When we examine the knee, we might need to see if there is swelling there. Patients may report to us that the knee is swollen, but it actually may not be. For example, it could be adipose tissue in the case of people who are overweight, or it could be hypertrophy (bone laid down around the knee joint). There are specific knee effusion tests to help us determine if there is actually excess synovial fluid or blood around the knee,
Understanding Knee Effusions
What is Knee Effusion?
Fluid on the knee, another name for knee effusion, is simply excess synovial fluid or blood that has accumulated in and around the knee joint. There’s lots of space for fluid to gather, such as above the knee cap, and behind the back of the knee. These spaces can be filled with excess fluid, giving you a knee effusion (sometimes called oedema).
Common Causes of Knee Effusion
Several conditions can lead to knee joint swelling, such as osteoarthritis, rheumatoid arthritis, blunt trauma (e.g. banging your knee hard), meniscus injury, fracture, or ligament tear. Anything that potentially tears of damages a structure could cause bleeding, which presents as swelling on the knee. Excess synovial fluid could be made too.
Symptoms Associated with Knee Effusion
You are going to find your knee is swollen and painful. You may be struggling to put weight through it, and you may have difficulty bending your knee to put socks on, or kneeling down. The knee is going to be stiff, because the fluid exerts a hydrostatic pressure on the joint which reduces the range of movement. In most cases, the knee will be stiff into flexion, so you might have difficulty bending and straightening your leg.
Importance of Knee Effusion Tests
Why Accurate Diagnosis is Crucial
Accurate diagnosis through a knee effusion is useful to find out if the swelling is actually fluid. As mentioned early, some people come to me and report swelling but when I examine the knee, I find no swelling. This can lead to disappointment from the patient who is expecting their knee to be aspirated. If there’s fluid, we’ll get it out.
The most accurate examination is the MRI scan – it shows almost everything in the knee. Although we don’t need an MRI scan before an aspiration or injection, it can be helpful to have the knowledge of any underlying issues with the knee. We do refer for MRI scan if needed, but it won’t stop us aspirating the knee.
Clinical Examination for Knee Effusion
Physical Examination Techniques
During a physical examination, physiotherapists will assess the knee for signs of fluid by observing its appearance and feeling for any swelling and heat. The presence of a balloon-like bulge in the knee area indicates fluid accumulation. We’ll also look at the range of motion of the knee, by checking flexion and extension. We want to see how far you can bend the knee. We also want to see if you can fully straighten your leg, as sometimes this will be restricted when there is knee effusion.
Identifying Visible Signs of Effusion
Visible signs of knee effusion include noticeable swelling, asymmetry between the affected and unaffected knee, and restricted range of motion. The fluctuation test and bulge test are common techniques used to confirm the presence of excess fluid. The fluctuation test involves applying pressure to the knee to feel the movement of fluid, while the bulge test looks for a visible shift of liquid when pressing on different parts of the knee.
Special Diagnostic Tests for Knee Effusion
Arthrocentesis Procedure
Arthrocentesis, or joint aspiration, is a procedure where a needle is inserted into the knee joint to withdraw synovial fluid for diagnostic purposes. This procedure not only helps relieve pressure and pain from fluid buildup but also provides a sample for synovial fluid analysis, which can determine the presence of infection, crystals, or other abnormalities.
Synovial Biopsy
In some cases, a synovial biopsy may be necessary to obtain a tissue sample from the lining of the joint. This test is performed when there is suspicion of chronic inflammatory diseases, infections, or malignancies that cannot be diagnosed through fluid analysis alone. The biopsy helps in detailed examination of the synovial membrane, aiding in accurate diagnosis.
Interpreting Test Results
Normal vs. Abnormal Findings
Interpreting the results of knee effusion tests involves comparing the findings to normal values. Normal synovial fluid is usually a golden yellow and straw-colored, whereas abnormal findings may include cloudy fluid, indicating infection or rheumatoid arthritis. In cases where there is blood, this indicates trauma.
Common Diagnostic Patterns
Certain patterns in test results can point to specific diagnoses. For example, high levels of calcium pyrophosphate crystals suggest pseudogout, while the presence of uric acid crystals indicates gout. Bacterial infections are confirmed by positive cultures from synovial fluid. These patterns help clinicians narrow down the possible causes of knee effusion and tailor treatment plans accordingly.
Follow-Up and Next Steps After Diagnosis
Developing a Plan
We often have a process of shared decision making with the patient. We discuss the options and what is best to do. In most cases, patients simply want the fluid out of the knee.
This is a good time to ask any questions about the procedure or what is involved.
Procedure for Aspiration
The aspiration process is fairly quick, depending on how much fluid there is, and will require a needle to go into the pocket of fluid or at multiple areas of the knee. The process can be mildly uncomfortable but most patients tolerate it very well, and local anaesthetic is not required.
We clean the skin with an alcohol wipe, and then insert the right-sized needle into the joint or around the joint depending on each case. The fluid is aspirated out.
Once out, most patients feel easier straight away with more range of movement and less discomfort. There will be pain from the actual needle insertion, but this wears off fairly quickly.