When should you see a doctor about knee swelling?

Swelling may be fluid inside the joint or inflammation in tissues around it. An ACL tear or meniscus tear can cause swelling after injury. Knee osteoarthritis may produce activity-related swelling, while gout can cause a sudden hot and painful joint. Bursitis affects a fluid sac near the knee, and a Baker's Cyst can create fullness behind it.

Septic arthritis is a serious infection of a joint. A hot, red, very painful knee with fever needs urgent assessment because symptoms can worsen quickly. The amount of swelling alone cannot show which cause is present.

Rest from impact and kneeling, elevate the leg when comfortable, and use a cold pack wrapped in cloth for short periods. Avoid massaging a hot or newly injured knee. Do not drain the swelling yourself. Note when it started, whether there was a twist or blow, and whether you have fever or pain in other joints.

Seek urgent care for sudden swelling after injury, inability to bear weight, deformity, severe pain, or a knee that is hot and red. Fever or feeling acutely unwell alongside swelling is especially concerning. A clinician may need to examine the joint and, in selected cases, consider joint aspiration to investigate or relieve fluid.

Ask whether the swelling is inside the joint, whether you need imaging or fluid testing, and which activities are safe. Ask how to recognise worsening infection or inflammation and when you should return for review.