To eliminate UHMWPE debris, hard-on-hard bearing surfaces are regaining favor, and metal-on-metal (MOM) is one such combination. To further improve the performance of MOM THRs, a titanium nitride (TiN) coating is sometimes applied through pressure vapor deposition to femoral heads and acetabular liners. This coating has sufficient hardness and therefore may resist abrasion and reduce overall wear, or at least not prohibitively compromise them. One such coating applied commercially was tested on a hip simulator, with coated and uncoated (control) implants supplied by the same implant manufacturer. This study investigates the wear rates of MOM THRs with and without the TiN coating over a 5 million cycle (Mc) in vitro wear test.

Six MOM THRs with 44mm diameter CoCr femoral heads, acetabular liners, and acetabular shells were simultaneously tested on a hip simulator (AMTI, Boston). Three of the six had heads and liners coated in TiN, and the remaining three were uncoated for control. The specimens were mounted anatomically and were lubricated with bovine serum diluted with deionized water to have 20g/l protein concentration at 37°C. The THR specimens were subjected to the loading and rotations of the walking cycle in ISO-14242-1 at 1Hz for 5Mc, without distraction. The loading and rotations were continually observed to ensure consistency with the desired waveforms. The femoral heads and acetabular liners were carefully cleansed and gravimetrically weighed at standard intervals.

Over 5Mc, the uncoated heads displayed a wear rate of 1.84±0.18mg/Mc while the coated femoral heads wore at 4.37±2.01mg/Mc. Wear results were similar in the case of the uncoated and coated metal acetabular liners (1.35±0.11mg/Mc and 4.16±2.06mg/Mc, respectively). However, most interesting was the observation that all three TiN coated THR specimens displayed a loss of coating on both the head and liner in the articulating region. The area where the coating wore away increased in size as the test progressed. The higher wear observed on the coated specimens was due to the removal of the coating, and perhaps the coating particles causing third body wear (evidenced by numerous scratches on coated components). The loss of coating occurred early in the experiment (after only 0.25Mc) in the case of one specimen which caused severe scratching and high wear to that specimen. After this “breakaway wear” occurred, the wear on that specimen stabilized. The difference in wear between the coated and uncoated femoral heads was not statistically significant. The difference in wear between the two types of metal acetabular liners was not statistically significant until 3Mc, after which it became marginally significant (p< 0.05).

Our simulator results confirm small wear overall for MOM THRs, however, we did find extreme “run-in” wear on one TiN coated specimen. The eventual loss of the TiN coating on all three coated specimens is of concern, as this coating is marketed commercially in some parts of the world. It is possible that the coating process was conducted improperly, which resulted in poor adhesion to the substrate, or perhaps resulted in thin application/deposition in the area where the coating did not last.