By: 10 July 2019
Biological Knee Replacement: the facts vs the fiction

Mr Ian McDermott, Consultant Orthopaedic Surgeon at London Sports Orthopaedics, looks at the future of knee replacement surgery.

Despite what some may say, knee replacement is actually a very good operation. 95 per cent of prostheses can be expected to ‘survive’ after five years, with 80 per cent survivorship after 20 years. Patient satisfaction rates of 80 to 85 per cent have been reported with standard off-the-shelf prostheses (which is not improved by the use of million-dollar robots). However, satisfaction rates of 95 per cent can be expected with the use of custom-made knee prostheses.

One of the main issues that we are facing, however, is that we are seeing younger and younger patients presenting with premature degenerative changes, or even with end-stage arthritis, and age is a definite issue when it comes to knee replacement surgery. The younger a patient is when they have a knee replacement, the more active they are and the higher their functional demands are likely to be. This means an increased rate of wear and tear on a knee prosthesis, which means that it may wear out faster, and hence fail sooner. Combined with this is the fact that younger patients tend to live longer, and hence this is a double-whammy, and there is a significantly higher risk of revision in patients who undergo knee replacement at younger ages. If a patient has a knee replacement in their 70s, then there’s only a 10 per cent risk of revision within their lifetime. However, if a patient has a knee replacement in their 50s, then they have a 50 per cent lifetime risk of revision. Does this matter? Yes: revision knee replacement is twice as technically complex as primary surgery, it has double the complication rates, it is associated with poorer outcomes and higher (re-)revision rates. 

So, if a revision can possibly be avoided, then it should be; and one of the best ways to avoid revision surgery is not to have your first knee replacement too young. (The other best way to prolong the
life of a knee prosthesis is to respect the prosthesis, and not to
abuse it by pounding it with heavy impact exercise, such as tennis or skiing.)

So, what do we do with this ever-increasing cohort of patients who present with premature knee arthritis?

If the patient has generalised primary osteoarthritis, with widespread damage within the joint, then there are few options available, apart from conservative management (painkillers, anti-inflammatories, physiotherapy, walking aids etc), intra-articular injections of corticosteroid (which is the only thing worth injecting into an arthritic joint), or arthroscopy, which in selected cases can reduce symptoms, help keep people keep their knee going for longer, buy them extra time and delay the eventual need for knee replacement surgery. In this cohort of patients, ‘biological reconstruction’ is not normally a viable option.

The patients that we are, however, sometimes able to help significantly, are those people with premature degenerative changes, predominantly in one compartment of their knee subsequent to previous injuries and previous operations, where they are developing premature post-traumatic secondary arthritis, and with the increase in popularity of high-impact type sports, we are seeing ever increasing numbers of these patients.

Meniscal tears

The meniscal cartilages are two crescent-shaped wedges of elastic fibrocartilage that sit in the knee between the femur and the tibia, acting as load sharers within the knee. The medial meniscus sits in the medial compartment and takes about 50 per cent of the load in that compartment; the lateral meniscus sits in the lateral side, and it takes about 70 per cent of the load passing through the lateral compartment. The posterior horn of the medial meniscus is also a secondary stabiliser against anterior tibial drawer, that is of particular importance in the ACL-deficient knee.

Meniscal tears are very common, with an approximate incidence of 65 per 100,000 population per year. It’s hard to give accurate numbers, but probably somewhere between 15 per cent to 25 per cent of meniscal tears may be repairable. In my practice, where I see a lot of younger sporty patients, I tend to repair anything up to about 33 per cent of the tears that I see. If a persistently symptomatic meniscal tear is not repairable, and if any ongoing symptoms are bad enough to justify surgery, then the symptoms can be ‘cured’ with an arthroscopic partial meniscectomy. However, this does not ‘cure’ the knee – it simply gets rid of the torn tissue and hence eliminates the symptoms of a meniscal tear. Importantly, the bigger the tear, the more tissue is lost, and therefore the less of a load sharer is then left in that compartment of the knee, causing a bigger potential risk of arthritis in that side of the joint in the longer-term future. To put a number on this, if a meniscus is completely removed then the relative risk of arthritis 21 years later is increased 14-fold (i.e. a 1400 per cent increased risk).

So, how can one mitigate against this?

The first answer is to always repair a meniscal tear whenever possible, in order to preserve the meniscus. Importantly, the sooner a meniscal tear is operated on in a young person, the better; as the more likely it is that the tear will then actually be repairable. The longer a tear is left (with ongoing multiple physio sessions) and if the knee is injected (especially repeatedly) with steroid, then the more damage there is likely to be and the less likely it is that the tear will end up being repairable – i.e. the more likely the patient will be to end up with a trim, and therefore be subjected to an increased risk of arthritis. So, symptomatic traumatic meniscal tears in younger patients should be treated by surgery, sooner rather than later. 

But what do we do for patients who have already lost their meniscus, and who are already beginning to develop premature degenerative changes?

The answer here is ‘simple’: we can simply replace the missing meniscus by meniscal allograft transplantation. 

A meniscal allograft comes from a cadaveric donor. The donors are screened for high-risk categories and tested for pathogens (viruses, bacteria and fungi), with any positives being discarded. The allograft itself is then tested for any contamination, and any positive grafts are discarded. Once a graft passes these tests, the tissue is still then sterilised and deep frozen, and it has been estimated that the risk of a contaminated graft is less than one in 1.6million. There are no living cells within the meniscal allograft, and the dead cellular material that is there is locked within a dense matrix of collagen. Therefore, meniscal allografts do not elicit an immune response, and they are referred to as ‘immunoprivileged’; anyone can have anyone else’s meniscus without the need for tissue typing, immunosuppressives or steroids, and there is no rejection reaction. All you have to do is to match left vs right, medial vs lateral and size match the donor to the recipient.

