Customised prosthetic knees: What are the advantages and who are they suitable for?

Prof. Dr. Markus P. Arnold, affiliated doctor at Klinik Birshof and knee specialist at Gruppenpraxis LEONARDO in Münchenstein, Basel is one of just a handful of doctors who use the 3D technology in Switzerland. Until a few years ago, 3D printing technology was most commonly associated with product designers and automotive manufacturers. 3D printers are increasingly being used by the medical profession to produce items such as customised prosthetic knees. For several years now, doctors in Switzerland have been modifying prostheses to suit individual knee joints and print personalised prostheses that match the patient’s unique anatomy. The orthopaedic surgeon explains who these tailor-made prostheses are suitable for and how they are created and implanted.

Der Läufer hält sein Knie, da es schmerzt.

What is wrong with standardised prosthetic knees that we now need customised implants produced using a 3D printer?

Prof. Dr. Markus P. Arnold: ‘Wrong’ is perhaps not the right word. After all, many patients are satisfied with their standard prostheses. However, studies have shown that one in five patients is unsatisfied after their prosthetic knee surgery. So we have to investigate the reasons. One reason could be that the ‘thing’ we implant is just a poor imitation of what the patient was used to having in their knee joint for 60 or 70 years.

This is how the knee works: the shape of the condyles (the bony part of the joint) and the points where the ligaments attach are all part of a unique, individual design. If I change just one factor – for instance, if I attach the cruciate ligament to a new spot or alter the shape of the condyle – the knee will no longer work properly. It will only work optimally with the original design, i.e. the structure in which it grew. Then we try to replace this ‘ideal’ design with a standardised prosthesis. In many cases it actually works very well, but in some cases it does not.

Until recently there were only standardised mass-produced prostheses available for knee replacement surgery. Sizes 1-10, at best they can also be slightly narrower or wider. Shoes are a good point of comparison: the standard shoe sizes fit most feet. Then there are feet that do not fit into the standard sizes. But tailor-made shoes always fit. Knee joints, like feet, come in many different shapes.

Plus with standard prostheses you usually only have an extreme choice between a totally prosthetic knee and a single part.

How are customised prosthetic knees better than conventional implants?

Prof. Dr. Markus P. Arnold: They can be adjusted to fit knees that don’t have a ‘standard shape’.

But that’s not even the most exciting aspect: often only part of the knee is damaged by osteoarthritis. The knee has three compartments: the interior, the exterior and the part between the kneecap and the thigh that forms the joint. Standard prostheses can either replace a single piece or the whole joint. However, two compartments are often affected by the disease. The necessary parts cannot be mass produced, since the anatomical shape, the connection between one part and the next, greatly differs from person to person. Until now such patients typically received a fully prosthetic knee.

That’s what makes tailor-made duo prostheses truly revolutionary, because they can be combined with several other partial prostheses to replace two thirds of the knee, for example. And we do not even have to touch ligaments! That means the mechanics of the knee, my way of moving, stays the way I am familiar with. The joint simply receives a new metal coating, a bit like a crown.

With standard prostheses, at the very least the front cruciate ligament is removed, although many surgeons remove the rear cruciate ligament as well. The thinking behind this is that “the mechanics are different now anyway; the prosthetic part will take on the role of stabilising the ligaments, so everything else is simply in the way and should be removed”. In contrast, with customised prostheses you can build around the patient’s system of ligaments. Previously, I was  sceptical about whether customised prostheses would catch on. However, these combinational duo prostheses, open up a really new way of thinking.

What type of patients are these tailor-made prostheses particularly suitable for?

Prof. Dr. Markus P. Arnold: They are ideal for younger osteoarthritis patients, who need a prosthesis, because significantly fewer bone is removed during the operation compared to standard prosthetic knees. If the patient develops more extensive osteoarthritis as they get older, for example, or if the existing prosthesis becomes loose or abrasion particles cause inflammation of the bone the patient will still have enough bone left if they need more work done in 10 to 20 years. That is to say, is easier to keep future treatment options on the table, a standard prosthesis can always be used later on.

What are the noticeable advantages of customised knee implants compared to standard prostheses?

Prof. Dr. Markus P. Arnold: The technology is still quite new, so unfortunately there are not yet many long-term studies available. But what is suggested and expect is that the knee starts to feel normal again very quickly. The sensation of having a foreign object in your knee is less pronounced: you do not even think about the fact that there is a prosthesis in your knee anymore.

In many instances the patient benefits from greater stability, because we leave the ligaments alone – particularly in the case of the partial prostheses. With standard prostheses, the surgeon often has to reach a compromise between stability and mobility, because the prosthesis does not quite fit right. The patient wants to retain the stability of their knee despite the prosthesis, because otherwise they will feel unsteady on their feet. However, they would also like to be able to clearly bend their knee over 90° and start riding a bike again, for example. The ideal combination is often difficult to achieve with standardised prostheses, whereas with customised prostheses there is no need to compromise, because you are copying the ideal knee, the person’s own.

What is the procedure for implanting a customised knee prosthesis?

Prof. Dr. Markus P. Arnold: It takes eight weeks, from the planning stage through to the operation. Prior to the operation we take a CT scan of the knee joint (link only available in German and French). These tomographic images are used to produce an individualised prosthesis with a 3D printer, which is then delivered to the hospital shortly before the surgery. Of course the suppliers do not tell us exactly how the parts are manufactured. But the end result is astonishing. I recently saw three knee prostheses and each one was completely different. It is really impressive!

Is the operation different to procedures involving standardised prostheses?

Prof. Dr. Markus P. Arnold: There are slight technical differences, there are modified procedures that the surgeons have to get used to. It is different and yet similar, like cycling and mountain biking.

What about the recovery time after the operation?

Prof. Dr. Markus P. Arnold: The initial stage is pretty much the same, after all, it is still a major operation. We need to be honest about that. At the beginning the condition of the soft tissue is particularly important, i.e. how much post-operative bleeding there was and how swollen the knee is.
After around four months I see a huge difference. Until recently you had to explain to prosthetic knee patients, “wait a year, then you’ll be happy with it.” After four months, around half the patients with the newer standard implants are fairly happy, but are still conscious of their prosthetic knee. Whereas all the patients with customised prostheses say to me after four months, “it feels very good.” And the annual check-up is just a formality.

Are prostheses for other joints already being produced using 3D printing?

Prof. Dr. Markus P. Arnold: I know there are also customised hip implants, but I personally only concentrate on knees.

In your opinion, is today’s trend generally heading towards a time when it will be possible to replace all joints with customised prostheses?

Prof. Dr. Markus P. Arnold: It is hard to say. One of the first ever hip implants (developed by John Charnley in the late 1950s) is still among the best available today. In contrast to the knee, the hip is a ‘more straightforward’ joint. The knee joint moves with six degrees of freedom and can be very precisely controlled. Therefore it contains an extremely large number of nerves. The intricate neuromuscular control required makes the knee relatively sensitive. I am sure this sensitivity contributes to the fact that some patients are not satisfied after the usual prosthetic knee operation. Even though I am not a hip surgeon, I believe the clinical success and demand for customised hips will remain relatively low.

However, I do see a lot of potential in the prosthetic ankles used by foot surgeons, because their success rate in terms of patient satisfaction has been relatively modest so far. I imagine 3D printing technology could offer new relevant opportunities for the field.

Thank you very much for your time.