Abstract
The relevance of fluid-film lubrication, elasto-hydrodynamic lubrication and ‘tribolayers’ for hip bearings has been the subject of much debate (Fisher 2012). However, knowledge of the thickness and distribution of proteins in and around the wear zone of metal-on-polyethylene (MPE) bearings is scant. The efficacy of protein lubrication with metal-on-metal bearings (MOM) is in discovery. This simulator study was designed to analyze film formation on MOM bearings using varied protein concentrations. The hypotheses were that increasing protein concentrations in the serum lubricant would result in 1) greater thickness of protein films, and 2) reduced MOM wear.
The hip simulator was run for 5 million cycles (5 Mc) duration using 28 mm MOM bearings (DJO Inc) run with the cups anatomical. Lubricant protein concentrations were 16.5, 33, and 66 mg/ml. At each test interval of 1 Mc, the proteins films on CoCr surfaces were analyzed by both interferometry and SEM imaging in main-wear, transition-wear and non-wear zones. Thickness of protein films was measured using non-contacting interferometry. Areas of wear zones were mapped and measured and the areas compared. MOM wear rates were assessed gravimetrically.
It was found that the proteins formed two types of film (Figure 1). Type-1 was visually hazy in appearance, grainy in structure, and most commonly found in the main-wear zone. This type of protein film was always present in the main-wear zone but its thickness (approximately 0.05 μm) did not increase with increase in the lubricant protein concentrations. Type-2 was visually rainbow-like in appearance, more gel-like with thick clumps appearing as islands on the CoCr surfaces, and more common in the transition zone. This type of film was always present (approximately 1 μm thick) and its thickness notably increased in cups with increased lubricant protein concentrations. This film remained relatively consistent on femoral heads and did not change with increased protein concentrations (Figure 2).
The type-1 protein films were always detectable in the actual wear zones but only the type-2 film showed a build-up with protein concentration and only inside the cups. This may be partially a response to the orbital simulator set up. In the Anatomical test mode, the cup is fixed with respect to the load axis and the head oscillates. Thus the main-wear zone on the head had a distributed type of wear patch and the main-wear zone in the cup was fixed. This configuration would allow the type-2 proteins to accumulate around the edge of the cup wear zone. In contrast, they would be scraped off the wear zone of the orbiting femoral head. This study showed that protein films endure even inside the main-wear zone of MOM bearings. In addition, collaborative studies have shown that the protein films are highly mobile and stream across the main-wear zones. Thus there is both an interaction with the CoCr surfaces and a degradation phenomenon that likely results in the protein-rich layers in the transition regions.
Figure 1: SEM images of type 1 and type 2 protein films.
Figure 2: Protein films on MOM bearings under three different protein concentrations.
