Total or partial knee replacement components have a limited lifespan. Wear and tear of the polyehilene spacer is the main reason for the limited lifetime of prostheses. This can be further affected by infection, aseptic loosening, inaccurate position of the implants and bone loss around the prosthesis. On average we expect that the currently implanted prostheses will last 15-20 years regardless of usage which means that not exercising or not using your knee will not extend the lifespan of the prosthesis.

One of the most important reasons for knee replacement revision surgery is infection. On average, we expect that 1-2% of all knee replacements will become infected. It is not completely clear how the infection actually gets into the knee joint; at the moment we believe that the most plausible theory is heamotogeneous spread during and after the initial procedure from a source in the body (such as an infected tooth, chest infection and urinary tract infection, which are not necessarily obvious during the surgery). The other possibility is contamination during the surgery. Aseptic loosening is also not completely understood. There are clinicians who believe that the prosthesis can become loose even if an infection is not present while some clinicians believe that all aseptic loosening events are actually infected but the infection has been caused by low-grade virulence bugs which can be quite difficult to identify during testing.    

Inaccurate position of implants is relatively common, certainly more common than we would like. There are significant attempts in this direction mainly by computer navigated and robotic surgery but the evidence for its advantage is currently lacking. Whilst there is no doubt that the computer can be more accurate than a human, it looks that the technology is still not where we would actually like it to be. When a component is put in at an angle which is not optimal as per the component's design, it is very likely that the forces acting on the component will not be neutralised leading to constant "overloading" of the component and result in a premature failure of the component/bone interface. Last but not least, polyethylene is a shock absorber between the tibial and femoral component and is exposed to significant forces during walking or even just during exercising. Over the years, it can change in its structure, become brittle and start delaminating which creates a lot of small debris particles which can move towards bone cement/bone interface (bone cement is a glue which fixes a prosthesis to the bone) and causes further reabsorption of the bone resulting in a loose component. The technology available today has improved mechanical properties of polyethylene significantly but it is still not at the stage where we would like it to be.

Anyone can develop a loose knee replacement which will require revision surgery. If you had a knee replacement done recently or a long time ago, it is possible that your knee replacement will need to be revised if the symptoms from your knee are worsening as time goes on and affecting your lifestyle.

Main symptoms that something may be wrong with your knee replacement are:

• Pain
• Swelling
• Difficulty Walking
• Instability
• Redness and Warmth
• Painful Clicking
• Reduced range of motion
• Progression of symptoms in time
• Activity related issues
  

First we will do a blood test. We are looking for inflammation markers in your blood (White cell count, ESR, CRP). This is always done at the same time as an X-ray of your knee replacement to look for any signs of loosening, instability, bone loss or bone collapse. If the X-ray is inconclusive, a CT scan of the knee replacement can be useful. MRI and nuclear medicine scans are of very limited importance. When those tests are completed we usually proceed with a knee aspiration to obtain fluid for microbiology tests. If the fluid is not available or it is not possible to aspirate the knee, will will obtain tissue samples from the knee using knee arthroscopy methods. Regardless of the method we use, it is extremely important that we minimise the risk of cross-contamination and that we obtain enough samples (usually we will go for 5 independent samples) for accuracy of the examination.

If your knee replacement is loose and causing symptoms it is very likely that you will need revision surgery. Revision surgery means that we will remove all the existing components and replace them with new ones depending on requirements if at all possible. If revision surgery is not possible, a knee fusion is certainly an option. In the worst case scenario an amputation has to be done, but this is rarely the case.

Around 5% of all patients with knee replacement will have certain symptoms coming from their knees and usually they describe them as pain with sensation of having a cuff around the knee which is squeezing the knee. This is usually a result of the scarring process and it is very difficult to positively affect as more surgery creates more scar tissue which then becomes even tighter and causes more pain.

Instability of the knee can be caused by either ligament damage (if there was a recent trauma to the knee) or thinning of the polyethylene. Unfortunately, in the majority of cases, they both require a form of surgical intervention.