By: 17 January 2018
Custom knees are the key to patient satisfaction with replacements

Ian McDermott discusses the success rates of knee replacement surgery and how to best judge ‘survivorship’ of the prosthesis

The most common question surrounding knee replacement surgery that I encounter is how long it will last. My response to this is always to tell my patients that they should actually be questioning the success rate of the replacement, not just the length of time that it will last.

95 per cent of knee replacements are still in place after 10 years of use, and won’t have failed, with 80 to 85 per cent still “surviving” after 20 years of use. “Survivorship” simply means that the knee prosthesis is still in the patient’s body and that it hasn’t had to be removed or revised. However, what if the prosthesis is still in but the patient is in agony, they’re barely able to walk and they’re deeply unhappy with their knee? Surely that can’t be considered a “success”?

It is well known that patient satisfaction rates after knee replacement are not as high as those seen after hip replacements, but this is because the knee is a far more biomechanically complex joint than the hip joint, which is just a simple ball and socket.

The complex movement of a knee is guided by a number of factors, including the shapes of the bones, the tension in the various ligaments inside and around the knee, and the pull of the muscles. The fact that it’s very hard to replicate these complex movements of the knee with an artificial joint means that for many knee replacement patients, their new knee never actually feels fully like a normal knee.

Currently, around 15 to 20 per cent of patients are unhappy after their knee replacement surgery, and with around 70,000 knee replacement surgeries taking place a year in the UK, this is a disturbingly high number of people.

Studies using real-time X-rays watching knee replacement patients walking post-operatively demonstrate that the artificial joints display very different kinematics compared to a normal knee joint, with the artificial joint sometimes sliding, rolling and clunking around quite considerably. This excessive irregular movement can contribute to increased wear and tear in the artificial joint, which may be a factor in early failure of the knee. Also, due to the shape as well as the movements of most artificial knees, post-operatively patients often find it difficult to flex the knee up fully.

These days, people are much more active, which is great, but this is placing much greater strain on joints, especially the knees, and it’s taking its toll, meaning that we’re operating on younger and younger patients. People’s expectations for the feel and function of their knee replacement are much higher nowadays too, and rightly so.

With a traditional knee replacement, various size options are available, but these options are “categorical”, meaning that there’s a size A, size B etc. However, what if you’re in-between two sizes? This means that you might get a prosthesis that’s either a bit too big for your knee or a bit too small. Also, knees are not all the same shape. The problem with knees is that they’re very complex, and individuals have not only different sized knees, but people’s knees can also have very different shapes and contours. With a standard knee replacement, however, the patient’s bone is cut to fit the prosthesis, which changes the natural shape and contours of the patient’s knee.

Importantly, the concept of “one size and one shape” will definitely not fit all, and this is where patient-specific custom implants fit in.

I have been using patient-specific custom-made ConforMIS knees for over five years now, and have never looked back. ConforMIS knees are designed to fit the exact contours and shape of each individual patient’s knee perfectly. Each prosthesis is designed specifically for each individual patient, using 3D-modelling from data obtained from a CT scan, which provides the patient’s exact anatomy. The implant fits the patient’s bone with millimetre precision, which means that the bone cuts are much smaller, which is beneficial on a number of levels: first, there’s more bone left should the patient potentially end up needing revision surgery later in life; and second, less bone removal means a larger surface area for the fixation of the prosthesis, and hence a stronger and better fixation between the bone and the implant. The blood lost during the procedure is also reduced with a custom knee because you can be less invasive, there are less exposed uncovered raw bleeding bone surfaces left in the knee at the end of the procedure (because of the better fit), and the procedure is actually slightly faster.

Patient-specific cutting blocks are 3D printed from nylon, and it takes six weeks from the time of the CT scan to the custom prosthesis arriving and being ready for surgical implantation. The actual procedure of fitting the prosthesis is frankly very easy. Particularly for the femur, you can actually put the cutting blocks on with your eyes closed, the fit is that good. That level of fit is not just making my life easier though, it also means it’s very hard to get anything wrong intra-operatively, because everything just slots into place.

The most important thing, however, is the patient outcomes. Patient satisfaction levels with a ConforMIS custom knee are significantly higher, compared to a standard off-the-shelf prosthesis. Data from Greg Martin and his team at Palm Beach have shown that patient satisfaction rates with a ConforMIS prosthesis are about 95 per cent, compared to about 85 per cent with a standard off-the-shelf prosthesis. Now, the way to look at that is that this is not just a 10 per cent increase in the proportion of happy patients; it’s actually a two-thirds reduction (from 15 per cent down to just 5 per cent) in unhappy patients, which is massive.

Patients of course want the best knee for them, that’s going to give them the best feel and the best function, and the best way to provide this is with a custom-made patient-specific prosthesis. I’ve seen a significant leap forwards in my patients’ outcomes after knee replacement surgery since I started using ConforMIS knees. The technology from ConforMIS is probably the biggest step forward in the evolution of knee replacement surgery in the last 30-odd years, and I now very rarely ever use anything other than a custom-made knee, as I simply want the very best for my patients.


Mr Ian McDermott is a consultant orthopaedic surgeon specialising in knees and sports injuries.