The potentials risks and possible complications of knee replacement surgery.

Knee replacement is a major surgical procedure, and inevitably there are a number of potential risks involved.

As surgeons, your safety is paramount, and we do our very best to try and ensure that every patient achieves the best possible outcome. However, no matter how many precautions we take and how careful we are, one can never completely eliminate all risks. It is therefore essential that every patient is fully informed about all of the potential risks of knee replacement surgery before they make the decision to actually proceed.

A discussion about the potential risks of surgery is an essential part of the pre-operative process, and informed patient consent is a prerequisite for any surgical procedure. If you would like any further information about anything relating to your potential knee replacement surgery, then it is imperative that you speak to your surgeon and that you have a full and detailed discussion prior to proceeding.


Infection is usually one of the biggest concerns of any surgeon or patient. There is always a risk of infection with any form of major surgery. The risk of infection is taken very seriously in knee replacement surgery, especially with the prevalence of new antibiotic resistant strains of bacteria, such as MRSA. The operation itself is therefore carried out in full aseptic conditions and in an operating theatre that uses ultra-filtered air with laminar flow (where the air comes from the ceiling and then down towards the floor and out of the theatre through low-level vents). In addition, patients are given 1 intravenous dose of prophylactic antibiotics at the start of the surgery followed by 2 intravenous doses post-operatively.

The risk of deep infection from knee replacement surgery is estimated to be in the region of about 1%.

If a wound becomes infected with just a superficial infection, then this can normally be treated effectively with appropriate antibiotics. However, if there is a deep infection, in or around the joint / the prosthesis itself, then this can be a major problem. Deep infections rarely ever settle down with just antibiotics alone. Sometimes is it possible to re-open up the joint, wash it out copiously (and also exchange the polyethylene tibial insert), treat the patient with a course of intravenous antibiotics followed by a course of oral antibiotics, and this may then be sufficient to eliminate the infection.

Unfortunately, however, if a knee replacement prosthesis does become infected, then the bacteria can stick to the metal of prosthesis and they form a protective ‘biofilm’, so that antibiotics cannot get to the ‘bugs’… in which case the whole joint prosthesis might end up needing to be removed and an antibiotic-loaded temporary spacer might need to be placed in the knee. The patient is then treated with intravenous antibiotics for a minimum of 6 weeks. Then, if the infection appears to have resolved, a new prosthesis can then be implanted back into the knee. However, even with this, there is no guarantee that the infection might not potentially still come back again. Also, outcomes after revision surgery for an infected knee replacement never tend to be as good as for a successful primary knee replacement.

Patients should also be aware that the risk of potential infection tends to be slightly higher in patients with diabetes, with obesity or with an medical condition that causes suppression of the immune response. In addition, it is important to ensure that one’s teeth are in good condition, as any dental infections (or infections elsewhere in the body, such as urinary tract infections) can cause ‘seeding’ of bacteria into the blood stream, which can lead to infections developing inside the knee.

So, this is why we take the risk of infection very seriously!

Nerve or blood vessel damage

All of the major nerves and blood vessels are round the back of the knee. With knee replacement surgery, we go into the knee joint through an incision at the front of the joint. Despite this, there is always a tiny potential risk of damage to the vessels. This can, in severe scenarios, result in a numb leg and foot, with complete loss of muscle power, or if there is severe damage to the major blood vessels then a patient could even end up needing an amputation and losing their leg! Thankfully, however, this is incredibly rare.

Numbness of the skin around the scar at the front of the knee is, however, quite common after knee replacement surgery. The cutaneous nerves within the skin are so small that you can’t even see them as you’re making the incision for the knee replacement surgery, and even if you could, you wouldn’t be able to avoid them anyway. However, most of the time any numbness around the front of the knee is only temporary, as the superficial nerves in the skin do normally tend to grow back, albeit very slowly, and this can take several months.

Very occasionally, a patient can develop a painful lump on a superficial nerve in or around a knee replacement scar, and this is called a neuroma. These can be very tender. Sometimes they might need to be injected with local anaesthetic and steroid. Very rarely, they might even need a small operation to remove the neuroma (much less than 1% risk).

