Selecting the right hip replacement

Joint replacement is one of the most successful major surgical interventions, being highly effective at relieving the pain and morbidity associated with arthritic joints. Hip replacement achieves improvements in general health comparable to coronary revascularisation and renal transplantation,1 is cost effective, and patients report high levels of satisfaction.2

Joint arthroplasty replaces an organic (but failed) self healing system with an inorganic system subject to fatigue and wear. Currently available prostheses cannot compare with a natural joint in terms of longevity. Ultimately, all components wear or fatigue; consequently, a further (more complex) operation may be needed. In the quest for enhanced longevity, researchers and manufacturers have used new materials and designs in particular patient groups whose replacements are known to fail earlier. Unfortunately, not all innovations have been successful, as was recently highlighted by the problems with metal-on-metal bearings.3

In a linked paper (doi:10.1136/bmj.f7592), Makela and colleagues review the methods by which an implant is attached to bone.4 They report a population study from the combined Nordic arthroplasty registers that assessed the use and outcome of cemented and uncemented fixation in total hip replacement in Scandinavia. Clearly, cemented implants performed better (lasted longer) in patients aged over 65 years in Nordic countries. However, cemented fixation is highly technique dependent, and the current results contrast with data from other international registries, which suggest that hybrid fixation (a combination of cemented and uncemented) works better in their populations.

Although Makela and colleagues sensibly advise caution in directly comparing their results with other registries, this discrepancy highlights the fact that the method of fixation to bone is just one variable in the equation. Head size and bearing surface are two key confounding variables related to the implant, while international differences in surgical technique and philosophy will also be important. Different solutions seem to work better in particular situations. Even the metal-on-metal designs that were recently pilloried have a role in the subgroup of younger male patients, with larger femoral head sizes.5 Implant fixation and bearing surfaces wear out in certain subgroups of patients (the physiologically elderly and those with a life expectancy greater than 20 years, respectively), and these problems are important when many patients survive well into the second decade after implantation.

Makela and colleagues’ paper raises interesting questions, which at face value reinforces the comment in an earlier BMJ editorial that, “shiny and new is not necessarily better.”6 We cannot condone the widespread use of unproved technology in small numbers of patients, which is neither scientific nor good practice. Kynaston-Pearson and colleagues performed a systematic review of the clinical evidence for hip prosthesis components recorded in the National Joint Registry in 2011.7 They reported that there was no published literature on 7% of implants used, although it was not clear how many of these implants were part of ongoing clinical trials. It is well known that, although surgeons who perform low volumes of arthroplasties using tried and tested techniques can achieve excellent results, high volume units generally achieve the most consistent results. This has led others to suggest that new techniques should be tested in high volume settings, before widespread release.8

Although registry data are generally excellent and important, it is easy to over-interpret such data. The research success of major arthroplasty registers lies in measuring the efficacy of a procedure in general use after the publication of smaller specific studies that evaluate a new implant in a select group of patients. Registries help us discern more general patterns, reporting outcomes that are the combined result of case selection, surgical experience, and implant choice. Although sophisticated case mix adjustment can be carried out on registry data, it is difficult to be comprehensive. For example, we know that backache and personality can influence patient reported outcomes after arthroplasty, but these factors are not included in techniques currently used to adjust for case mix.

Registries are an excellent way to reflect the overall population outcomes. They are most useful when looking at large representative groups of patients, as in hospital statistics, or large numbers of the same implant with longer term follow-up. For example, comprehensive registry data show clearly that mortality after joint replacement has fallen significantly in England and Wales over the past few years.9 But confounding covariables make it difficult to relate population statistics to specific situations or to individual cases; this is where the art of medicine meets the science of statistics. A consistent surgical technique, careful patient selection, and evidence based implant selection are needed to achieve good long term results, such as those reported in the current study. This is the challenge for the orthopaedic community. Given the excellent longevity of joint arthroplasty, the how, when, and who are as important as the what. Registers allow individual surgeons’ outcome data and overall care to be independently assessed and compared with those of their peers. This is driving change.

Perhaps the real importance of these registers, and publications such as Makela and colleagues’ study, is that they allow the public to see that the orthopaedic community is reviewing its results in an open forum, setting standards for accountability and leadership in medicine.