Wear debris from metal on metal (M/M) hip resurfacing and metal on polyethylene (M/P) total hip replacements have different biocompatibilities. M/P wear particles have a foreign body effect¹. M/M wear particles cause hypersensitivity², DNA damage^3,4 and white blood cell suppression⁵.

M/P wear debris contains nickel and M/M wear debris contains cobalt. Nickel and cobalt are both heavy metals, required as trace elements for some bacteria but potentially toxic to bacteria in high concentrations.

Cobalt kills Helicobacter Pylori at concentrations as low as 30 ppb⁶, substantially lower than the concentration in prosthetic joints. Nickel/cobalt permease membrane transporters are found in a wide range of microorganisms⁷ including Staphylococcus Aureus and Coagulase Negative Staphylococci, which commonly infect prosthetic joints.

The purpose was to investigate the effects of nanoparticulate wear debris and their heavy metal constituents on bacterial growth.

Samples of Coagulase Negative Staphylococci (CNS), Staphylococcus Aureus and MRSA were cultivated to compare their growth in M/M wear debris, M/P wear debris, nickel, cobalt and control culture mediums over 48 hours. Nickel was toxic to CNS (p=0.006) and MRSA (p=0.048). Cobalt also retarded the growth of all three bacteria. M/M wear debris increased the growth of CNS 183 times at 48 hours (p=0.044), Staphylococcus Aureus and MRSA. M/P wear increased the growth of Staphylococcus Aureus 120 times at 48 hours (p=0.021), CNS and MRSA.

M/M and M/P nanoparticulate wear debris accelerate the rate of growth of common organisms that infect prosthetic hip joints, the clinical significance of which is uncertain. In isolation, the heavy metals contained within wear debris retard bacterial growth, particularly nickel.

Nanoparticulate wear debris does not possess the toxic effects of its constituent metals on bacteria. This reinforces the safety of wear debris but also shows the potential for anti-bacterial effects to be harnessed.

The biological significance of cobalt-chromium wear particles from metal-on-metal hip replacements may be different to the effects of the constituent metal ions in solution. Bacteria may be able to discriminate between particulate and ionic forms of these metals because of a transmembrane nickel/cobalt-permease. It is not known whether wear particles are bacteriocidal.

We compared the doubling time of coagulase negative staphylococcus, Staphylococcus aureus and methicillin resistant S. aureus when cultured in either wear particles from a metal-on-metal hip simulator, wear particles from a metal-on-polyethylene hip simulator, metal ions in solution or a control.

Doubling time halved in metal-on-metal (p = 0.003) and metal-on-polyethylene (p = 0.131) particulate debris compared with the control.

Bacterial nickel/cobalt-transporters allow metal ions but not wear particles to cross bacterial membranes. This may be useful for testing the biological characteristics of different wear debris. This experiment also shows that metal-on-metal hip wear debris is not bacteriocidal.