Popliteal Cyst (Baker's Cyst)

Patients describe a lump in the posteromedial popliteal fossa, often fluctuating in size with activity and rest. Posterior knee tightness or fullness on full flexion is common, but many cysts are painless or only mildly symptomatic. The cyst may be the presenting complaint or discovered incidentally on knee imaging. Always ask about the underlying knee disorder: OA symptoms, mechanical features of meniscal tear, inflammatory joint disease, or previous knee injury.

A ruptured cyst presents as sudden-onset calf swelling, pain, and tenderness, often after activity. The patient may notice the popliteal lump has disappeared as the calf swelled - this disappearing lump with simultaneous calf swelling is highly characteristic.

Examination: the cyst is best palpated in the posteromedial popliteal fossa with the knee in slight flexion - typically smooth, well-circumscribed, and fluctuant. Foucher sign: firmer in extension, softer in flexion. Always assess the knee joint for effusion, joint-line tenderness, crepitus, ligamentous stability, and synovial thickening - the underlying disorder is the management priority. For a ruptured cyst, look for diffuse calf swelling and tenderness, with the popliteal lump often resolved.

Practical posterior knee mass assessment: a fluctuant cyst with knee pathology is most likely Baker's; a pulsatile mass suggests popliteal artery aneurysm and must not be aspirated; a firm, fixed, non-fluctuant, or enlarging mass should prompt MRI and specialist referral (NICE NG12) to exclude sarcoma.

A typical Baker's cyst in an adult with known knee OA can usually be diagnosed clinically. Investigations confirm the diagnosis, exclude differentials, and assess the underlying cause.

Ultrasound is the first-line investigation. It confirms cystic nature, demonstrates the communication neck, assesses size, and excludes popliteal artery aneurysm or solid mass. For suspected ruptured Baker's cyst, venous duplex USS is the key investigation - it simultaneously confirms or excludes DVT and identifies the ruptured cyst. D-dimer may be elevated from the ruptured cyst and does not reliably distinguish from DVT, so duplex is preferred.

Plain weight-bearing radiographs do not visualise the cyst but assess the underlying cause (OA changes, chondrocalcinosis, loose bodies). MRI is reserved for atypical masses or detailed intra-articular assessment. Bloods are not routine but are indicated if inflammatory arthritis is suspected. USS-guided aspiration can provide fluid for analysis and symptomatic relief; always confirm cystic nature on USS before aspirating any popliteal mass.

In adults, identify and optimise management of the underlying knee disorder - the cyst is the consequence. Asymptomatic cysts usually do not need direct treatment (NICE CKS). The management hierarchy is: confirm it is a cyst rather than vascular or solid mass, treat the underlying knee disorder, and only then consider direct cyst management if symptoms persist.

Treat the underlying cause. Knee OA per NICE NG226 (exercise, weight management, topical then oral NSAIDs, intra-articular corticosteroid where indicated). Meniscal tears conservatively for degenerative patterns or surgically for traumatic repairable ones. Inflammatory arthritis via rheumatology referral and disease-modifying treatment.

Direct cyst management: observation suits asymptomatic or mildly symptomatic cysts when the cause is being managed. USS-guided aspiration provides symptomatic relief but recurs if the intra-articular driver is not addressed. Intra-articular corticosteroid injection into the knee may be more logical because it targets the synovitis driving the cyst at source; evidence supports this for cyst reduction in OA and inflammatory arthritis. Surgical excision is rarely required and is reserved for persistent symptomatic cysts despite optimisation or neurovascular compression.

Ruptured Baker's cyst: exclude DVT via the NICE suspected DVT pathway (NG158) with venous duplex USS - the two can coexist because large cysts can compress the popliteal vein. Once DVT is excluded, manage with rest, elevation, oral NSAIDs, and compression. Treat the underlying intra-articular pathology.

In children, observation and reassurance is the standard - most resolve spontaneously within 1-2 years.

Rehabilitation is directed at the underlying knee disorder, not the cyst.

For knee OA, per NICE NG226: quadriceps strengthening is the priority, with hamstring and hip abductor strengthening, range-of-motion work (maintaining full extension matters), aerobic fitness, and weight management. For underlying meniscal pathology: quadriceps strengthening, range of motion, and graduated functional loading. For inflammatory arthritis: maintain range and strength alongside joint protection and aerobic fitness.

Post-aspiration: continue rehabilitation for the underlying condition, with a compression bandage for 24-48 hours and progression as tolerated. The general principle is that optimising quadriceps strength, knee range, and (where relevant) body weight reduces effusion volume and stabilises or shrinks the cyst over time.

• •NICE CKS: Baker's cyst - In adults the cyst is almost always secondary to underlying knee pathology; treat the underlying cause and observe asymptomatic cysts.
• •NICE NG226 (2022) - Osteoarthritis: structured exercise plus weight management is first-line; intra-articular corticosteroid injection into the knee targets the effusion driving Baker's cyst.
• •NICE NG158 - DVT diagnosis pathway: venous duplex USS is the investigation of choice for any acute calf swelling, including suspected ruptured Baker's cyst.
• •NICE NG12 - Suspected cancer recognition: firm, non-fluctuant, fixed or enlarging posterior knee mass should trigger urgent MRI and specialist referral.
• •Acebes et al. (2006) - USS-guided intra-articular corticosteroid injection reduces Baker's cyst size by treating the synovitis at source.

• •Baker's cyst in adults is almost always secondary to intra-articular pathology (OA most common); the management priority is the underlying disorder. Aspiration alone recurs.
• •A ruptured Baker's cyst mimics DVT - acute calf swelling after the popliteal lump has disappeared. Crescent sign (medial malleolar ecchymosis) may take 24-48 hours.
• •A Baker's cyst can compress the popliteal vein and cause true secondary DVT; the two conditions can coexist. Venous duplex USS is the investigation of choice, not D-dimer (elevated by the rupture itself).
• •Always check for pulsation before aspirating any popliteal mass - popliteal artery aneurysm is dangerous to aspirate.
• •A firm, fixed, non-fluctuant, or enlarging posterior knee mass should prompt urgent MRI and specialist referral (NICE NG12) to exclude sarcoma.
• •Intra-articular corticosteroid injection into the knee may be more logical than aspirating the cyst because it targets the synovitis driving it.