Swelling and bruising behind the knee: what to know

Bruising may come from bleeding under the skin after a knock, twist, or muscle strain. Fluid from an irritated knee joint can collect toward the back and form a Baker's cyst, producing fullness or tightness when the knee bends. A meniscus tear or ACL tear may cause swelling after a twisting injury, while knee bursitis can create a more localised tender area. Gout and knee osteoarthritis can also inflame the joint, although bruising is not a defining feature. A physical examination helps separate these patterns.

Reduce activities that increase pain, use a cold pack wrapped in cloth for short periods, and rest the leg in a comfortable elevated position. Avoid forceful stretching or deep massage over the bruise. Use walking support only if advised, and follow any injury plan. Note when the swelling began, whether the knee locks or gives way, and whether the bruise is spreading. A clinician may consider imaging or joint aspiration when fluid needs assessment.

Arrange prompt review for rapidly expanding bruising, inability to bear weight, a locked knee, or swelling that follows a significant twist. Seek emergency help for sudden calf or leg swelling, warmth, severe pain, breathlessness, chest pain, faintness, or a cold or pale foot. Fever with a hot, very painful knee also needs urgent assessment because infection must be considered.

Ask whether the pattern fits a Baker's cyst, ligament or meniscus injury, bursitis, or joint inflammation. Ask whether you need an examination or scan, which activities are safe, and what changes in the bruise or swelling should bring you back sooner.