Four distinct types of cysts can form in and around the knee requiring medical evaluation and treatment. In this article, surgeons from the New York University Hospital for Joint Diseases in New York City provide a detailed presentation of each one. Background, clinical presentation, differential diagnosis, evaluation, and management are included for each one.
A cyst is defined as an abnormal closed, bladderlike sac (membrane) containing fluid, semifluid, or semisolid matter. The four most common cysts found associated with the knee are popliteal (Baker) cysts, meniscal cysts, proximal tibiofibular joint cysts, and cruciate ligament ganglion cysts. The fluid usually comes from a channel (pathway or connection) between the synovial fluid inside the joint and the cyst.
Each of these cysts may have a unique cause and formation but the clinical presentation is usually swelling with limited knee motion. Patient history is important. For example, a previous knee injury or known trauma to the knee cartilage (e.g., meniscus) or any of the knee ligaments has been linked with meniscal cysts.
The patient may or may not have knee pain. When pain is present, it is usually confined to the joint line, which aids the examiner in looking for the cause. Sometimes, there is a palpable mass but more often a distinct entity cannot be felt outside of the joint. If the cyst causes meniscal degeneration or erodes the bone, knee instability may be present.
Because knee swelling can be a symptom of bursitis, tumors, blood clots, hematomas, and aneurysms (or other vascular problems), it cannot be assumed that the problem is a cyst. Clinical tests specific for each type of cysts may be performed. But the final diagnosis often requires special tests. Diagnostic tests available to the physician include X-rays, arteriography, ultrasound, CT scans, or MRIs. The pros and cons of each test procedure are reviewed for each type of cyst. Examples of each type of imaging study used to diagnose different types of cysts are presented.
Cysts can be very benign (do not cause serious problems) and self-limiting (go away without treatment). But more often, they cause enough symptoms that treatment is required. Arthroscopic surgery may be needed to remove the cyst, end the connection between the cysts and the joint, and repair damage to the knee. In some cases, invasive surgery can be avoided. The physician can aspirate (use a needle to withdraw the fluid) the cyst.
The prognosis for most knee cysts is good. With aspiration or surgical removal, most cysts do not recur. The key to good recovery is an accurate diagnosis. Management depends on the type of cyst and of course, making sure there isn’t a more serious underlying problem (e.g., tumor). The diagnosis can be very challenging. The detailed information in this article differentiating one cyst from another will aid the physician.