No abstract

There has been a revived interest in metal-metal total hip replacements because of their potential for improved wear performance compared with conventional metal-polyethylene implants. The aim of the present study was to characterize metal wear particles isolated from metal-metal hip simulator testing of various clinically relevant alloys and to analyze the effects of these alloys and the number of loading cycles on wear particle characteristics. Implants were manufactured using medical-grade cobalt-chromium-molybdenum (CoCrMo) alloys that were high-carbon wrought, low-carbon wrought, or cast (with solution annealing). Testing was performed in a MATCO orbital bearing hip simulator in 95% bovine calf serum. The wear particles were isolated from the serum at test periods of 0-0.25 million cycles (Mc) (run-in wear) and 1.75-2 Mc (steady-state wear) using an enzymatic protocol previously optimized to minimize particle changes due to reagents. Isolated particles embedded in epoxy resin were characterized by transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDXA). The EDXA results revealed the predominance of "lighter" particles containing Cr and O (most likely chromium oxide particles from the passivation layer) and fewer darker CoCrMo particles, with varying ratios of Co and Cr (possibly from carbides and from implant matrix material). More CoCrMo particles were observed with the low-carbon wrought alloy, but the majority of the particles for all three alloys was chromium oxides, especially for the 1.75-2 Mc test period. Image analysis of TEM micrographs revealed that for 0-0.25 Mc, there was up to 21% needle-shaped particles but that the majority remained round to oval in shape, reflecting the predominance of chromium oxide particles. Particle length averaged about 52 +/- 4 nm, with only small differences due to the alloy. For 1.75-2 Mc, most particles were round to oval in shape. They were even less needle-shaped than at 0.25 Mc, and they had a slightly smaller length, averaging 46 +/- 3 nm. In addition to characterizing the size and shape of particles from a MATCO simulator, this study is the first to demonstrate that particles that do not contain Co (presumably chromium oxides) can be predominant in the wear of metal-metal hip implants. It is therefore recommended that future in vitro and in vivo studies include the effects of these particles rather than just the effects of CoCrMo particles on the overall tissue response.

The purpose of the present study was to compare wear particles isolated from metal-metal (MM) hip implants worn in an orbital bearing simulator with particles from similar MM total hip replacement (THR) implants worn in vivo. Comparison of these particles is important because it will help to assess the overall suitability of this type of hip simulator for reproducing in vivo wear and for producing physiological wear particles suitable for biological studies of in vitro cellular response. Commercial grade components made of ASTM F75 (cast) alloy were evaluated. Simulator tests were performed in 95% bovine calf serum with a 28-mm-diameter implant. Wear particles were collected from 0 to 0.25 million cycles (run-in wear period) and 1.75 to 2 million cycles (steady-state wear period). Tissues from seven patients with MM implants (surface replacement or stem type) were harvested at revision surgeries (after 1-43 months). Metal wear particles were isolated from serum lubricant or tissues using an enzymatic protocol that was previously optimized to minimize particle changes due to reagents. After isolation, particles were centrifuged, embedded in epoxy resin, and characterized by transmission electron microscopy (TEM) and energy dispersive X-ray analysis (EDXA). Results of EDXA on particles from the hip simulator primarily indicated a predominance of particles containing Cr and O but no Co (most likely chromium oxide particles), and fewer CoCrMo particles presenting varying ratios of Co and Cr. Image analysis of TEM micrographs demonstrated that the majority of the particles from the simulator were round to oval, but a substantial number of needle-shaped particles were also found, especially from 0 to 0.25 Mc. The particles generated from 0 to 0.25 Mc had an average length of 53 nm, whereas those generated from 1.75 to 2 Mc had an average length of 43 nm. In vivo, EDXA and TEM analysis of particles that were retrieved from two patients at 23 and 43 months respectively, revealed that they were the most comparable in composition, average length (57 nm), and shape to particles generated in the hip simulator during the run-in wear period. Because a large clinical retrieval study in the literature suggested that a run-in wear regime might occur in vivo for some 6-36 months, the fidelity of the simulator of the present study was strongly supported. However, some uncertainties existed, including the finding that the particles isolated from the other five patients generated from 1 month up to 15 months (shorter implantation times than the other two patients) were smaller and mostly contained only Cr and O (no Co). In the opinion of the authors, this particular very short term patient group was somewhat atypical. Therefore, despite these uncertainties, the present study was deemed to support the ability of the orbital bearing hip simulator to produce physiological wear particles.

No abstract

Isolation of metal wear particles from hip simulator lubricants or tissues surrounding implants is a challenging problem because of small particle size, their tendency to agglomerate, and their potential for chemical degradation by digestion reagents. To provide realistic measurements of size, shape, and composition of metal wear particles, it is important to optimize particle isolation and minimize particle changes due to the effects of the reagents. In this study (Part I of II), transmission electron microscopy (TEM) was used to examine and compare the effects of different isolation protocols, using enzymes or alkaline solutions, on the size and shape of three different types of cobalt-based alloy particles produced from metal-metal bearings. The effect on particle composition was examined in a subsequent study (Part II). Large particles (<1200 nm) were generated by dry abrasion of CoCrMo alloy against itself and small particles (<300 nm) were generated by hip simulator testing of a metal-metal implant pair in the presence of either distilled-deionized water or a 95% bovine serum solution. The reagents changed particle size and to a lesser extent particle shape. For both large particles and small particles generated in water, the changes in size were more extensive after alkaline than after enzymatic protocols and increased with alkaline concentration and time in solution, up to twofold at 2 h and threefold at 48 h. However, when isolating particles from 95% serum, an initial protective effect of serum proteins and/or lipids was observed. Because of this protective effect, there was no significant difference in particle size and shape for both oval and needle-shaped particles after 2 h in 2N KOH and after enzymatic treatments. However, round particles were significantly smaller after 2 h in 2N KOH than after enzymatic treatments. Particle composition may also have been affected by the 2N KOH treatment, as suggested by a difference in particle contrast under TEM, an issue examined in detail in Part II.