Characterization and comparison of metal-metal particles isolated from a hip simulator and in tissues from failed THR. Does a hip simulator produce metal-metal wear particles the same as in vivo?

Large volumes of polyethylene wear particles from conventional metal-polyethylene total hip replacement (THR) bearings can elicit an inflammatory response leading to bone resorption and aseptic implant loosening. As a result, there has been a revived interest in alternate bearings such as metal-metal (MM) because of their demonstrated low volumetric wear in hip simulators. Detailed characterization of wear particles is necessary to help predict and study the periprosthetic tissue response. MM wear particles produced by a hip simulator have yet to be compared to those found in MM periprosthetic tissues from similar implants. Comparison of these particles will help to assess the overall suitability of hip simulators for reproducing in vivo wear and being considered as an adequate model for testing new implants and predict periprosthetic osteolysis. The purpose of the present study was to compare wear particles isolated from MM implants worn in a hip simulator with particles from similar MM THR worn in vivo.

Commercial grade implants made of ASTM F75 (cast) alloy were evaluated. Simulator testing on 28 mm diameter parts was carried out in a Matco hip simulator for 0.25 million cycles (0.25 Mc) and 2 Mc in 95% bovine calf serum. Tissues from 7 patients with MM THR implants were harvested at revision surgeries at various postoperative time periods. Metal wear particles were isolated from serum lubricant or tissues using an enzymatic protocol previously optimized to minimize particle changes due to reagents. After isolation, centrifugation was used to pelletize the particles, which were then embedded in epoxy resin. Ultrathin sections were analyzed by transmission electron microscopy with energy dispersive X-ray spectrophotometry (EDAX) for particle identification. EDAX analysis of particles from the hip simulator primarily revealed the presence of chromium oxide particles with occasional CoCrMo particles. The majority of particles were round to oval in shape, mainly reflecting the majority of chromium oxide particles, and averaged about 53 nm at 0.25 Mc and 43 nm at 2 Mc. EDAX analysis of particles in human tissues revealed the same types of particles that were produced in the hip simulator. However, for all cases of less than 18 months duration, particles appeared to be different in composition with almost only chromium oxide particles, and were statistically significantly smaller in size. Particles from cases beyond 18 months appeared to be the most comparable to particles generated in the hip simulator in terms of size, shape and composition.

The results of this study reveal comparable results in hip simulator and tissues, more so for the longer as opposed to the shorter implantation times. Within the context of these data, this study demonstrated that the hip simulator used in this study can be suitable for reproducing in vivo wear particles, but at appropriate periods of comparison.