Swelling at the back of the knee: possible causes

A soft or tense lump may become clearer when standing or straightening the knee. Because a clot or infection can also cause concerning swelling, sudden symptoms with calf changes, fever or marked illness need prompt evaluation.

A Baker's cyst is a pocket of joint fluid that can produce fullness behind the knee, often when the joint is irritated by arthritis or internal damage. Knee osteoarthritis may present with stiffness, aching and swelling, while a meniscus or ligament injury may present with swelling after a twist, instability, catching or reduced movement.

Gout can present with a rapidly painful, hot and swollen joint. Bursitis more often creates localised tenderness near a bursa. Appearance alone cannot reliably distinguish these possibilities, so the history and examination matter.

Reduce activities that increase pressure or pain, such as deep squatting, and use comfortable gentle movement instead of repeatedly testing the lump. A wrapped cool pack and leg elevation may ease a recent flare. Do not puncture, squeeze or vigorously massage swelling behind the knee.

Note when the swelling began, whether an injury occurred, and whether the knee locks, gives way or feels hot. A clinician may compare both legs, examine the joint and decide whether imaging or joint-fluid testing would change treatment.

Get urgent help if swelling is sudden and the calf is painful, warm or enlarged, especially with chest pain, faintness or shortness of breath. A hot red knee with fever or feeling very unwell also requires urgent assessment.

Prompt evaluation is appropriate after a significant injury, when you cannot bear weight, or when the foot becomes cold, pale, numb or weak. Book a non-urgent visit for a persistent lump, recurring swelling, restricted movement or unexplained discomfort.

Ask whether the swelling is inside the joint or in nearby tissue, what may be driving fluid production, which activities are suitable, and whether aspiration would diagnose or relieve the problem. Also clarify which recurrence or calf symptoms should prompt faster follow-up.