Blood clots

Studies have shown that the risk of developing some kind of blood clot in one of the leg veins after knee replacement surgery may actually be quite high, with estimates ranging from 40% up to 80%. However, the very large percentage of these clots are ‘sub-clinical’, meaning that they don’t cause any perceptible symptoms or long-term problems. If a clot is large, however, and if it blocks off one of the major veins at the back of the leg, then this is referred to as a Deep Vein Thrombosis (a DVT). A significant DVT can cause pain and swelling in the calf and foot, and it some instances this can be associated with skin problems (due to impaired circulation) in the longer-term (post-phlebitic syndrome).

Smaller DVTs below the level of the knee often don’t need any kind of treatment. However, with larger DVTs, and particularly those that extend up above the level of the knee joint, there is a risk that bits of clot may break off and go into the circulation, and end up getting lodged in the lungs, which is called a Pulmonary Embolus (PE). Symptomatic PE occurs in probably under 1% of patients; however, large PEs can, very rarely, actually be fatal.

Again, this is why we take the risk of blood clots very seriously with knee replacement surgery.

There is a lot of uncertainty and debate about what the best methods might actually be for reducing the risks of potential blood clots with knee replacement surgery. The author’s protocols are:

  • patients are risk-assessed prior to surgery as part of their pre-admission assessment,
  • we use intermittent calf compression pumps on the other, non-operated leg during the actual surgery,
  • we strongly encourage early mobilisation after the surgery, and patients are told to keep moving as much as possible,
  • TED (Thrombo-Embolic Device) compression stocking use,
  • Clexane (low-molecular-weight heparin) blood thinning injections are given under the skin once a day whilst in hospital, and
  • patients are discharged from hospital with 2-week supply of Rivaroxaban tablets, which is an oral anticoagulant (blood-thinner).

Your medical team will ensure that all of the above precautions are taken; however, it is very much down to you, the patient, to ensure that you get going as quickly as possible after your surgery and that you spend as little time as possible just lying immobile in bed.


If you don’t get your knee moving quickly enough after your knee replacement surgery, then there is a risk that excessive scar tissue might form… The longer scar tissue is left, the thicker, stronger and less elastic it becomes, and hence the longer you leave a stiff knee, the harder it becomes to actually get it moving. The stiffness may be a lack of extension (getting the knee fully straight) or a lack of flexion (bending the joint), or potentially both.

Normally, we would want to see patient bending their knee up to at 90 degrees, if not more, before they leave hospital after their knee replacement surgery. Ideally, we would want to see a 100+ degree bend by 2 weeks post-op, a 120-degree bend by 6 weeks and 135% by 3 months (particularly in younger patients and patients with skinnier legs, but perhaps less than this in patients with bulkier legs).

If a patient’s knee is too stiff post-operatively, and if they are not making sufficient progress with their physio rehab treatments, then it is sometimes necessary to perform a Manipulation Under Anaesthetic (an MUA) of the joint. This involves bringing the patient back into hospital, giving them a very quick anaesthetic and forcing the knee to straighten and bend, which breaks down the scar tissue. The patient is then kept in hospital for a day or two for pain relief and intensive inpatient physio, to try and ensure that the increased range of motion is maintained, and after discharge the patient then has to work twice as hard to keep the knee moving and to avoid further scar tissue simply building up again. An MUA is required in roughly 1% or so of patients.

Kneeling after knee replacement surgery

One other important thing to note about knee replacements, is that only about 50% of patients are actually happy and comfortable to kneel on the front of their new knee. It is impossible to predict who will or who won’t feel comfortable kneeling, and it’s a roughly 50:50 thing. Therefore, some patients do end up needing to use a cushion to be able knee comfortably, once they’re actually mobile enough to get down into and up from a kneeling position.

Is it possible to be allergic to a knee replacement?

