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

TLDR

Frictional heating in hip simulator tests with bovine serum can precipitate proteins, forming a solid layer that may either protect or increase wear by depleting lubricants. The study aimed to determine transient surface temperatures during sliding, hypothesizing that maximum surface temperature drives protein precipitation. Thermocouples embedded in UHMWPE cups and metal or ceramic balls were used during 1‑cycle/s Paul‑type loading at 2030 N, with data fed into a finite‑element temperature model. Surface temperatures ranged from 45 °C (alumina) to 99 °C (zirconia), correlating with the amount of protein precipitated, while bulk lubricant temperatures remained near physiological levels.

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.