Fluid on the knee, occasionally referred to "water on the knee," is basically a swollen knee. It’s very common. The accumulation of fluid in the knee joint can lead to significant knee pain, making everyday activities a real pain. The knee joint is a large joint primarily designed to bend and straighten, and thus help our gait and movement. The problem with fluid on the knee is getting it off. Gravity does not help. While many try ice, and anti-inflammatories, this can help and in some cases the swelling disappears on its own. But in some cases, the knee stays chronically swollen and inflamed, and this is often where the trouble happens. So what causes fluid on the knee?
What Causes Fluid on the Knee?
Multiple factors can cause fluid accumulation and knee swelling. Let’s look in detail at some of the common causes.
ACL injury
The anterior cruciate ligament (ACL) injury is a particularly unpleasant and problematic injury. A tear can cause bleeding and significant joint swelling. This type of injury often occurs in athletes and is usually caused by twisting the knee badly, or impact with another player. Skiers are also very prone to ACL tears during a fall.
Symptoms of an ACL injury include swelling, pain, and the accumulation of fluid around the knee joint. If you experience an ACL injury, you'll likely notice that your knee feels unstable and painful when bearing weight on the leg, and you’ll have significant difficulty walking.
Other Ligament Tears
Other ligament tears can be to the MCL (medial collateral ligament), LCL (lateral collateral ligament) and PCL (posterior cruciate ligament). The PCL tear is quite unusual, and normally only occurs on a sports field when the leg is hyper-extended from a blunt trauma backwards on the leg. LCL injury is the next least common, and MCL’s are much more common. The MCL runs on the inside of the knee, and can be torn from twisting, landing badly after a jump, or trauma from impact with another player.
Torn Meniscus
A torn meniscus is a common knee injury that can cause significant knee pain and swelling. The meniscus is a C-shaped shock-absorber pad in your knee that acts as a cushion in the knee. There are two – one on the inside (known as the medial meniscus) and one on the outside (known as the lateral meniscus). A torn meniscus is commonly referred to as a cartilage tear, but strictly this is not true. The cartilage is a much tougher structure lining the end of the bones.
A tear in the meniscus can occur during activities that forcefully twist or rotate the knee, especially if the foot is planted while the knee is bent. When the meniscus is torn, it can lead to instability in the knee joint, making it difficult to move without experiencing pain. People can report the knee giving way, or locking. Additionally, a torn meniscus can cause the knee to lock or feel like it's giving way. Of course, there’ll be significant joint effusion or knee swelling.
There is another type of meniscus tear that is actually more common. This is known as a degenerative meniscus tear. This occurs through often quite benign movements, and the meniscus is degenerated from years of force going through the knee joint. This also causes knee swelling.
Knee Bursitis
Knee bursitis is a common cause of a swollen knee, often leading to significant pain and inflammation. I’ve seen many, and they always look remarkable and interesting.
This condition occurs when the small fluid-filled sacs, known as bursae, become irritated and then get much bigger. The inflammation of these sacs can cause the knee to swell, further exacerbating pain and discomfort. If you've got a swollen knee, it might be due to buritis, which can arise from repetitive knee movements or prolonged pressure. Activities like kneeling for extended periods or direct trauma to the knee can also lead to bursitis.
Let’s look in more detail at the various types of knee bursitis:
- Prepatellar Bursitis (Housemaid's Knee): This type of bursitis is situated right on top of the patella (knee cap). It is often caused by frequent kneeling, such as housekeeping or carpet laying, or roofing. Hence the term, housemaid’s knee. Fairly common.
- Infrapatellar Bursitis (Clergyman's Knee): This affects the bursa located just below the kneecap. It is often associated with repetitive stress or kneeling on hard surfaces. You guessed it, a clergyman kneeling to pray. Not very common.
- Pes Anserine Bursitis (Goosefoot Bursitis): This type of bursitis affects the bursa located on the inner side of the knee, below the joint. It often occurs in people who are overweight, have osteoarthritis, or participate in sports that involve a lot of running. Not very common.
- Suprapatellar Bursitis: This involves the bursa located above the kneecap, beneath the quadriceps muscle. It can be caused by repetitive strain. More common.
- Popliteal bursitis (Baker’s Cyst): Names after the British surgeon, Dr William Baker, it’s a swelling behind the back of the knee. The term cyst is a misleading, it’s not actually a cyst in the traditional sense. Its an accumulation of fluid at the back of the knee, in the popliteal fossa (a dip in the back of your knee). This is very common.
