By: 15 May 2019
State of the art technology and surgery with 3D pre-op planning, 3D printing and custom-made implants is already in clinical practice today

Mr Ian McDermott, Consultant Knee Surgeon at London Sports Orthopaedics, believes 3D technology is well and truly already here in the field of orthopaedics, and it looks set to evolve and expand significantly in years to come

My first encounter with 3D technology was several years ago, when I did hips as well as knees (for a number of years now I’ve been purely a dedicated specialist knee surgeon). Everyone with any sense of 3D awareness knows that it’s impossible to judge femoral neck angles and offset from just plain x-rays, where rotation cannot be controlled properly. Therefore, when Symbios came out with 3D CT planning software that was able to measure every hip angle and distance with millimetre precision, this was a big step forward. From CT data it was then apparent that hip angles and distances vary enormously between individuals, and that most of the angles and distances can vary independently of each other. Thus, to replicate hip biomechanics accurately, one has to be able to account for each of these variables accurately and independently. Symbios therefore developed a modular stemmed prosthesis with a large number of different options covering the majority of hip anatomies encountered. For the roughly 5 per cent of people whose hip geometry falls outside of the range that can accurately be replicated with a modular stem, Symbios are able to provide a custom-made prosthesis that matches the patient perfectly.

But does any of this really matter?

Yes. If the femoral offset is altered by as little as 15 per cent (which is just a few millimetres) then this significantly weakens the hip abductors. At best, this can lead to early muscle fatigue with walking; at worst, this can leave the patient with weakness and a Trendelenberg gait. 

But does it actually make a difference?

My personal experience of doing Symbios hips was that they were a joy. Every hip had precise pre-op planning and there were never any nasty unexpected intra-operative surprises. No eyeballing. No guessing. Proper pre-op planning led to excellent patient outcomes, and for those patients with badly deformed hips, it transformed what could have been a really difficult operation into a really easy straight forward procedure: something that’s good for the surgeon and the patient alike.

However, I’m no longer a hip surgeon. I’m a specialist knee surgeon. So, I’ll leave it up to others to debate the pros and cons of 3D planning and custom-made prostheses in hip surgery. However, I can happily talk as an expert about knees.

Several years ago I was struggling to find an acceptable solution for that small cohort of patients with isolated lateral unicompartmental osteoarthritis who might benefit from a partial knee replacement, as none of the prostheses on the market were particularly easy to implant or gave predictably good results for the lateral compartment. However, in 2012, Conformis entered the UK market with its custom-made knee prostheses. The process of Conformis knee replacement surgery involves the patient having a CT scan, with a scannogram including the hip and the ankle and with fine-cut imaging of the knee itself. From this, the engineers over in Boston design and manufacture a patient-specific prosthesis that exactly matches the precise size and shape of the patient’s own knee. I implanted the UK’s first iUni prosthesis in 2012 in an ex-professional footballer from Man City. He did scarily well. I say scarily, because his recovery was so good and so fast that I spent most of my time telling him to slow down, and within just a few weeks he was out cycling 15 miles a day. I’d never seen anything quite like it. Was it me? Was it because I’m such a fantastic surgeon? Sadly, no – the only thing I’d changed was my prosthesis. So, was this just a fluke and was this just a one-off? No – I found exactly the same outstanding results from my next iUni case (also a lateral) with the patient squatting fully and normally by just five weeks post-op, and looking like he’d not even had any surgery.

So, why? How can this ‘miracle’ result be so? With a standard knee prosthesis, designers look at large numbers of different knees and they then try and formulate a model that best encompasses the widest range of ‘average’. However, 1) size is a grey-scale and a continuous variable, not a categorical thing, and 2) people vary enormously in shape, not just size. A size ‘A’ prosthesis will cover a certain range. Size ‘B’ will cover the next size-range up from this. This is fine if you’re in the middle of the ‘A’ range or the middle of the ‘B’ range; but what if you’re half way between the two? Then you’ll get something that’s either a bit too small or a bit too big. It’s exactly the same as suit sizes. The problem with poor sizing is that under-sizing leaves exposed bare bone surfaces and leaves the prosthesis resting on softer cancellous bone. Over-sizing leaves prominent metal edges jutting out, and an edge that protrudes as little as just 3mm can cause significant symptoms from impingement. So, the first answer clearly lies in proper detailed pre-operative 3D planning. On top of this, however, people’s knee anatomy can vary enormously, too. Some people have a ‘dog-faced’ (wider) distal femur and some people’s distal femur can be ‘horse-faced’ (narrower and taller). Some people have a significant offset between the distal level of their femoral condyles. Some have a very tight curve on their lateral femoral condyle. Therefore, the only way to accurately replicate the patient’s anatomy, and hence their kinematics, and give them a knee that feels ‘normal’ (normal to them) is to use a custom-made / patient-specific prosthesis that matches every aspect of their bony anatomy precisely, and this is the second answer as to how best to replace a knee.

