Baker’s cyst

Image type: Baker’s cyst

Body Site: Knees

Description: A Baker’s cyst is a fluid-filled cyst that causes a bulge and a feeling of tightness behind the knee. The pain can worsen when flexing or extending the knee or when active.

A Baker’s cyst, also called a popliteal (pop-luh-TEE-ul) cyst, is usually the result of a problem with the knee joint, such as arthritis or a cartilage tear. Both conditions can cause the knee to produce too much fluid, which can lead to a Baker’s cyst.

Although a Baker’s cyst may cause swelling and discomfort, treating the probable underlying problem usually provides relief. In some cases, a Baker’s cyst causes no pain, and it may go unnoticed. If signs and symptoms are present, they might include:

  • Swelling behind the knee, and sometimes in the leg
  • Knee pain
  • Stiffness and inability to fully flex the knee
  • Symptoms may worsen after being active or standing for a long time.

Causes: A lubricating fluid called synovial (sih-NO-vee-ul) fluid helps the leg swing smoothly and reduces friction between the moving parts of the knee. However, sometimes the knee produces too much synovial fluid, resulting in a buildup of fluid in the area behind the knee (popliteal bursa), causing a Baker’s cyst. This can happen due to:

  • Inflammation of the knee joint, such as occurs with various types of arthritis
  • A knee injury, such as a cartilage tear

Differential diagnosis: The differential diagnosis of a Baker cyst includes:


The difficulty in distinguishing cellulitis from DVT stems from the tendency of a cellulitis patient’s leg or arm to appear swollen due to edema secondary to localized infection. This edema and swelling, in turn, cause discomfort when an examiner squeezes the leg or arm. Laboratory tests may not be specific enough to adequately differentiate between the two disease entities; in fact, there are no diagnostic lab tests for DVT. Furthermore, the white blood cell count may not be elevated in cellulitis and may be elevated with DVT. It is considerably helpful when the area of erythema is far removed from the path of any deep veins in the leg, such as the anterior lower leg. However, when this is not the case, there can be doubt in distinguishing between DVT and cellulitis clinically. Hence, there is often little choice but to seek an imaging study for guidance toward a diagnosis.

Superficial thrombophlebitis

Thrombosis of a superficial vein, such as the great saphenous, can be quite uncomfortable for a patient. The course of the great saphenous vein is close enough to deep veins in several regions of the leg to make symptom locations overlap. It is important to remember that, while treatment in most cases may only require simple measures, like heat and aspirin, full anticoagulation may occasionally be indicated. In cases where the thrombus is seen in the proximal great saphenous vein and is at the point of inflow into the femoral vein, or for those patients with significant, known hypercoagulability, treatment is similar to proximal DVT. The use of duplex ultrasound is sometimes necessary to confirm the thrombosis of a superficial versus a deep vein. As previously noted, it may behoove the EP to confirm the location of the superficial thrombosis in high-risk patients, especially considering that nearly 25% of all patients with superficial phlebitis were found to have involvement of the deep system in one study. Older patients — particularly males over 60 — patients with systemic infections, and those subjected to bed rest run a higher risk of progression from superficial phlebitis to DVT. Intravenous drug users can also develop septic thrombophlebitis from skin penetration under nonsterile conditions.

Trousseau’s syndrome

Also known as Nygaard-Brown syndrome, Trousseau’s syndrome refers to the association sometimes seen between visceral cancer and DVT in both the lower and upper extremity. Classically, the association is seen with pancreatic tumors, but other cancers, such as adenocarcinomas, can cause it.

Musculoskeletal pain

Popliteus tendonitis, bursitis of the knee, rupture of the soleus, Achilles tendon rupture, osteoarthritis, bone fracture, and muscle strain can all mimic DVT pain. The Thompson squeeze test involves compression of the calf muscle halfway down the lower leg in a patient who is lying prone, with the knee flexed and ankle relaxed. A positive test, when the foot fails to passively plantarflex, indicates a complete rupture of the Achilles tendon. However, it is important to note that, in many cases, enough fibers of a tendon will remain intact, despite severe injury, to give a false-negative test. When in doubt, ultrasound is a rapid and highly accurate diagnostic tool and can even be performed at the bedside. It is not uncommon for patients to present several days after trauma to the lower extremity. Patients with a history of “trauma” to a lower extremity and pain in a suggestive location should raise one’s suspicion for DVT. For patients who are on anticoagulants, as well as the occasional serious athlete, a hemorrhage into the calf musculature can present with calf pain and significant swelling. Although history is usually helpful, ultimately, many of these limbs will still require imaging.

Any venous stasis can lead to lower extremity swelling and sometimes pain. This is especially likely for patients who have previously had a DVT and who present with an acute post-thrombotic syndrome or a worsening of chronic symptoms. Differentiating either of these from acute DVT is difficult based on a physical examination alone, and typically, ancillary testing is required.

A Baker’s cyst can often be diagnosed with a physical exam. However, because some of the signs and symptoms of a Baker’s cyst mimic those of more serious conditions, such as a blood clot, aneurysm, or tumor, noninvasive imaging tests would be ordered, including:

  • Ultrasound
  • X-ray
  • Magnetic resonance imaging (MRI)

Treatment: Treatment of a Baker’s cyst typically focuses on addressing the underlying cause, such as arthritis or a knee injury. Conservative measures such as rest, ice, compression, and elevation (RICE) may help alleviate symptoms. In some cases, aspiration of the cyst or corticosteroid injections may be recommended to reduce swelling and discomfort. Surgery to remove the cyst may be considered if conservative measures are ineffective or if the cyst is causing significant symptoms. Physical therapy may also be beneficial to improve knee function and prevent recurrence.