Baker's Cyst

A Baker's cyst (also know as a popliteal cyst) is a fluctuant swelling located in the popliteal space. The term is a misnomer as the swelling is the result of synovial fluid distending the gastrocnemio-semimembranosus bursa, rather than being a true cyst.¹

A primary cyst usually communicates with the joint through a slit-like opening at the posteromedial aspect of the knee capsule just superior to the joint line. This acts as a valve and it has been postulated that synovial fluid and fibrin are pumped from the knee joint to the cyst in a one-way direction.² The synovial fluid becomes very viscous and scarring may develop within the walls of the membrane. These cysts usually occur in young people and are symptomless.

A secondary cyst communicates freely with the knee joint and contains fluid of normal viscosity. These types or cysts occur in older people, often cause symptoms and may be associated with underlying articular disorders.³

A wide range of figures can be found in the literature, because this varies depending on the type of imaging used. Between 5% and 32% of patients presenting with knee problems have been found to have these cysts, with two age-incidence peaks of 4 to 7 years and 35 to 70 years.⁴ There is no predilection for race and sex.¹

• Popliteal mass: this is the commonest presenting symptom (78% in one study).
• Pseudothrombophlebitis syndrome: this is a syndrome in which symptoms simulate those of deep venous thrombosis (DVT). Exclude a DVT in patients with Baker's cyst and leg swelling.
• Thrombophlebitis: the anatomical site of a Baker's cyst means that there can be an increased risk of thrombophlebitis, so this should be excluded in all Baker's cyst patients who have leg swelling.
• Other presentations include aching, knee effusion, clicking of the knee, buckling of the knee and locking.

Examination

• This reveals a transilluminable swelling in the posteromedial aspect of the knee.⁵
• Foucher's sign can often be demonstrated in which the swelling becomes tense on extension and soft on flexion.⁶
• If presenting symptoms suggest any unusual features, e.g. rapid growth, severe pain, fever or night pain, greater attention should be paid to examination to exclude other causes.
  • Overlying skin changes may suggest a superficial haemangioma, lymphangiosarcoma, dermatofibrosarcoma or Kaposi's sarcoma.
  • Rhabdomyosarcoma may give the appearance of a muscle bulging through the skin.⁷
  • If the mass is larger than 5 cm or deep to the subcutaneous tissue, it is more likely to be malignant.⁸ However, a third of soft-tissue sarcomas are located in the subcutaneous tissue.⁹ Warmth as a sign of hyperemia can be seen with soft-tissue sarcomas and inflammatory processes and is rare with benign neoplasms.⁷

• Deep vein thrombosis
• Vascular masses - popliteal artery aneurysm, cystic adventitial degeneration of popliteal artery (Erdheim mucoid degeneration), haemangioma
• Inflammatory arthritides
• Septic arthritis
• Postoperative changes (seroma, haematoma, abscess)
• Haemophilic arthropathy
• Benign soft tissue tumour - peripheral nerve sheath tumours (neurolemmoma)
• Malignant - myxoid liposarcoma (adults), lipoblastoma (children, especially < 5 years), lymphangiosarcoma, dermatofibrosarcoma, Kaposi's sarcoma, rhabdomyosarcoma
• Meniscal cyst (occur more commonly laterally, but medial cysts have been identified)
• Ganglion cyst
• Traumatic tear of gastrocnemius muscle

As a general rule, plain X-ray is a useful cheap initial investigation. If it is unhelpful, ultrasound may be required. Referral to secondary care for MRI scan or arthrography may be necessary in complicated cases.

• Plain radiology^1,7 - a Baker's cyst appears as a soft tissue mass in the posteromedial knee joint and accompanying arthritides may be identified. Occasionally, multiple calcified loose bodies within the cyst may be seen. Rarely, a solitary loose body within a popliteal cyst may mimic a fabella (sesamoid bone of lateral gastrocnemius¹⁰) on a lateral radiograph of the knee. This can be differentiated on frontal radiograph when the calcified body within the Baker cyst is located behind the medial femoral condyle (a fabella will be present behind the lateral femoral condyle).¹
  • Increase in fat density may suggest a lipoma or liposarcoma whilst calcification may represent phleboliths of a haemangioma, loose bodies associated with synovial chondromatosis or mineral deposition within a sarcoma.
  • Ossification within the soft tissue about the knee may be due to myositis ossificans or soft tissue osteosarcomas.
  • Juxta-articular erosions on both sides of the joint in the presence of a suprapatellar pouch and/or Baker's cyst filled with boggy soft tissue or loose bodies should suggest pigmented villonodular synovitis¹¹ or synovial chondromatosis.
  • Cortical hyperostosis may accompany deep intramuscular haemangiomas juxtaposed to the bone.
• Ultrasound - this may be helpful in differentiating purely cystic masses from more solid lesions.¹² A purely cystic mass about the knee, located superficially and less than 5 cm in size, is highly unlikely to be malignant.¹³
• MRI - this is not routinely indicated although it is seen as the investigation of choice by some clinicians.¹⁴ It may be helpful in cases of diagnostic difficulty, particularly to exclude rare malignancies,¹ to assess a potential concomitant intra-articular disorder or prior to surgery.¹⁵ On MRI, a Baker's cyst appears as a high signal intensity homogeneous cystic mass behind the medial femoral condyle. The thin fluid-filled neck between the tendons of the medial head of the gastrocnemius and semimembranosus muscles can often be visualised.¹
• Intravenous gadolinium arthrography - this is sometimes used as a further imaging enhancement and is especially useful if a concomitant menisceal tear is suspected.¹⁶