With a Conformis knee, the patient has their CT scan, the data is sent to Boston and it then takes the team six weeks to design, manufacture and deliver the custom-made prosthesis back to us in the UK, ready for implantation. The kit comes in a small box, with radically less instruments and trays than is normally required for a standard off-the-shelf prosthesis. 3D-printed nylon cutting blocks are provided, which ensures the highest degree of accuracy for all the required cuts (no, you don’t need some gimmicky million-pound robot). Also, there is no breaching of the femoral intramedullary canal plus significantly less bone is removed (which means less intra- and peri-operative bleeding, more bone for better fixation, and less bone removal for any potential future revision surgery). The surgery itself is easy and straight-forward, with less ‘eyeballing’ than with standard knee replacement surgery. There are a number of tips and tricks to follow, but these are outside of the scope of this article.

The main question is, does it actually work?

Yes. Early data from the Beyond Compliance register is showing lower than average / lower than expected early revision rates compared to other prostheses on the market. Long-term prosthetic survivorship figures are still awaited. However, what’s the point of having a prosthesis that ‘survives’ for years and years if the patient’s in pain, unhappy with their knee and generally miserable? What really counts and what has always tended to give knee replacement a bit of a bad name is patient satisfaction. Published figures show that patient satisfaction after knee replacement is probably somewhere in the region of about 80 per cent. What this means is that 20 per cent of the 70,000 or so knee replacement patients per year in the UK are unhappy with their new knee. This is a catastrophic statistic. 

Importantly, high quality studies have shown drastically higher patient satisfaction rates with a custom-made Conformis knee compared to standard off-the-shelf prostheses. Probably the best study to-date is that from Katthagen, presented at the 2015 World Arthroplasty Congress: they demonstrated 74.2 per cent patient satisfaction with a standard off-the-shelf prosthesis, but 94.3 per cent patient satisfaction with a Conformis custom-made knee. That shouldn’t be viewed as a 20 per cent improvement in satisfied patients: it’s actually an 80 per cent reduction in unhappy patients. This is massive.

I’ve heard a few surgeons say negative things about Conformis knees: these have always been surgeons who have never actually done one. From those surgeons who have done Conformis knees, every single one that I’ve spoken to has been a convert and has ended up an enthusiast, like myself. In our practice at London Sports Orthopaedics, we are now prospectively gathering pre- and post-operative PROMS data on all our knee replacement patients. This includes detailed functional testing on a Biodex Isokinometer and video gait analysis on the KneeKG system from Emovi. Assessments are undertaken by our Clinical Nurse Specialist and two dedicated Research Physiotherapists in our Research and Outcomes Centre at our clinic at 31 Old Broad Street, in collaboration with London Bridge Hospital. We are gathering data and looking at our outcomes, and I’m convinced that increasingly compelling data will become available to support the wider use of 3D pre-op planning and custom-made prostheses in the future, so that in time the benefits of this technology will become available to all patients, not just the lucky few.

Symbios and Conformis are not the only companies providing amazing 3D technology in orthopaedics. In our practice we are now using the custom-made KineMatch patellofemoral arthroplasty prosthesis from Kinamed (distributed in the UK by Exactech), and the Episealer custom-made focal resurfacing implant from Episurf, with both giving outstanding clinical results.

3D technology is well and truly here in the world of orthopaedics, particularly in the field of knees. Much like the fact that SatNav is now available as standard in most run-of-the-mill cars, precise 3D planning will no doubt eventually become the norm for all patients. Whether the NHS will ever have the resources to provide access for all patients to the best custom-made implants remains to be seen; but for those people who do have potential access, the evidence is increasingly proving that this is quite simply a no-brainer.

About the author

Mr Ian McDermott is a full-time private Consultant Orthopaedic Surgeon based in Central London, specialising purely in knees. He is the founder of London Sports Orthopaedics, he is an Honorary Professor Associate at Brunel University, he is the President of the UK Biological Knee Society and he is also the Vice-Chairman of The Federation of Independent Practitioners Organisations. For further info or to contact Ian, please visit: or