Why Coating Technologies for Hip Replacement Systems, and the Importance of Testing Them In Vitro

The rationale behind novel surface engineering and coating technologies for implant articular surfaces is described especially for the significant number of metal-sensitive patients with total hip replacement. We list the important coating characteristics, and emphasize adhesion as the most challenging. We report on 2 large scale experimental hip wear simulator studies we conducted on samples of 2 such coating technologies implemented by external experts in their respective fields. One was a titanium nitride ceramic coating on a metal-on-metal total hip replacement, which aimed at wear reduction and a barrier against metal-ion diffusion. The other was a nanocrystalline homometallic surface treatment where same element metals were vapor-deposited in a novel ion-beam-assisted deposition process onto CoCr alloy femoral heads in CoCr metal on ultra high molecular weight polyethylene (UHMWPE) hips. The gradual reduction in crystal size toward the surface into nano size aimed at increasing hardness and reducing wear. Both test series were conducted on an AMTI hip simulator for 5 million cycles under the 14242-1 ISO standard, simulating walking gait, without any deliberate edge-loading or harsh conditions, and using uncoated specimens of identical design and materials as controls in each test. Both tests resulted in some of the coating eventually rubbing away (separating) to various levels on all specimens. The wear rate of the coated specimens was generally higher not lower than the uncoated controls, but not multiply so. Our discussion compares and contrasts our results with others, and we speculate whether a metal-sensitive arthritic patient who badly needs a hip replacement may still benefit from a coated implant despite the risk.