Arthritides - osteoarthritis is the commonest condition associated with a Baker's cyst. Rheumatoid arthritis is another common association. In one study Baker's cysts were demonstrated in 48% of patients.¹⁷ In children, the condition may be linked to juvenile arthritis,¹⁸ septic arthritis and osteochondritis dissecans.¹⁷

Other reported associations include gout, intramuscular cavernous haemangioma, pigmented villonodular synovitis, Reiter syndrome, psoriasis, systemic lupus erythematosus, meniscal and anterior cruciate ligament tears, osteochondral fractures, tuberculosis, chronic dialysis, haemophilia, hypothyroidism and sarcoidosis.

• If the cyst is asymptomatic, no treatment may be necessary. Spontaneous resolution is not uncommon, particularly in younger age groups, but may take 10-20 months.¹⁹
• Patients with a Baker's cyst and calf swelling should be referred urgently to secondary care to exclude a DVT.
• Non-steroidal anti-inflammatory drugs, ice and assisted weight bearing may help with symptoms whilst spontaneous resolution is awaited.¹
• Aspiration is sometimes attempted, occasionally with instillation of corticosteroid,⁴ but recurrence is frequent.¹
• Successful resolution after sclerotherapy using sodium morrhuate has been reported.²⁰
• Radioactive synoviorthesis (synovectomy using radiation) can be used to treat inflammatory arthritides and haemophilia and often results in resolution of an associated Baker's cyst.²¹ Arthrography is required prior to this procedure to exclude a leaking cyst which would release radionuclide agent outside the knee joint.¹
• Arthroscopic treatment of underlying knee arthropathy sometimes results in resolution of an associated Baker's cyst. Anterior synovectomy has been used to good effect in rheumatoid arthritis patients.²²
• Arthroscopic treatment of the cyst per se has also been tried with some success.²³ One technique involves suture of the communication between the gastrocnemius semimembranous bursa and the cyst.²⁴
• Open surgical excision of the cyst is indicated if conservative measures or arthroscopic intervention fail. A stalk leading from the cyst down to the joint can often be located and sutured over or cauterised, after which the cyst can be removed.²⁵ The recurrence rate can be quite high, particularly if an articular lesion remains uncorrected, but can be reduced by various techniques.²⁶

• Rupture or dissection of fluid into the adjacent proximal gastrocnemius muscle belly is the commonest complication, resulting in a clinical picture which looks very much like a DVT (pseudothrombophlebitis syndrome). A ruptured cyst may present as an enlarging mass in the calf. The incidence of rupture is 3.4-10%.¹ The diagnosis is not easy because Baker's cyst is a risk factor for DVT due to its anatomical position and both conditions can co-exist.²⁷
• Pulmonary embolism has been reported as a complication.
• Haemorrhage into a cyst has been reported, particularly if there is concomitant bleeding diathesis (e.g. haemophilia).
• Infection occurs rarely. The presentation is with a fever. FBC shows a raised white blood cell count and the ESR is elevated.¹ Candidal infection has also been reported.²⁸
• Posterior compartment syndrome can be caused by a ruptured Baker's cyst. Misdiagnosis as a DVT and treatment with anticoagulants can lead to a worsening of symptoms.²⁹
• Trapped calcified bodies in Bakers cysts are common, with an incidence of 6% in one series. They may derive from trauma, arthropathy or synovial osteochondromatosis.
• Pressure from the cyst on the common peroneal and tibial nerves has been reported.³⁰

This depends on whether there is any underlying knee pathology, how treatable it is and the age of the patient at presentation. Simple Baker's cyst in children and young adults is likely to resolve spontaneously. Prospective studies have found a high rate of recurrence after arthroscopy, particularly if osteoarthritis is a causative factor.³ The same principles apply to excision.³