Meniscal transplantation is a fairly major complex procedure, and it is one of the most technically demanding procedures within the field of soft tissue reconstructive surgery of the knee. Not surprisingly, it is considered a highly specialised procedure; it should not be undertaken on an occasional ‘have a go’ basis, and, rightly, it tends to lie within the remit of just a small number of specialised surgeons / centres, where there is particular expertise and interest.

The surgical procedure of meniscal transplantation normally takes about 2.5 hours, with the patient staying in hospital overnight. The patient is then on crutches and in a knee brace for the first six weeks post-op, before slowly getting going with cautious physio rehab treatments. It tends to take about 6 to 9 months for patients to fully plateau in their recovery from the surgery.

As for the results, success rates in the region of about 85 per cent can be expected at five-year follow-up, although it should be noted that the concept of ‘success’ depends very much on your actual definition of success. Importantly, this should be considered as just salvage surgery, not ‘restorative’: a donor meniscus is better than no meniscus, but it’s never as good as the patient’s own meniscus. Also, although meniscal transplantation can reduce pain, improve function and help keep people’s knees going for longer, delaying potential future knee replacement surgery, it does not make any of the worn / thin / missing articular cartilage in the joint actually grow back at all. Therefore, in my opinion, it is wrong to offer meniscal transplantation to patients as a procedure to get them back to impact exercise or sport, as activity modification should be a mandatory part of the overall treatment and advice given to the patient. 

There is evidence that the outcomes of meniscal transplantation surgery depend heavily on the general state of the joint as a whole: the more badly damaged the knee is, the lower the probability of a good result. If issues are left unaddressed, then these will adversely impact upon the probability of ‘success’. Therefore:

If there is partial thickness articular cartilage damage, then this should be smoothed off and stabilised by radiofrequency coblation chondroplasty.

If there is full-thickness articular cartilage loss in a small surface area (<2cm2), then this should be treated by microfracture.

If there are larger areas of full-thickness cartilage loss (>2cm2), then these should be treated by articular cartilage grafting, with whichever cartilage procedure the individual surgeon most favours (e.g. ACI / MACI or AMIC / Chondrotissue / Chondro-Guide etc).

If there is ligamentous instability, then the deficient ligament(s) should be reconstructed, and

If there is malalignment, then this should be corrected with a realignment osteotomy.

Biological Knee Replacement 

And finally, this brings us on to the concept of ‘Biological Knee Replacement’: this is simply a slightly cheesy Americanism that indicates that multiple reconstructive / soft tissue replacement procedures are being undertaken within a knee joint together, all at the same time, e.g. meniscal transplantation +/- cartilage grafting +/- ligament reconstruction +/- realignment osteotomy.

Some of the biggest procedures that I, personally, have undertaken to-date include:

meniscal transplantation + articular cartilage grafting + ACL reconstruction (including revision ACL surgery),

meniscal transplantation combined with simultaneous realignment osteotomy, and

combined medial + lateral meniscal transplantation with simultaneous articular cartilage grafting.

However, such cases are not for the faint-hearted or for the novice. It’s taken me more than a decade of hard work, focus, determination and dedication (and practice) to hone my personal skillset to the level where I’m able to take on cases of this kind, and yet in clinic I still tend to turn away more cases than I actually end up listing for surgery, due to people presenting with knees that are just too far gone and just not appropriate for the surgery. And after all these years of research and practice, I’m still continuing to evolve my practice and my techniques, and I am sure that I will continue to do so for as long as I continue to practice surgery.

The latest addition to my reconstructive armamentarium is the use of Vivostat PRF, which is autologous platelet-rich fibrin ‘glue’. I use this as a bioactive sealant over my cartilage grafts, as the PRF releases growth factors, and I also use this around the circumference and under the tibial under-surface of my meniscal allografts, to promote more rapid healing and graft incorporation. 

The future vs current risks and scams

As the understanding and popularity of biological knee surgery will continue to grow, the next step in the evolution of the field of ‘orthobiologics’ will undoubtedly be in the use of cells and growth factors. It’s only a matter of time until true ‘tissue engineering’ comes to the fore, with 3D-printed custom-made biological scaffolds seeded with living cells and soaked in growth factors being available to-order. 

What’s important in the meantime, however, is that enthusiasm is tempered by science and safety, and that patients aren’t given false claims about ‘stem cell injections’ into joints, where osteoarthritis can allegedly be treated by just a single injection of cells or fat into a joint. It simply can’t. If you inject cells into a joint then they simply wash around within the synovial fluid and they are then mopped up by the synovial membrane. Cells don’t swim towards defects, congregate and then decide to rebuild new articular cartilage or regrow a meniscus. That’s just fantasy, not science. There is zero clinical evidence to back up or justify such fanciful claims, and it is beholden to all of us, collectively, not to tolerate practitioners who prey on the gullible and the vulnerable. 

The UK Biological Knee Society

In 2014, a small of group of friends / colleagues with a particular specialist interest in biological knee surgery set up The UK Biological Knee Society, and I have the honour of being the President of the Society. Over the years, the Society has flourished, and we have held an Annual Meeting every year that has grown in popularity and numbers, with our last meeting having been at Man Utd’s stadium at Old Trafford, with about 100 attendees listening to cutting-edge presentations from national and international experts in the field of biological knee surgery. This is a forum for surgeons with a specialist interest in the field to discuss ideas and surgical techniques, and to present their results, to collaborate on projects and to promote the science of biological reconstruction. 

The future is exciting, but it will likely be just a continuation of further baby-steps in the evolution of ‘regenerative surgery’, which is a path that will inevitably require many further years of hard scientific and clinical work to develop.

Further info:

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 him, visit: or