Osteoarthritis
Osteoarthritis is a degenerative joint disease which affects the knee joint by wearing down the protective cartilage, resulting in severe pain and stiffness. As cartilage deteriorates, the bones in the knee joint rub together, causing swelling and inflammation. Hence why the knee will be swollen.
Osteoarthritis often leads to weight gain because the pain and stiffness make it difficult to maintain an active lifestyle. The added weight puts extra pressure on the knee joint, further causing the pain and swelling. Exercises can help a lot, as can weight loss and physiotherapy. Patients often come to see us for Ostenil Plus injections too – a lubricant for the knee to help reduce abrasion.
Rheumatoid Arthritis
Rheumatoid arthritis is an autoimmune disorder which leads to inflammation in the joints, causing the joint fluid to build up and making the knee appear swollen. The inflammation results in stiffness, especially in the morning, and can severely affect daily activities. People with rheumatoid arthritis often experience affected movements due to pain and stiffness in the knee joints. Knees are often swollen, and when we aspirate them, the fluid can be cloudy or flaky.
Managing rheumatoid arthritis involves the care of a rheumatologist. Without proper treatment, the affected joints can suffer from chronic knee pain and persistent stiffness, making it difficult for individuals to maintain mobility. Blood tests may help to detect RA.
Gout
Gout is a type of arthritis that can cause sudden and severe knee pain, primarily due to the buildup of uric acid crystals within the knee joint. It more commonly affects the big toe, but I’ve seen it several times in the knee too.
This condition often leads to inflammation, resulting in significant swelling of the knee. The joint fluid within the knee can become inflamed, red, and extremely sore. Individuals suffering from gout commonly experience intense pain in the knee joint, especially during flare-ups. Blood tests can be used to detect uric acid.
Pseudogout
Pseudogout is another less common cause of inflammation in the knee joint, leading to significant pain and discomfort. This condition occurs when calcium pyrophosphate dihydrate (CPPD) crystals accumulate in the joint fluid, resulting in swelling and irritation. Unlike gout, pseudogout isn’t caused by uric acid but still leads to painful flare-ups and knee stiffness. These symptoms can mimic those of other knee conditions, making it difficult to diagnose. Management typically includes medications to reduce inflammation and alleviate pain, and aspiration can be very helpful, as can steroid injections.
Osgood-Schlatter Disease
Osgood-Schlatter disease is a common cause of knee pain in adolescents experiencing growth spurts. In almost all cases of Osgood-Schlatter, the kids have been highly active. My personal belief is that the cause is from shortened quadriceps that become even more short as the femur (leg bone) elongates during a growth spurt.
This condition results in inflammation where the patellar tendon attaches to the shinbone. At this spot, just below the knee cap, there can be some mild swelling. A lump can also appear, and this is a bony lump from the traction to the bone.
We don’t aspirate knees with Osgood-Schlatter, but our team of therapists treat it.
Infection
When an infection occurs, it can lead to severe knee pain and inflammation, causing the joint to swell and become tender. Bacterial infections, such as septic arthritis, are particularly notorious for causing abrupt and intense episodes of knee pain. This type of infection is a medical emergency, and if left untreated, it can damage the joint permanently. Other pathogens can also cause infections in the knees that might not be as aggressive but still contribute to swelling and discomfort. Symptoms often include redness, warmth, fever, and significant pain in the knee. If you suspect an infection, go to A&E. Immediate medical attention is essential to prevent further complications and ensure the joint's health.
Tumours
Thank goodness we don’t see them very often. Bone tumours are a concern, and we are always mindful of this when treating children.
Tumours can be benign, or cancerous, and if cancerous, they can be very problematic. If this is the case with your knee, it will have been very hard to predict you would have a bone cancer (e.g. an osteosarcoma).
We don’t aspirate if we suspect a bone tumour, but we can refer for MRI.
Baker's Cyst
Please see information higher up on knee bursitis. Baker’s cysts are a real pain, and the number of times patients want to hug me when I drain them is quite amusing. Patients usually feel immediate relief.
I once aspirated 2ml out of one lady, so a fairly small amount. She got off the table, and straight away knew her knee felt better. She told me within five minutes that her knee had not felt this good for years. Even aspirating small amounts can make the knee feel very much better.
Final Thoughts on Causes of Fluid on the Knee
A multitude of factors can lead to a fluid on the knee, from injury to more chronic issues and diseases. Getting a clear diagnosis is key. And then get the fluid off. The best way is to aspirate it. We aspirate knees daily, and people travel hundreds of miles to see our team because they struggle to get local NHS or private treatment.