"How long will it last, doctor?"

Is a question that many patients will ask their surgeon before hip replacement. To give an accurate reply is impossible, but most orthopaedic surgeons will advise that an average of ten years is a realistic estimate. It should be remembered that this is an average and not a guarantee.

Irrespective of the type of replacement performed, more than 95% of patients are highly satisfied with the results of surgery in the immediate postoperative period. As time goes by the components can begin to loosen and so the level of satisfaction declines.

Many factors determine the lifespan of a total hip replacement. The age of the patient is important. The younger the patient, the less time a hip replacement is likely to last. An American study in 1983 looked at more than 100 patients under the age of 45 years at the time of hip replacement. Averages of 4½ years after the procedure only 76% of the replacements were still satisfactory. Another study from Sweden in 2004, of more than 3000 total hip replacements, reported a 73.5% implant survival 13 years after surgery in men aged less than 50 years. For this reason great care is needed if total hip replacements are performed for such young patients.

The failure of hip replacements does appear to increase with time. Some studies suggest that the failure rate at 15 years is more than twice that at 10 years. Also, different parts of the total hip replacement behave in different ways. The femoral component usually lasts longer than the acetabular component. Indeed, it is the problem of failure of the cemented acetabular component that led orthopaedic surgeons to look at the use of cementless acetabular components, which are so widely used today.

Surgical technique plays a significant part in the longevity of a total hip replacement. Results for the same design of component, inserted in different orthopaedic centres, show failure rates that vary between 1 and 24%, with some very old-fashioned designs having done well in the long-term, perhaps because of the expertise with which they were inserted.

One would not expect a revision hip replacement to last as long as a primary one and this is indeed the case, although some centres claim that the results of a first revision are as good as the results of a primary replacement. Certainly, the early results of revision surgery can be as good as the results of a primary procedure but, overall, the failure rate of a revision replacement is higher. For revisions of revisions, the failure rate can be as high as 60%.
When it comes to assessing the result of a total hip replacement it is difficult to know against what the result should be compared. In the United Kingdom, the 'gold standard' prosthesis has for a long time been the Charnley prosthesis even though the original design of component has long since ceased to be used. Another prosthesis that is widely regarded as a gold standard is the Exeter hip replacement. This is still being implanted although in a slightly modified form compared with the original.

As well as advocating the use of bone cement in the late 1950s, John Charnley also realised the importance of following patients up over the years. This is a critical part of hip replacement surgery and widely recommended. Only in this way can failures be identified and avoiding action taken before errors are compounded. Consequently, it is possible to say that for patients over the age of 65 years approximately 90% of Charnley components will still be going strong at 15 years after implantation. The same can be said of the Exeter hip replacement. These are excellent results but not necessarily applicable to younger age groups, nor for categories of patient who are heavy users and extremely active.

The problem with predicting results for a hip replacement is the necessity to wait so long before any sense can be made of a component’s performance. Only once a component has been followed through for 15 years can it compete with the Charnley or Exeter designs. A number of advanced research techniques (e.g. RSA or radiostereophotogrammetric analysis) have been developed in an attempt to predict in the short term how a total hip replacement will do in the long term. These techniques are extremely helpful but not always as accurate as one might like.