Chapter 5: Lower Limb – part 2

Intro

Popliteal Fossa and Leg

Popliteal Region

The popliteal fossa is a mostly fat-filled compartment of the lower limb. Superficially, when the knee is flexed, the popliteal fossa is evident as a diamond-shaped depression posterior to the knee joint (Fig. 5.49). The size of the gap between the hamstring and gastrocnemius muscles is misleading, however, in terms of the actual size and extent of the fossa. Deeply, it is much larger than the superficial depression indicates because the heads of the gastrocnemius forming the inferior boundary superficially form a roof over the inferior half of the deep part. When the knee is extended, the fat within the fossa protrudes through the gap between muscles, producing a rounded elevation flanked by shallow, longitudinal grooves overlying the hamstring tendons. In dissection, if the heads of the gastrocnemius are separated and retracted (Fig. 5.50), a much larger space is revealed.

Figure 5.49. Superficial popliteal region.

Superficial popliteal region

A.Numbers on the surface anatomy refer to structures identified in B. The diamond-shaped gap in the roof of the popliteal fossa, formed by the overlying muscles, is outlined. B.Superficial dissection of the popliteal region showing the muscles that cover most of the popliteal fossa.

Figure 5.50. Exposure of popliteal fossa and nerves in it.

Exposure of popliteal fossa and nerves in it

The two heads of the gastrocnemius muscle have been separated and are being retracted. The sciatic nerve separates into its components at the apex of the popliteal fossa (or higher; Fig. 5.43B). The common fibular nerve courses along the medial border of the biceps femoris. All the motor branches arising from the tibial nerve, except one, arise from the lateral side; consequently, in surgery it is safer to dissect on the medial side. The level at which the medial sural cutaneous nerve and sural communicating branch merge to form the sural nerve—occurring high here—is quite variable; it may even occur at the level of the ankle.

Superficially, the popliteal fossa is bounded:

  • Superolaterally by the biceps femoris (superolateral border).
  • Superomedially by the semimembranosus, lateral to which is the semitendinosus (superomedial border).
  • Inferolaterally and inferomedially by the lateral and medial heads of the gastrocnemius, respectively (inferolateral and inferomedial borders).
  • Posteriorly by skin and popliteal fascia (roof)

Deeply, the superior boundaries are formed by the diverging medial and lateral supracondylar lines of the femur. The inferior boundary is formed by the soleal line of the tibia (Fig. 5.4B). These boundaries surround a relatively large diamond-shaped floor (anterior wall), formed by the popliteal surface of the femur superiorly, the posterior aspect of the joint capsule of the knee joint centrally, and the investing popliteus fascia covering the popliteus muscle inferiorly (Fig. 5.51).

Figure 5.51. Deep dissection of popliteal fossa.

deep-dissection-of-popliteal-fossa

The popliteal artery runs on the floor of the fossa, formed by the popliteal surface of the femur, the joint capsule of the knee, and the investing fascia of the popliteus.

The contents of the popliteal fossa (Figs. 5.49B, 5.50, and 5.51) include the:

  • Termination of the small saphenous vein.
  • Popliteal arteries and veins and their branches and tributaries.
  • Tibial and common fibular nerves.
  • Posterior cutaneous nerve of thigh (see Fig. 5.42B).
  • Popliteal lymph nodes and lymphatic vessels (see Fig. 5.15B).

Fascia of Popliteal Fossa

The subcutaneous tissue (superficial fascia) overlying the popliteal fossa contains the small saphenous vein (Fig. 5.14B—unless it has penetrated the deep fascia of the leg at a more inferior level) and three cutaneous nerves: the terminal branch(es) of theposterior cutaneous nerve of the thigh and the medial and lateral sural cutaneous nerves (Fig. 5.49B).

The popliteal fascia is a strong sheet of deep fascia, continuous superiorly with the fascia lata and inferiorly with the deep fascia of the leg (Fig. 5.13B). The popliteal fascia forms a protective covering for neurovascular structures passing from the thigh through the popliteal fossa to the leg, and a relatively loose but functional retaining “retinaculum” (retaining band) for the hamstring tendons. Often the fascia is pierced by the small saphenous vein.

