A valve. An explanation of the formation of popliteal cysts.

Popliteal and, to a lesser extent, calf cysts are a common manifestation in patients suffering from rheumatoid arthritis. They may, however, be found in association with any chronic knee effusion. Morrant Baker (1877) described cysts in the popliteal fossa and calf. Particular note was made that popliteal cysts were accompanied by knee joint pathology and that, in those subjected to dissection, a communication was found to exist between the posterior aspect of the knee and the cyst. Although a communication existed, the fluid could not be pressed from the cyst into the knee joint. Both Morrant Baker (1877) and Adams (1840) suggested that there must be a valvular opening between the cyst and the knee. Jayson and Dixon (1970) recorded the pressures obtained in the knee joint and in popliteal cysts. They found that the pressure in the cyst was always higher than that in the knee. We have injected radio-opaque dye into popliteal cysts, and have demonstrated that it does not pass into the knee joint, although the reverse is the rule. It is thus apparent on clinical, experimental, and radiological evidence that a valvular mechanism exists between the posterior aspect of the knee joint and the popliteal cyst. In an attempt to find an anatomical explanation for this valve-like effect, fifty unselected knees were examined post mortem. In nineteen of these knees a defect was found in the posterior capsule, forming a communication between the knee joint and the normally existing bursa found deep to the medial head of the gastrocnemius. This bursa often formed a composite bursa with that associated with the semimembranosus tendon-called the gastrocnemius semimembranosus bursa (Wilson, Eyre-Brook, and Francis, 1938). The opening in the posterior capsule was constant in position. It was always found deep to the medial head of the gastrocnemius high up in the posterior capsule opposite the medial femoral condyle. On its medial side is the tendon of the semimembranosus muscle. To obtain a clear view of this opening, the edges of which were usually smooth, the knee had to be flexed a little and the medial head of the gastrocnemius strongly retracted laterally. It is significant that in only one knee was there any evidence ofmacroscopic pathology, and that in nonewas there an indication that this opening had been produced by a rupture of the posterior capsule. The true nature of the opening does not become apparent, however, until the posterior capsule is viewed from the inner aspect ofthe knee. Ineach specimen the posterior joint capsule was removed in toto and then examined as if viewed from the inside of the joint. It was seen that the opening is not just a simple communication, but that its lower aspect has a valvular arrangement (Fig. 1). It is concluded that this opening in the medial aspect of the posterior capsule is an anatomical feature and not acquired, and that it occurs in roughly 40 per cent. of adult knees.