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I have knee swelling – should I be worried?

Knee swelling is common in runnersKnee swelling often occurs in conjunction with other symptoms, such as pain. If the swelling is mild or there are no other symptoms, it may go unnoticed. The nature of the swelling may give a clue as to it’s cause. Swelling is usually a sign that there is something wrong and this should be investigated. Don’t panic, though – not all causes of knee swelling require surgery and often there is a simple solution to the problem…

One of the routine questions I ask patients who see me for a knee problem is if they have noticed any swelling. It surprises me how many people say no, and when I examine them I discover a large swelling in the knee. Knee swelling is not normal. The nature of the swelling gives valuable clues as to the cause and subsequent treatment. I find it useful to divide knee swelling into two groups: swelling inside the knee joint and swelling outside the knee joint.

Knee swelling inside the joint

Meniscal tears are performed using arthroscopyThe knee joint has a normal amount of fluid in it. This is called synovial fluid, It assists with lubrication and nourishment of cartilage. A normal amount of synovial fluid does not cause swelling of the knee and will not be detected as swelling during a routine examination.

Synovial fluid is produced by the inner lining of the knee joint (synovium). The synovium may produce excessive fluid as a result of irritation or inflammation. It is thought that this fluid is produced as a protective measure. The accumulation of fluid in the joint will result in swelling, called an effusion. This may be a small enough to be detected only by the most experienced examiner, or it may be big enough to be seen by everyone in the room.

The synovium itself may cause swelling if it is inflamed and thickened (synovitis). There will usually be some fluid associated with this type of swelling.

Besides normal synovial fluid, blood which accumulates in the joint can also result in swelling. This is seen in conditions such as haemophilia and obviously in acute knee injuries. It is also seen after surgery. I will not discuss these conditions here.

There are many potential causes for a swelling inside the knee joint. Instead of giving you an exhaustive list I would prefer to tell you about some of the common conditions I see daily.

Chondromalacia patella

Chondromalacia patella can cause a crunching sensation in the kneeI really dislike this term, but as it is a common condition I find myself using it often. It is one of the most common causes of anterior knee pain (pain in the front of the knee). It is sometimes called “runners knee” for obvious reasons. I sometimes refer to it as “back ache of the knee”. The term chondromalacia patella means “softening of the cartilage underneath the kneecap” or patella. It usually arises due to long-standing muscle imbalance and mal-tracking issues of the patella. Initially the cartilage underneath the patella become soft. This may progress to fissuring and the cartilage surface becomes rough. This causes a crunching noise in the knee called crepitus. The symptoms are typically worse when using stairs.

Swelling is usually associated with more severe cases where there is significant crepitus and fissuring.

The good news is that this condition does not normally require surgery. It normally responds to a good rehabilitation programme, focusing on core muscle strengthening exercises and strengthening of the quadriceps muscle.

Meniscal tears

Meniscus tear requiring meniscectomy

An arthroscopic view of a torn meniscal cartilage in the knee

The meniscus is a C-shaped cartilage structure in the knee. There is a medial and a lateral meniscus in each knee. They are like cushions or “shock absorbers”, and also assist with stability and proprioception (position sense). A tear in the meniscus can result in pain along the sides of the joint, clicking, and sometimes locking. Often, however, it is the finding of mild swelling in association with these features that leads me to the diagnosis. A meniscal tear is usually treated surgically by arthroscopy or minimally invasive surgery unless there is significant arthritis in the joint already (read: Three signs that you may have a meniscus tear).


Knee pain may have many causes requiring surgeryArthritis is another common cause of knee swelling. There are different types of arthritis which can cause any pain and swelling. The commonest type that I encounter is osteoarthritis. Osteoarthritis is a condition where the normal cartilage in the knee is worn away through wear and tear. In the most severe form the cartilage may be worn right down to the level of the bone. The synovium may become inflamed and produce increased amounts of synovial fluid as described. This will result in an effusion (fluid in the joint). Osteoarthritis is generally seen in older age groups, but may develop in younger people, especially if the knee has had significant trauma.

An inflammatory arthritis such as rheumatoid arthritis will also cause swelling of the knee. Swelling may be the first feature of these conditions.

