A number of potential bearing surfaces can be considered in a hip replacement.
Traditionally, a high molecular weight polyethylene acetabular component has been used against a metal femoral head. This bearing has, over years, given excellent pain relief and improvement in function. There are many series in the literature that report implant survival in excess of fifteen to twenty years with this bearing surface. Typically, in younger, more active patients, as the years progress and activity continues, the wear debris that results from this bearing can lead to a local soft tissue reaction that, over time, can be associated with loosening of the implant and subsequently with damage to the surrounding bone. Both can lead to failure of the reconstruction, possibly requiring revision surgery. Indeed, if the wear is allowed to continue, the wear products and local reaction to them can result in quite considerable bone loss requiring more complex reconstruction at the time of revision.
In an attempt to improve the outcome, various changes have been made over the years to the polyethylene bearing surface. More recently a technique known as ‘cross-linking’ of the polyethylene has been utilised. Cross-linked polyethylene has been used as a relatively standard bearing surface over the course of the last eight to ten years with encouraging early clinical results showing a reduction in wear rates. For this reason, I routinely use cross-linked polyethylene with an uncemented socket in my practice.
For the above reasons, it has always been my practice to arrange regular ‘follow up’ for my patients with clinical and x-ray review to ensure that all continues to progress well over time with the ongoing activity.
More recently other bearing surfaces have been introduced.
The use of ‘metal on metal’ bearings, specifically in hip resurfacing, is described elsewhere. ‘Metal on metal’ hip replacements have been used in orthopaedics since the 1950s and 1960s and indeed many of the original implants utilised this technology. Good results with this bearing have been published though the use of ‘metal on metal’ hip replacement has reduced more recently as a consequence of the concerns surrounding local soft tissue reactions, discussed in the section on hip resurfacing. This reduction has, in my opinion, also been influenced by ongoing improvement in other potential bearing surface options.
‘Ceramic on ceramic’ bearings have been used widely in orthopaedics for a number of years. More recent improvements in the composition and preparation of ceramics has resulted in improved clinical function. This has encouraged increased popularity in ceramic bearings. Ceramic is a harder material and has better lubrication characteristics. The resulting reduced wear rate allows the surgeon to use a significantly larger diameter bearing surface. Increased head size has been shown to reduce the rate of dislocation of the hip replacement.
In addition to benefits surrounding a reduced rate of dislocation, the wear rate of ‘ceramic on ceramic’ bearings is considerably lower than any of the other bearing surfaces that have been tried in hip surgery.
One of the concerns that has been raised with ceramic bearings has been the fact that the material is slightly brittle. There have been situations in which the ceramic femoral head or ceramic liner used has failed. Unfortunately a number of the ceramic liners that have failed have been implanted in suboptimal alignment or mal-positioned with respect to the shell. Similarly, the vast majority of the ceramic heads that failed were made of early generation materials.
Fracture of a first generation ceramic femoral head
Ceramic debris is seen within the surrounding soft tissues
More recent developments in ceramic technology have reduced the rate of femoral head fracture enormously. Similarly, an understanding of just how important it is that ceramic bearings are perfectly aligned and that the ceramic liners are perfectly positioned inside the metal shell within the pelvis has led to reduced concern of ceramic bearing failure.
Theoretically, therefore, the overall effect of a ‘ceramic on ceramic’ bearing is a better, safer range of movement with a reduced wear rate.
It is my opinion that any surgeon who carries out any number of hip replacements should have experience and understanding of all of the different bearing surface options to allow the patient and surgeon in consultation to make a sound, evidence-based decision in each and every circumstance.
© Andrew R J Manktelow – September 2011