Swollen knee: what’s the cause?

The increase in the amount of intra-articular fluid is often referred to as knee effusion. The severity and urgency of the clinical picture will depend on the original cause.

What are the main causes of knee effusion?

This clinical picture may result from different causes, including direct or indirect trauma (knee sprains or twists), synovial diseases, the existence of a degenerative disease and infections, or septic arthritis.

In the case of post-traumatic knee effusion, sprains or physical overexertion, the cause may be:

  • Torn knee ligaments
  • Meniscal injuries
  • Degenerative diseases
  • Joint fractures

Secondary knee effusion can also emerge from systemic diseases (diagnosed or undiagnosed) such as:

  • Rheumatoid arthritis
  • Septic arthritis
  • Gout
  • Pseudogout
  • Transient synovitis

What to do when joint effusion occurs

After a sprain: If the effusion appears after trauma or spraining, it is advisable to rest the limb immediately, use crutches to walk, and apply a cold pack to the knee every two to three hours, while avoiding direct contact with the ice to prevent burns on the skin.

The patient should then seek emergency care as soon as possible, where they will perform necessary examinations and additional tests.

If severe effusion occurs, joint aspiration will control the pain significantly. It will also help determine a possible diagnosis, based on the contents of the evacuated knee (haematic, synovial, etc).

Postoperatively: Sometimes, as with postoperative processes, inflammation or knee effusion can occur, which may also result in an increased local temperature of the knee and the body. This clinical picture is to be expected, in isolation, within the first seven to ten days after the operation. It not recommended to take any antibiotics without first seeking medical care to undergo, when necessary, a joint aspiration (arthrocentesis) and a joint fluid culture. The appearance of the symptoms described above is often a result of the postoperative inflammatory process, without any relation to an infectious process. It is recommended to monitor the situation closely and remain vigilant, while also regularly monitoring the body temperature.

In degenerative knees: When a patient has a history of degenerative joint disease that leads to knee effusion, they should rest, take the weight off the joint by using crutches and apply ice to the injured area. If effusion occurs when the knee is under tension, the patient should see a specialist, undergo joint aspiration and monitor the progress of the joint.

Diseases of synovial membrane: There are different diseases that affect the synovial membrane and that frequently result in knee effusion, whether synovial fluid or with haematic content (hemarthrosis). When this occurs and in the case of it being practically spontaneous with no apparent cause for joint effusion, a specialist should undergo a puncture of the knee joint. Additionally, images should be taken for review, such as X-rays and MRI to find a specific diagnosis. Occasionally, it may be necessary to refer the patient to rheumatology.

How to avoid effusion when a degenerative joint is involved

When joint effusion is associated with a degenerative process, there are measures to protect the joint and reduce the risk of the knee swelling.

These are:

  • Keeping one’s body weight under control
  • Avoiding high-impact exercise
  • Strengthening periarticular muscles, performing stretching exercises, doing kinaesthetic exercises and maintaining proper joint balance
  • Apply a cold pack to the local area after exercise.