Frictional heating of bearing materials tested in a hip joint wear simulator

Abstract

In a hip simulator wear test using bovine serum as a lubricant, the heat generated by ball-cup friction may cause precipitation of the proteins from the lubricant. The resultant accumulation of a solid layer of precipitated protein between the ball and cup could artificially protect the bearing surfaces from wear, in a manner that does not occur in vivo. Alternatively, the gradual depletion of the soluble proteins could interfere with their ability to act as boundary lubricants on the bearing surfaces, thereby artificially increasing the wear rate. Because the rate of protein precipitation may depend on the maximum temperature at the bearing surfaces during sliding, rather than the mean temperature of the bulk lubricant, this study determined the transient surface temperatures using an array of thermocouples embedded in acetabular cups of GUR 415 ultra-high molecular weight polyethylene (UHMWPE) and femoral balls of metal or ceramic, in conjunction with a finite element model of the temperature distribution. The prostheses were tested at one cycle/s under a Paul-type, physiological load profile with 2030 N maximum force, with the load cycle synchronized to the motion cycle. The steady state temperatures of the bulk lubricant were 38 degrees C for the zirconia balls, 36 degrees C for the cobalt-chromium and 33 degrees C for the alumina. However, the corresponding surface temperatures of the polyethylene, calculated with the finite element model, were 99 degrees C with zirconia ceramic, 60 degrees C with cobalt-chromium alloy, and 45 degrees C with alumina ceramic. The rank order of the surface temperatures corresponded to the relative amounts of protein that were precipitated in the test chambers during wear tests with these materials.

The effects of lubricant composition on in vitro wear testing of polymeric acetabular components

“…32,33 Antibiotics and fungicides are often used to retard bacterial and fungal growth in lubricants but there is no evidence indicating that this is effective in reducing the overall degradation of serum. On the contrary, Liao and co-workers 34,35 and Lu and McKellop 36 have shown that serum degradation is caused predominantly by frictional heating. These authors found that higher cycling rates, continuous motion, and lower thermal conductivity of the femoral head, which all contribute to a greater temperature rise in the serum, led to more severe degradation of the proteins and consequently, lower wear rates for UHMWPE.…”

The use of self-mating PEEK as an alternative bearing material for cervical disc arthroplasty: a comparison of different simulator inputs and tribological environments

“…This may be the result of thermal heating at the articulating surfaces when testing at 2 Hz. PEEK is considered a thermal insulator (thermal conductivity of 0.25 W/mK) as compared to the more common material of CoCr (thermal conductivity of 18 W/mK), and this may have resulted in denaturization of the proteinaceous lubricant, potentially forming a protective triobolayer [27][28][29]. This is supported by the results for Group 3, where a reduced protein content of 5 g/L and a decrease in the lubricant temperature to 23°C was performed to reduce any thermal artifact and further minimize protein denaturization.…”

The Biotribology of PEEK-on-HXLPE Bearings Is Comparable to Traditional Bearings on a Multidirectional Pin-on-disk Tester

“…It is also important to analyze the frictional heat generated in all-polymer bearings, because these testing artifacts may lead to lubricant protein precipitation [15]. The precipitation may result in a solid layer of precipitated proteins and protect the articulating surfaces from wear or it may result in the depletion of soluble proteins in the lubricant and result in increased wear.…”