Some people have ‘allergies’ to some metals, particularly to nickel. However, these ‘allergies’ have actually been shown to be skin sensitivity. There is no convincing scientific evidence that one can develop an allergy to a metal implant that’s placed inside the body.

Some knee replacements are made from Titanium. Conformis knees are made from Cobalt-Chrome alloy. A recent review of the published literature concluded that there is no point in performing pre-operative skin patch-testing prior to knee replacement surgery, because it is too unreliable, with too many false positives. Also, the latest published professional advice is that there is no justification for using special ‘hypoallergenic’ prostheses (made of Oxinium, which is anodised metal, where the surface layer is treated) as there is no evidence that these make any difference.

However, if you have any concerns about potential metal allergies, then you must specifically discuss this with your surgeon, particularly in advance of having a Conformis custom-made implant or any other Cobalt-Chrome prosthesis, as these do tend to contain trace amounts of Nickel.

Other potential risks

Other negative things that can potentially occur during or after knee replacement surgery, albeit only very rarely (<1%) include:

  • bone fractures of the femur or the tibia (mainly in people with very weak bones, e.g. severe osteoporosis)
  • fracture of the patella (very rare indeed)
  • clicking or clunking (which can be due to scar tissue catching inside the joint)
  • rupture of the quads tendon (normally only if there is significant trauma, like a bad fall post-operatively)
  • urinary retention (in ‘men of a certain age’ who may have an enlarged prostate gland), some people might sometimes struggle to pass water in the early post-operative period, in which case a temporary urinary catheter might potentially need to be inserted.

Wear / failure / revision

All artificial joints will eventually wear out and fail if the patient lives long enough.

If the plastic tibial spacer in the knee wears out but the metal prostheses are still fixed firmly in place, then it is sometimes possible to replace just the plastic spacer.

Often, however, ‘failure’ from ‘wear and tear’ tends to be from a process called ‘aseptic loosening’… The metal surfaces in a knee replacement are harder-wearing than the plastic spacer, and hence with time tiny plastic wear particles tend to form. These plastic particles excite and activate some of the white cells, which then elicit an inflammatory reaction. This reaction causes pain and swelling; however, it also activates the osteoclast cells within the bones, which are the cells that absorb bone. This can cause the bone to shrink back from the metal prosthesis, so that the prosthesis becomes increasingly loose, thus causing increasing pain. If this happens, then the whole prosthesis normally ends up having to be replaced / re-done – and this is called a revision knee replacement.

Roughly speaking, 95%+ of knee replacements are still working after 10 years, with about 80% still working after 20 years.

Importantly, however, the younger you are when you have a knee replacement then the more active you are likely to be. The more active one is, the faster the rate of any wear and tear on the prosthesis will be, and the quicker it is likely to wear out… and hence the more likely it is that the knee will fail within your lifetime. Also, the younger you are, the longer you are likely to live, and hence again, the more likely it is that the prosthesis will wear out and fail within your lifetime. A bit of a ‘double-whammy’!


  • if you have a knee replacement in your 70’s, then there’s only an approximately 10% chance of you ending up needing a revision within your lifetime; however,
  • if you have a knee replacement in your 50’s, then there a 50% chance of you needing a revision within your lifetime.

This is one of the important factors to consider when deciding on when the right time might be to actually go ahead with knee replacement surgery.

Just not happy!

Even if there haven’t been any specific complications per se, some patients end up simply ‘not happy’ with their knee after knee replacement surgery. The joint may feel too stiff or tight, there might still be some ongoing discomfort or even some ongoing pain. Or, the patient just might not be happy with their function.

Overall, with a standard off-the-shelf prosthesis, about 80 to 85% of patients report that they are happy with their new knee. This means that up to 15 to 20% are not!

This is one of justifications for using custom-made knee replacements, because studies have shown that with custom-made knees the patient satisfaction rates normally tend to be in the region of about 95%. This is still not 100%, but it does represent a 2/3 to 3/4 reduction in the number of patients likely to end up unhappy with their knee replacement, which is clearly significant!