- Are associated with decreased wear rate compared to conventional polyethylene
- Mixed film lubrication appears to be the operative mechanism in most metal-on-metal hip joints
- With metal-on-metal bearings, in contrast to polyethylene bearings, a larger-diameter bearing actually produces lower wear rates than does a smaller-diameter bearing
- Larger-diameter bearings have a greater arc of motion(and thus better ROM), which decreases the risk of impingement and also lessens the incidence of dislocation
- Alloys of cobalt (Co) and chromium (Cr) have been preferred for MOM bearings in THR because of their hardness.
- High chromium content provides good corrosion resistance
- There are two types of wear particles with MoM bearings: Co-Cr-Mo particles and chromium oxide particles
- It has been hypothesized that the Co-Cr-Mo particles are produced by the wear of the carbides on the bearing surfaces and the prosthesis matrix, and that the chromium oxide particles come from the passivation layer on the implant surface and possibly from oxidized chromium carbides
Metal hypersensitivity:
- all metals in a biological environment corrode; the ions released can combine with proteins and activate the immune system as antigens and elicit hypersensitivity responses
- the incidence of hypersensitivity is approximately 2 per 10,000
- are generally delayed cell-mediated responses
- ALVAL: are Lymphocytic infiltrations in the subsurface layer of the lining tissues, which were either diffuse or aggregated around small postcapillary vessels
- delayed type hypersensitivity should be considered when a patient with a well-fixed implant experiences chronic, aching pain with evidence of synovitis (an irritable range of motion) but has no objective evidence of infection
- If a modular MOM bearing is being considered, the use of substrates without Co-Cr (e.g., titanium) will allow revision of only the bearings in cases were hypersensitivity develops
What is New in Hip Resurfacing(Metal on Metal)?
-S. Glyn-Jones and H. Pandit reported in Dec 2009 JBJS B that incidence of ALVAL is higher in women especially if performed under 40 years of age.
-Factors significantly associated with an increase in revision rate were female gender, age under 40, dysplasia and small components.
-the higher revision rates in women could be due to increased prevalence of allergy in women because of wearing jewellery, increased ROM, which would be more likely to cause impingement, edge loading or different gait patterns
