Size, shape, and composition of wear particles from metal–metal hip simulator testing: Effects of alloy and number of loading cycles

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 entrapment of corrosion products from CoCr implant alloys in the deposits of calcium phosphate: A comparison of serum, synovial fluid, albumin, EDTA, and water

“…Wear of this oxide-phosphate layer produces particles predominantly of metal orthophosphate, oxides, and hydroxides, which is in agreement with published work on the nature of wear debris in vivo. [4][5][6][7][8][9][10][11][12][13] Wear at the joint interface may produce metal particles but dissemination to nonlocal tissues and organs 2,13 exposes the particles to nonsynovial environments. Transport is typically via vascular and lymph systems, which are both predominantly composed of serum.…”

“…The organic matrix surrounding the particles has been removed by different techniques, including the use of nitric acid, 5 potassium hydroxide, 11,12 sodium hypochlorite, 13 sodium sulphite, 6 and enzymes. 6,7 These methods are extremely aggressive, and likely to dissolve the calcium phosphate component from the particles. The recovered particles are often then exposed to aggressive physical techniques such as boiling 7 and ultrasonic water bath.…”

“…6,7 These methods are extremely aggressive, and likely to dissolve the calcium phosphate component from the particles. The recovered particles are often then exposed to aggressive physical techniques such as boiling 7 and ultrasonic water bath. 11 Despite the isolation techniques, the analyses have emphasized the importance of the metal compounds, but inevitably they have been underestimated.…”

“…Particles originating from CoCr alloy implants are often Co deficient, and are frequently oxidized to hydroxides, phosphates, and oxides. [4][5][6][7][8][9][10][11][12][13] Transition metals, especially those in the trivalent state such as chromium, have a tendency of hydrolysis, the degree of which is dependent upon the pH of the environment where the metal is oxidized. Thus, where Jacobs et al 10 and Urban et al 13 infer the presence of particles of chromium orthophosphate hydrate-rich material from their energy dispersive X-ray spectroscopy (EDX), they are only stating the presence of the ions Cr 3þ and PO 4 3À in the material.…”

Investigating the Immunologic Effects of CoCr Nanoparticles

“…Although studies of APC function have concentrated on dendritic cells (DCs), as a result of their ability to effectively activate naïve, unstimulated T cells [17], B cells can act as potent APCs if activated appropriately [24]. One explanation for the differences in biologic effects between the implants may be the result of the difference in particle sizes shed from these implants, with M/P implants producing micron-sized wear debris as opposed to the nanoparticles produced from M/M articulations [4,6,7]. Differences in particle size can alter the cell interaction with particles and their resulting cellular distribution, for example, smaller particles (less than 200 nm) may gain access to cells through pinocytosis, a general feature of all cells, as opposed to larger particles (greater than 200 nm) that can only be internalized by phagocytosis, a feature of APCs such as DCs [2,25].…”