The treatment of osteoarthritis begins with lifestyle modification, and the use of anti-inflammatories and physiotherapy. Steroid (or other) injections into the joint may be required for more advanced cases. Someone with arthritis who does not respond to these interventions may need a total knee replacement.

Baker’s cyst

A Baker’s cyst is a swelling felt at the back of the knee. It is extremely common. It is caused by fluid in the joint which puts pressure on a weak area in the back of the knee. This causes a bulging of the joint capsule in this area. It is no different to the swelling caused by arthritis – they are one and the same. The fluid is not usually drained from a Baker’s cyst just for the sake of draining the fluid as it will simply re-accumulate. The best treatment of a Baker’s cyst is to treat the underlying cause, which is the arthritis (see above).

Knee swelling outside the joint

There are conditions which can cause knee swelling which do not involve an accumulation of fluid inside the joint itself. This kind of swelling is usually related to a specific area of the knee which makes diagnosis easier.


A bursa is a pocket or sack of tissue which normally has a small amount of fluid in it. They are found throughout the body, especially where the skin moves over a prominent bony area, such as the knee and elbow. They assist with gliding movements of the skin over the bone. There are three bursas in the front of the knee. The prepatellar bursa (in front of the patella); the suprapatellar bursa (above the patella); and the infrapatella bursa below the patella.

Normally you cannot feel a bursa through the skin. If it becomes inflamed or irritated it may fill with fluid and cause a lump. This may happen because of trauma such as a knock to the knee. It may also occur as a result of a repetitive activity. Another name for a prepatellar bursitis is “housemaid’s knee”, supposedly named after a housemaid who develops this swelling due to long hours spent on her knees scrubbing floors. Another name for infrapatellar bursitis is the “clergyman’s knee” – the way a clergyman would kneel to pray would put pressure on the infrapatella bursa.

Bursitis may be treated by anti-inflammatories or cortisone injection. Occasionally a compression brace may help. If the bursal swelling becomes established it may form a thick wall and become a large lump which swells repeatedly every time the knee takes a knock. These are best excised.

Sometimes a bursal swelling can become infected or an infection can result in a bursitis. These should be treated with antibiotics and may require surgical drainage if the infection is severe.

Popliteal aneurysm

An aneurysm is an abnormal swelling in an artery. The major artery which courses down the back of your knee is called the popliteal artery. A swelling or dilatation of this blood vessel is called a popliteal aneurysm. It can be confused with a Baker’s cyst. This aneurysm is outside the knee joint, however, and it is pulsatile. It goes without saying that an attempt to drain this kind of swelling with a needle should NOT be attempted. Popliteal aneurysms may become large and can cause blood clots which can compromise the blood supply to the foot. They are best assessed and treated by a vascular surgeon.

And Gout?

Oh, yes…never forget about gout. I nearly did, which is why it is last on the list.  Gout is a common condition where crystals of uric acid accumulate in a joint. It can result in sudden attacks of pain and swelling. The pain can be quite severe, and sufferers say that it is even too painful to have a bedsheet resting on the joint. In the long term, uncontrolled gout can cause chalky white deposits in tissues. Gout has a tendency to invade through tissue planes and can affect the knee both inside and outside the joint.

Gout can mimic or be found in addition to many of the conditions listed above. The focus of treating gout is to control the body’s uric acid levels through diet, lifestyle modification and/or medication. Gout commonly causes bursitis as described above, and may require surgery. Surgery may also be needed if long-standing gout has resulted in an arthritic knee or a meniscal tear.

There are other causes of knee swelling, both inside and outside the joint. Tumours and rare conditions of the synovial lining come to mind. Fortunately these are not seen commonly and won’t be discussed further.

In conclusion…

A swollen knee is not normal, and should always be investigated properly to establish the cause and to rule out a serious condition which may require surgery.

Yours in good health,

Dr Warren Matthee

MB BCh (Wits), MRCS (England), MMed (Ortho Surg), FC Orth (SA)

Orthopaedic Surgeon

(011) 304 6784

About the author…

I am an orthopaedic surgeon with a special interest in sports injuries and minimally invasive (arthroscopic) surgery of the knee and shoulder. I treat patients at Waterfall Hospital in Waterfall/Midrand and Morningside Mediclinic in Sandton. Besides these, I also treat patients from other areas as well, including Rosebank, Sunninghill and Fourways.

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