The subpopliteal fat body.

Introduction: The knee is likely to get hurt due to its excessive weight-bearing, for which it is surrounded by strongly tensioned ligaments, connectives and muscles. These highly active structures are imbedded in fatty tissue. The Fatty and loose connective tissue of the knee recently gained a renaissance in research. While the Hoffa fat body in the ventral knee attracted attention over the last years, we have investigated a smaller, dorsal fat body, ventral to the popliteus muscle. This fat body has not been described before.

Materials And Methods: 11 knees of 11 fresh specimens were investigated. All muscles but the popliteus muscle were removed. The popliteus was released from its tibial origin and dissected towards its tendinous insertion. Thereby, a subpopliteal fat body (SFB) was shown. The related vessels and nerves were evaluated. The size of the body was measured. Examples of histological slices were stained with HE and immunostained against neurofilament.

Results: The SFB lies ventral of the popliteus muscle at the concave posterior tibia and attaches to the periosteum and the popliteus muscle. It is not attached to the posterior cruciate ligament. It is separated from the subpopliteal recess by a lamella deriving from the fibular head. Arterial and venous vessels are seen entering the SFB, deriving from the popliteal artery or the anterior tibial artery. A subbranch of the tibial nerve was seen to reach the SFB. The SFB could be identified in MRI scans and in plastinations.

Discussion: Primarily, the SFB provides a gliding space for the mobile part of the popliteus muscle over the tibia. The SFB lies within the tibial concavity, ventral to the popliteus muscle. This is exactly where embryologically, the popliteal artery passes through, before its involution in later stages. Therefore, the SFB may show the former perivascular autonomic nerves which encompass the embryologically created arteries, from which we have seen the arterial remnants. The nerves seen here form neurovascular bundles which could be a source of pain, when compressed. This anatomy may explain the autonomic component of pain in the deep lateral region of the knee. The SFB is functional fat, comparable to the Hoffa's fat pad in the ventral knee.