In the presence of posterior hernia of the knee joint the patient's symptoms suggest an internal derangement of the knee. This condition may be suspected when a cystic swelling is found in the popliteal space, particularly if located in the mid-line and, in greater part, distal to the skin-flexion crease in this region.
Clinically, the chief symptoms vary from aching discomfort to severe, binding pain in the knee region, often associated with intermittent effusion into the knee joint. There is a moderate to marked degree of disability.
The hernial sac communicates with the knee joint usually by a pedicle, one to two centimeters in length, arising either above or below the oblique popliteal ligament. The sac may occasionally dissect its way distally to the mid-calf region, lying on top of the gastrocnemius muscle and beneath the deep fascia.
On histological examination, the lining of the hernial sac is found to be a true mesothelium which cannot be distinguished from the synovial membrane of the knee joint. There is acute and chronic inflammation of the serosa, as well as round-cell infiltration of the subserous layers.