Baker cyst and knee effusion

A knee effusion is a sign, while a Baker cyst is a common consequence of excess synovial fluid. Ask whether swelling was immediate after trauma, developed over hours, fluctuates with activity or appeared with fever. Establish acute hot attacks, inflammatory disease, osteoarthritis, locking, instability, anticoagulants, bleeding disorder and previous aspiration or injection. For a popliteal lump ask about change with flexion, calf rupture symptoms, neurological complaints and distal swelling. Never assume every posterior mass is a cyst.

Assess observations and gait, compare both knees and identify intra-articular effusion using bulge or patellar-tap methods appropriate to size. Measure warmth, range, passive pain, joint lines and stability. Palpate the popliteal space cautiously for a posteromedial, often flexion-dependent fullness and do not puncture a pulsatile or atypical mass. Examine calf circumference and tenderness, pitting oedema, pulses and tibial and peroneal nerve function. Dyspnoea or chest pain redirects to pulmonary-embolism assessment.

Urgent aspiration is for an unexplained hot or systemically concerning effusion, not for every quiet Baker cyst. Use sterile technique and send synovial cell count and differential, Gram stain, aerobic and anaerobic cultures and crystals, prioritising culture if volume is small. In a stable patient sample before antibiotics; in sepsis obtain rapid blood cultures and give intravenous treatment without waiting for delayed aspiration. Bloody fluid prompts trauma, anticoagulation, tumour and pigmented synovial disease review.

Plain radiographs identify osteoarthritis, fracture, chondrocalcinosis, loose body and tumour clues. Ultrasound confirms fluid in the expected bursa, distinguishes solid or vascular lesions and guides joint or cyst aspiration when appropriate. It also supports the venous pathway for suspected DVT. MRI shows cyst communication, synovitis, meniscus, cartilage and atypical mass detail but is selective. A popliteal artery aneurysm requires vascular imaging and must never be aspirated.

Ruptured Baker cyst releases fluid down tissue planes and can produce calf erythema, tenderness and swelling. These features overlap substantially with DVT; a known cyst does not remove thrombosis probability. Apply NICE NG158 clinical probability and D-dimer or proximal-leg-vein ultrasound sequence according to the pathway, including interim anticoagulation when indicated and safe. Ultrasound may show both venous patency and dissecting fluid. Consider that cyst and thrombosis can coexist.

Manage the driver. For osteoarthritis, use education, progressive knee and hip strengthening, aerobic activity, weight support and suitable topical or short oral NSAID treatment. Inflammatory disease needs disease-modifying specialist care. Mechanical locking or instability follows its own soft-tissue pathway. Most cysts settle or become tolerable when synovitis and loading improve. Reassure only after red flags and atypical features have been addressed.

Ultrasound-guided aspiration or corticosteroid injection may temporarily reduce symptoms in selected cases but recurrence is common if the joint continues to produce fluid. Exclude infection and vascular lesions, assess anticoagulation and discuss transient hyperglycaemia and rare septic arthritis. Surgical excision alone has a recurrence risk and is uncommon; persistent disability, compression or a treatable intra-articular lesion requires orthopaedic planning directed at both communication and cause.

Follow calf and neurovascular symptoms, effusion, range, quadriceps strength and recurrence. Recurrent unexplained haemarthrosis or rapidly enlarging mass needs specialist imaging. After septic arthritis, follow source control and cartilage function; after DVT, follow the venous pathway rather than attributing residual swelling to the cyst. Teach urgent return for fever, inability to bear weight, rapidly increasing calf swelling, chest symptoms, cold foot or new weakness.