Wear of total joint replacements is determined gravimetrically in simulator studies. A mix of bovine serum, distilled water, and additives is intended to replicate the lubrication conditions in vivo. Load soak stations are used to correct for weight gain due to fluid absorption. Although the implementation of a soak station is considered only as an option for wear testing, this study indicated that fluid absorption depends on the lubricant composition as well as on the applied load profile. Two different fluid recipes were tested with three load scenarios each. Standard ultra-high molecular weight polyethylene (UHMWPE) pins acted against commercially available ceramic hip balls. Dynamic (0 to 1000 N), static (1000 N), and no load were applied for 23.1 days (or 2.106 cycles). Both lubricant recipes based on the same mix of deionized water and newborn calf serum (final protein content: 30 g/L), and differed in the chemicals admixed to the fluid to minimize bacterial growths and to prevent protein precipitation. After test completion, the group of samples tested in fluid B gained 2.4-, 3.6-, and 4.2-fold the weight of samples submersed in fluid A for the dynamically, statically, and nonloaded samples, respectively. The specimens showed a significantly higher weight gain under dynamic load than subjected to static or no load for both fluids. The results of this study suggest the implementation of a load-soak control station for every wear test when UHMWPE materials are examined.

Total knee arthroplasties are subjected to high slip velocities and constantly changing velocity vector directions, as they mimic the anatomical conditions of the natural joint. Together with fluid film thickness and applied load, the velocity difference between the two bodies defines the amount of energy that is induced into the contact. Particularly for polyethylene (PE), it is known that the angle describing the difference in motion direction is an important variable for material loss. In this study, a wheel-on-flat simulator scenario was used to apply a slip velocity vector with a constant magnitude and changing direction over the length of a polyethylene sample. The difference in vector orientation ranged from zero to 20.4°, while a constant axial load of 1600 N was applied, and the contact area was submersed in physiological testing fluid. After 500k cycles, the surface of the PE specimen was visually examined and scanned with a video-based measurement system to analyze the wear profile. Polishing was the predominant wear pattern, and minor striations were found in the end zone of the wear area. The wear depth increased with higher velocity angles—this relationship appeared to be linear. This study supports earlier publications that reported a correlation between cross-shear motion and wear. The presented experimental results will help in attempts to numerically simulate wear generation.