The convention in measuring the clinical success of hip replacements is to measure the number of years an implant survives before it needs replacing, so the success (or “survivorship”) of any given implant is usually measured at ten years, for example. However, this convention does not consider all the factors involved.

Joint survival is inextricably linked to joint activity, and the best way of measuring this is the number of “cycles” the joint has performed, in other words, the number of times the leg has moved through one complete movement. The problem is that Hip Resurfacing devices are typically implanted in much younger (and therefore more active) patients than a conventional Total Hip Replacement (THR).

One study reports a variation from 0.5 million cycles per year in elderly patients to 5.4 million cycles in active younger patients(1). Another study quotes an average of 1.7 million cycles per year in a population of subjects between the ages of 25 and 74(2).  A third quotes a 45-fold variation in a subject group ranging from 23 to 82 years old(3).

There is obviously a massive variation in activity levels depending on age, fitness and occupation.

To simplify this, lets look at an analogy that we are all familiar with. The tyres on my wife’s car typically last five years – so at two years the “survivability” is 100%. However, my car tyres last me only one year – so at two years the survivability is 0%. If we applied conventional methods of measuring the success of hip replacements to car tyres, then my car tyres would be banned, and my wife’s car tyres would be the Gold Standard.

Strangely, the tyres are identical!

It’s just that I do 20,000 miles a year in my car, whilst my wife only does 4,000 miles a year. The tyres last the same number of cycles on both cars, but different lengths of time according to levels of activity.

It’s not fair to measure the life of a car tyre in years, any more than it is for hip replacements!

Hip Resurfacing v Total Hip Replacement

If we take the Charnley Total Hip Replacement as one of the Gold Standards in conventional hip replacement, this shows 96% survivorship at seven years(4). The problem is that typically these are put into older patients with much lower activity levels.

Reported clinical results in Corin Hip Resurfacing patients show 97.4% survivorship at six to seven years(5), well on course to match or better the results of the Charnley device. However, the average age of the Cormet patients was under 50 and so we could assume that the Cormet devices had seen many more times the use than that of the equivalent Charnley devices in the same period of time. If we assume that the activity level of the Cormet patients was only ten times higher than the typical, older THR patient, then in seven years, their number of cycles would have been the equivalent of 70 years for a conventional hip replacement device in an older patient!

The Cormet device demonstrates as good or better survivorship using conventional criteria than ‘Gold Standard’ implants despite being implanted in a younger, more active age group. This is a remarkable testament to the quality of the engineering of the Cormet device, and an encouraging thought for those patients who have had them implanted.

References

1. Seedhom BB, Dowson D, Wright WW: Wear of solid phase formed high density polyethylene in relation to the life of artificial hips and knees. WEAR 24:52, 1973.

2. Sequeira MM, Rickrenbach M, Wietlisbach V et al: Physical Activity assessment using a pedometer and its comparison with a questionnaire in a large population survey. Am J Epidemiol 142:989,1995.

3. Schamlzreid TP, Szuszczwicz ES, Northfield MR etc al: Quantitative assessment of walking activity after total hip or knee replacement. J Bone Joint Surg Am 80:54, 1998.

4. Onsten I, Besjakov J, Carlsson AS: Improved radiographic survival of the Charnley prosthesis in rheumatoid arthritis and osteoarthritis. J Arthroplasty, No1, 1994.

5. McMinn D, Pynsent P. Metal/metal hip resurfacing with hybrid fixation: Results of 1,000 cases – a personal series. Trans Am Academy of Orth Surgeons, 2001, San Francisco