Hip Resurfacing: A Good Idea?

Steven J.M. MacDonald, M.D, FRCSC
London, ON

Hip resurfacing implants enjoyed a brief period of popularity 25-30 years ago. The results at that time, however, were clearly inferior to conventional total hip replacements and most, but not all, centres abandoned these implants.

Over the last five years or so there has been a growing interest in the newer generation of resurfacing implants and, clearly, a resurgence in their use. The Australian Orthopaedic Association National Joint Registry1 reported that resurfacing implants accounted for 6.7% of all primary hip arthroplasties performed in 2003. In areas of Europe, and particularly in the United Kingdom, the popularity of this implant grows at a rapid pace.

macdonald fig. 1.jpg

Figure 1 An anteroposterior radiograph of a resurfacing implant functioning well at one-year follow-up.

There have been many purported advantages to resurfacing implants; however, to date, there has not been a single prospective randomized clinical trial comparing a resurfacing implant to a total hip replacement with specific outcome measures. Suggested advantages include bone preservation, reduced wear rates due to the use of a metal-on-metal bearing as opposed to metal-on-polyethylene, greater stability and range of motion of the hip joint and greater activity levels including return to sports. Critically evaluating these potential advantages is difficult due to a lack of published data to support many of the claims.

While no one doubts that there is bone stock preservation on the femoral side, historically, the acetabular components have had to be of a slightly larger size than conventional cementless acetabular components. Newer designs; however, claim to have addressed this issue. The advantage of a metal-on-metal bearing is significantly reduced wear rates; in the order of 20 to 25 times less compared with a conventional cobalt chrome head on a polyethylene implant, and this has been confirmed with retrieved implants. Theoretically, a larger diameter head, such as is used in resurfacing implants, allows for fluid film lubrication, thereby resulting in negligible bearing wear. It should be emphasized however that large diameter femoral head implants can now be used in conjunction with several conventional femoral components that have proven clinical track records. These can be coupled with either a metal bearing (currently available in both a one-piece acetabular component or a modular acetabular shell with a metal insert) or a highly cross-linked polyethylene liner. The surgeon therefore does not have to use a resurfacing femoral component to gain the advantages of a large diameter femoral head. The claims with respect to greater stability and range of motion once again relate to the larger diameter femoral heads that can be used with either a resurfacing or a more conventional total hip arthroplasty system. Reports of greater activity levels and easier return to sports are clearly anecdotal and would be strongly dependent upon patient selection criteria.

Clinical results to date have been mixed. Two recent publications report somewhat different results. Daniel et al2 report on a series of 446 Birmingham (Smith and Nephew, Memphis, TN) resurfacing implants with a mean follow-up of 3.3 years. They report only one revision, leading to a failure rate of 0.2%. Amstutz et al3 discuss the results of 400 Conserve (Wright Medical Technologies, Arlington, TN) resurfacings, with a mean of 3.5 years follow-up, with a 3% conversion rate to a total hip replacement predominantly due to femoral component loosening and femoral neck fracture. This series most closely represents the experience that has been seen in Australia with the introduction of resurfacing implants. The Australian Orthopaedic Association National Joint Registry reports their results in revisions per one hundred cases per observed component years with rates of 0.8 for a cemented socket and stem, 1.3 for a cementless socket and stem and 1.7 for resurfacing implants. While clearly there are differences in patient selection, it should be noted that resurfacing implants do demonstrate the highest rates of early component revision in this registry data. It is interesting to note that the primary reason for early revision in total hip arthroplasty is secondary to instability whereas with resurfacing it is secondary to early femoral neck fractures.

Weighed against the suggested advantages of hip resurfacing are the published disadvantages. Currently all hip resurfacing implants employ metal-on-metal bearing couples. Metal-on-metal bearings produce elevated metal ions4 with their theoretical concerns related to local and systemic effects. While resurfacing implants with their larger diameter femoral heads should produce lower wear rates, publications to date report equal, if not higher, metal ion levels5. This issue however is very complex with factors such as head size, metallurgy, carbon content, surface carbides, bearing clearances and component modularity all having potentially significant influences on bearing wear and metal ion production.

Another significant disadvantage is that current generation resurfacing implants have no long-term data to support their use. The same holds true however for almost every total hip implant, and specifically alternate bearings, that are used in clinical practice today across Canada and internationally. While a recent RSA publication6 did report on the inherent stability of the femoral component in 20 patients following resurfacing implants, other authors have reported increased femoral component loosening rates. Resurfacing implants also have a unique complication with associated femoral neck fractures that is obviously not seen in total hip arthroplasty. These fractures, in combination with higher published component loosening rates, results in higher failure rates than one would expect with a conventional total hip replacement. As well, proponents of this implant do clearly indicate that significant leg length discrepancies cannot be corrected with hip resurfacing and it is not recommended in patients with significant osteopenia and, therefore, in older women. Additionally the cost of these implants in most Canadian institutions is two to three times greater than a more conventional total hip arthroplasty.

At the present time there are many unanswered questions surrounding the current generation of hip resurfacing implants. It would appear from several sources that early failure rates exceed those of conventional total hip replacements at comparable follow-up intervals. Clearly this could be secondary to a learning curve, however, with no data at all to suggest superiority to a total hip replacement, the indication for this implant is unclear. Prospective randomized clinical trials will obviously aid in answering specific questions however it will be national joint registries, with their large numbers of patients, that will provide the data required to support or refute the claims made surrounding this implant choice.


References

1. Australian Orthopaedic Association National Joint Replacement Registry, 2003 Annual Report. www.aoa.org.au

2. Daniel J., Pynsent P.B., McMinn D.J.: Metal-on-metal resurfacings of the hip in patients under the age of 55 years with osteoarthritis. J Bone Joint Surg (Br), Mar:86(2):177-84, 2004.

3. Amstutz H.C., Beaule P.E., Dorey F.J., Le Duff M.J., Campbell P.A., Gruen T.: Metal-on-metal hybrid surface arthroplasty: two to six-year followup study. J Bone Joint Surg (Am), Jan:86(1):28-39, 2004.

4. MacDonald S.J., McCalden R.W., Chess D.G., Bourne R.B., Rorabeck C.H., Cleland D., Leung F.: Metal-on-metal versus polyethylene in hip arthroplasty: a randomized clinical trial. Clin Orthop, Jan:406:282-96, 2003

5. Clarke M.T., Lee P.T., Arora A., Villar R.N.: Levels of metal ions after small- and large-diameter metal-on-metal hip arthroplasty. J Bone joint Surg (Br), Aug:85(6):913-7, 2003

6. Glyn-Jones S., Gill H.S., McLardy-Smith P., Murray D.W.: Roentgen stereophotogrammetric analysis of the Birmingham hip resurfacing arthroplasty. A two-year study. J Bone Joint Surg, Mar:86(2):172-6, 2004.

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