Knee Replacement Surgery

I will begin this discussion by saying that in general knee replacement surgery outcomes are not as good as hip replacement outcomes.  It is an interesting fact that this is possibly because the expectations of patients who have the replacements are not in line with the expectations that we as surgeons have for knee replacements.  Unlike a hip replacement, a knee replacement is not as functional a replacement as that of the hip.  A knee replacement is primarily for pain relief.  A knee replacement is not going to allow you to play singles tennis or even doubles tennis in many cases.  So it is really a joint replacement that will take your pain away and allow you to go for walks, to swim with freestyle kicking for example, to do light cycling and play golf.  To a large measure the marketing by numerous implant companies often overstates the functional outcome for knee replacements and this adds to the unrealistic expectation that many patients have with respect to having this surgery.

In addition, a knee replacement is a far more painful experience when compared to a hip replacement.  This is primarily because there is a lot more bone cutting required during knee replacement surgery.  The approach is through the front and has not changed for many decades.  What has changed for knee replacement surgery is the anaesthetic side of things. We spend some time injecting large amounts of local anaesthetic and other painkilling medication around the knee during surgery in order to prevent pain postoperatively and facilitate early movement.  With respect to the latter, all my patients are placed in a continuous passive motion machine that immediately moves the knee after surgery.  This may sound cruel but in fact allows you to become used to the idea of early movement.  It improves circulation of the limb and it minimises initial adhesion formation within the joint.  This is relevant as one of the complications of knee replacement surgery is an excessively stiff joint after the procedure.  In addition, this early mobilisation allows a patient to get in and out of bed and mobilise up and down stairs with their crutches within 3 to 4 days.  I have to say that knee replacement surgery is conceivably more technically demanding than a hip replacement in some respects.  As a surgeon I have to pay attention to keeping the joint line where it is supposed to be as well as doing the correct bony cuts in order to facilitate very precise ligament balance around the knee.  The nature of osteoarthritis of the knee is that there is often deformity, one may be bowlegged or knock-kneed and often one is unable to straighten the knee prior to the operation.  Sometimes bone outgrowths are noted which we call osteophytes and these limit over many years the range of motion of the knee.  At the time of surgery we carefully remove all of these osteophytes in order to facilitate an improved range of motion of the knee.  However, if you read the evidence in the scientific studies on this point, often the range of motion after surgery is dictated by the range of motion prior to surgery.  This is probably due to the fact that when the knee becomes very stiff there is an excessive amount of muscle wasting and contracture which we cannot alleviate at the time of doing the knee replacement.  It is hoped however that with dedicated effort on the part of the patient, a better range can be obtained after the operation.

There are numerous designs of the knee replacement on the market, however, if you run through all the Meta analyses and review articles on the subject and despite what the different implant companies will tell you, the result of the knee replacement is primarily based on the preoperative state of the limb and by extension the patient as well as the skills of the surgeon.

In terms of materials, the highly cross-linked polyethylene forms the basis of the knee replacement and the metal components are titanium alloy as with a hip replacement for the lower leg (tibial) side and a cobalt chrome stainless steel alloy for the Upper leg (femur) side. I use a ceramicised metal component for the femur side which is called Oxinium, which is my preferred knee replacement material.  The reason for this is the very smooth surface and ultimately a lower wear rate for the bearing surfaces and a lower allergy risk.  The majority of knee replacements require a bone cement interface in order to fix the implants onto the bone.  This bone cement contains antibiotic which assists in lowering the rate of infection following surgery.  Knee replacements, due to the fact that they are relatively close to the skin have a slightly higher rate of infection when compared to hip replacements.  One of the biggest concerns following a knee replacement is bleeding around the knee, because collections of blood are sources of food for bacteria.  I take great care to obtain as much control or otherwise known as haemostasis in respect of bleeding around the knee.  I do not use a tourniquet during my knee replacements because this allows me to see the bleeders at the time of surgery and to cauterise the bleeding blood vessels so that there are no surprises post-surgery.  Many of the deformities are correctable at the time of the operation as we aim to produce the normal mechanical axis for your leg.  The mechanical axis is a plumb line from the middle of the hip to the middle of the ankle and this line should in most patients run just inside the middle of the knee.  This is very important because it has an impact on how the weight is distributed through the knee replacement as well is how the patella moves in the groove of the knee replacement.  Sometimes we also have to resurface the patella or kneecap.  This is not always necessary, however, in very large patients or patients with deformed kneecaps or those with very soft bone, for example rheumatoid patients, it is wise to do a patellar resurfacing at the time of the knee replacement.  This subject however is also one of the unresolved discussion points amongst knee replacement surgeons.

