Volume 3, Issue 8 (September 2006)
Load Profile and Fluid Composition Influence the Soak Behavior of UHMWPE Implants
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.