Baker's Cyst: Posterior Herniation of the Knee Joint

Abstract

There are numerous references in the literature to cysts of the popliteal region. Adams (1) in 1840 presented one of the first descriptions of herniation of the capsule of the knee joint. He considered the popliteal swelling to be a “dropsy” of the joint. Gruber (2) described enlarged bursae communicating with the knee joint. Foucher (3) made a study of 11 dissected specimens of popliteal cysts and 19 clinical cases. Baker (4) described 10 cases, discussed the formation of synovial cysts, and differentiated them from diseases of the knee joint. His original belief was that these cysts were the result of osteo-arthritis. In 3 of his patients examination after amputation indicated that there was a true herniation through the posterior wall of the joint capsule. He wrote: “To be a true hernia, the sac must be connected with the joint. It must be lined with synovia and connected with the joint by either a normal or abnormal opening.” Haggart (7) presented 12 cases in which operation was done. Nine of these were true herniations of the knee joint; in one there was a lipoma of the popliteal space, in another a hyperplastic subcutaneous fat pad, and the remaining patient had an enlarged semimembranosus bursa. Wilson, Eyre-Brook and Francis (8) presented a thorough survey of the anatomical, operative, and pathological aspects of 21 cases. They made roentgenologic pneumograms in their last 3 patients and describe aspiration of the cyst and its injection with air. Cravener (5) reported a single case in which air was injected directly into the joint and demonstrated air in the cyst roentgenographically. The syndrome is fairly definite. Usually the hernia develops insidiously. Occasionally, because of trauma or inflammation, an acute effusion occurs and the sac is pushed backward along the path of least resistance. In the early stages, there may be only vague pain and a sense of fullness, which may be intermittent. The symptoms may suggest an internal derangement of the knee. As the cyst increases in size, the swelling extends down the popliteal space, under the deep fascia. When the patient is in the upright position and the knee is completely extended, the cystic swelling may appear as a bulging mass on the medial aspect of the popliteal area distal to the popliteal skin creases. Examination usually shows a soft, non-tender, fluctuant mass along the lateral aspect of the semimembranosus tendon. This cyst must be differentiated from a number of other entities, including lipoma, hyperplastic bursitis, fibrosarcoma, angioma, and arteriovenous aneurysm. The purpose of this report is again to call attention to the method of pneumographic diagnosis. In many patients who are suspected of having a popliteal cyst a definite diagnosis can be made by pneumograms of the knee joint. Plain soft-tissue roentgenograms will usually show the abnormal swelling but will not demonstrate its communication with the joint (Fig. 1).