After surgery as with hip replacements, you will need to be on a blood thinner in order to prevent deep-vein thrombosis.

The list of possible complications with respect to knee replacement surgery is similar to that of a hip replacement, however, as mentioned before, excessive scarring around the knee replacement can result in stiffness which may require subsequent manipulations under anaesthetic.  In addition, the knee replacement is far more susceptible to infections and infection in a knee replacement is a very serious complication.  The blood vessels and nerves are also at risk, in particular if there has been previous surgery to the knee due to scarring which often tethers these structures close to the bone and places them at increased risk at the time of the knee replacement.  Instability of knee replacements can be a complication although it is rare.  Subsequent long-term concerns for knee replacements as for hip replacements is loosening of the implants and wear of the polyethylene surface which may require revision surgery.  As with hip replacements we are working with bone and during the implantation, sometimes bone can break.  This is normally a problem in osteoporotic bone but can be managed appropriately as once again bone itself has the potential to heal back to its original state unlike any other organ in the body which heals with scar tissue.

As always, we are endeavouring to improve the functional outcome especially for knee replacements.  We are already able to improve the pain component but we would like to try and make the knee replacement almost as functional as a hip replacement.  In this regard there are some designs which maintain the cruciate ligaments.  Historically they have not done well decades ago, however the newer designs may well do a lot better. But, we do not have enough accumulated data in this regard yet so not a proven option.  Other advances are the use of patient specific templates.  These are 3D printed templates of the patient’s anatomy where we use preoperative planning on a computer to design the cutting guides that fit onto the anatomy of the patient to give us a more reproducible and precise alignment of the implants following surgery. These template are however most useful in cases with unusual anatomy. Interestingly for normal osteoarthritic knees, we have not yet shown that 3D templates improve outcomes over and above the normal techniques of doing a knee replacement.  There is of course added expense and many of the medical aids refrain from paying for these templates, specifically when there are global fixed fee initiatives associated with joint replacement surgery nowadays. I have used these techniques and lectured on their use but have had to stop due to the above financial constraints and available evidence.

One of the more modern endeavours is the use of robotic computer assisted surgeries.  This allows the surgeon to very carefully cut the bone, again improving the alignment reproducibility and more precisely sizing and positioning implants with a view to improving the balance of the ligaments and soft tissues around the knee.

Again this is still new and we do not have data that fully supports the use of these modalities, even though intuitively we feel that they would be benefit.  The problem however is that there is always a slightly increased length of surgical time associated with these modern techniques and based on evidence, the one thing that does increase the risk of infection is the time of the operation where there is an open wound.  So we have to balance our scientific and academic pursuits with the possible risks of these procedures.  What is however interesting is that there is no consistent high grade evidence that shows that these modern techniques improve long-term outcomes in knee replacements done by experienced surgeons using the normal instrumented techniques.

In up to 10% of patients with osteoarthritis of the knee the arthritis is located to a single compartment of the knee, either the inner, the outer or the patellofemoral aspect of the knee. In these cases one can consider having only that portion of the knee replaced. The biggest advantage is that the cruciate ligaments are maintained and this provides for a more functional outcome. It is however still a knee replacement, and is more technically demanding than a total knee replacement and a proportion of patients will in future need conversion to a total knee replacement as the rest of the knee continues to wear out.

In my practice, I do not believe that a unicompartmental knee replacement is any less risky than a total knee replacement. I also do not think that it is an option without very careful consideration as its indications are a very unique set of patients with localised disease in the knee. In my practice I would rather perform surgeries that stimulate a biological healing process if at all possible in order to stave off the use of artificial implants in the knee to a later age. It must be remembered that when putting in any form of knee replacement there is inevitably a degree of bone loss which one can never get back. Saying this however, when correctly indicated, it is an excellent operation and the functional outcomes are slightly better than a total knee replacement. The recovery times are also slightly quicker and the postoperative pain slightly less because of the need for less bony cuts.