuploidy, etc.). In defining implant biocompatibility it is essential to consider the biological response both to an altered mechanical environment and to the liberation of particulate debris.