Bearing surfaces in total joint replacement
Total hip and knee replacements most commonly utilise a bearing surface consisting of highly polished metal articulating with a strong plastic (polyethylene). Although the implants are designed to be very durable, over long periods of time they can slowly wear out. As this wear occurs, microscopic wear particles can cause areas of bone loss around the prosthesis and this can ultimately interfere with the fixation of the implant to the bone, resulting in the implant becoming loose and requiring revision surgery (see Revision Hip and Knee Replacement).
Metal and polyethylene bearings wear predictably and very slowly over time. With appropriate follow-up in the years and decades following joint replacement such predictable wear can be recognised and managed appropriately before catastrophic failure occurs. Because of this, my preferred bearing for total hip and knee replacements is a highly polished cobalt-chrome alloy (metal) articulating on a toughened highly cross-linked polyethylene (modified polyethylene). The results of this bearing combination outperform all other bearing combinations at 13 years post surgery, as documented in the Australian National Joint Replacement Registry, 2014. Alternative bearing surfaces that may have wear rates lower than metal and modified polyethylene are available, but these have their own unique problems which may result in catastrophic failure, and overall the revision rates of hip replacements with these bearings is higher than hip replacements with metal and modified polyethylene.
I do not routinely recommend the use of ceramic-on-ceramic bearings. Hard-on-hard bearings may cause a squeak with joint movement; in some reports up to 20% of people with a ceramic bearing experience squeak. Ceramic bearings can also fail catastrophically due to component facture, because the ceramic material is hard and brittle. When ceramic bearings are used for hip replacements they are relatively unforgiving and may be intolerant to small variations in implant alignment; such small variations in alignment may be difficult to recognise during surgery and may increase the risk of both squeak and catastrophic failure. Hip replacements with ceramic-on-ceramic bearings have a higher rate of revision at 13 years post surgery than hip replacements with a metal on modified polyethylene bearing, as documented in the Australian National Joint Replacement Registry, 2014. The registry does not currently demonstrate a difference in 12 year revision rates between ceramic-on-modified polyethylene and metal-on-modified polyethylene bearing surfaces.
I do not recommend the use of metal-on-metal bearings. Metal-on-metal bearings, especially large diameter bearings for hip replacement and hip resurfacing were initially promising, offering a theoretical reduction in implant wear as well as increased hip joint stability. Mid-term results, however, have been less favourable, with metal debris potentially causing significant destruction of the tissue around the joint and elevated metal ion levels in the bloodstream. As a large metal on metal bearing is the only bearing option for hip resurfacing surgery, I do not routinely recommend hip resurfacing surgery, although in a small subset of patients the 10 year results of hip resurfacing is approximately equal to the results of total hip replacement. Aside from this subset of patients, the revision rate of large metal on metal bearings is up to four times greater than that of metal on modified polyethylene in some patient groups, as documented in the Australian National Joint Replacement Registry, 2014. I have not implanted any large metal-on-metal bearings and have not been involved in any operations involving the recalled ASR large bearing metal-on-metal hip.
The Australian National Joint Replacement Registry is available here (opens in a new page).
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