When the leg extends, the fat within the fossa is relatively compressed as the popliteal fascia becomes taut, and the semimembranosus muscle moves laterally, providing further protection to the contents of the fossa.

The contents, most important the popliteal artery and lymph nodes, are most easily palpated with the knee semiflexed. Because of the deep fascial roof and osseofibrous floor, the fossa is a relatively confined space. Many disorders produce swelling of the fossa, making knee extension painful. (See the blue boxes “Popliteal Abscess and Tumor” and “Popliteal Aneurysm and Hemorrhage”, and “Popliteal Cysts”.)

Neurovascular Structures and Relationships in Popliteal Fossa

All important neurovascular structures that pass from the thigh to the leg do so by traversing the popliteal fossa. Progressing from superficial to deep (posterior to anterior) within the fossa, as in dissection, the nerves are encountered first, then the veins. The arteries lie deepest, directly on the popliteal surface of the femur, joint capsule, and investing fascia of the popliteus forming the floor of the fossa (Fig. 5.51).

Nerves in Popliteal Fossa

The sciatic nerve usually ends at the superior angle of the popliteal fossa by dividing into the tibial and common fibular nerves (Figs. 5.49B, 5.50, and 5.51).

The tibial nerve is the medial, larger terminal branch of the sciatic nerve derived from anterior (preaxial) divisions of the anterior rami of the L4–S3 spinal nerves. The tibial nerve is the most superficial of the three main central components of the popliteal fossa (i.e., nerve, vein, and artery); however, it is still in a deep and protected position. The tibial nerve bisects the fossa as it passes from its superior to its inferior angle.

While in the fossa, the tibial nerve gives branches to the soleus, gastrocnemius, plantaris, and popliteus muscles. The medial sural cutaneous nerve is also derived from the tibial nerve in the popliteal fossa. It is joined by the sural communicating branch of the common fibular nerve at a highly variable level to form the sural nerve. This nerve supplies the lateral side of the leg and ankle.

The common fibular (peroneal) nerve is the lateral, smaller terminal branch of the sciatic nerve derived from posterior (postaxial) divisions of the anterior rami of the L4–S2 spinal nerves. The common fibular nerve begins at the superior angle of the popliteal fossa and follows closely the medial border of the biceps femoris and its tendon along the superolateral boundary of the fossa. The nerve leaves the fossa by passing superficial to the lateral head of the gastrocnemius and then passes over the posterior aspect of the head of the fibula. The common fibular nerve winds around the neck of the fibula and divides into its terminal branches.

The most inferior branches of the posterior cutaneous nerve of the thigh supply the skin that overlies the popliteal fossa (see Fig 5.42B). The nerve traverses most of the length of the posterior compartment of the thigh deep to the fascia lata; only its terminal branches enter the subcutaneous tissue as cutaneous nerves.

Blood Vessels in Popliteal Fossa

The popliteal artery, the continuation of the femoral artery (Figs. 5.51 and 5.52), begins when the latter passes through the adductor hiatus (see Fig. 5.30A). The popliteal artery passes inferolaterally through the fossa and ends at the inferior border of the popliteus by dividing into the anterior and posterior tibial arteries. The deepest (most anterior) structure in the fossa, the popliteal artery, runs in close proximity to the joint capsule of the knee as it spans the intercondylar fossa.

Figure 5.52. Genicular anastomosis.

Genicular anastomosis

The many arteries making up the peri-articular anastomosis around the knee provide an important collateral circulation for bypassing the popliteal artery when the knee joint has been maintained too long in a fully flexed position or when the vessels are narrowed or occluded.

Five genicular branches of the popliteal artery supply the capsule and ligaments of the knee joint. The genicular arteries are the superior lateral, superior medial, middle, inferior lateral, and inferior medial genicular arteries (Fig. 5.52). They participate in the formation of the peri-articular genicular anastomosis, a network of vessels surrounding the knee that provides collateral circulation capable of maintaining blood supply to the leg during full knee flexion, which may kink the popliteal artery. Other contributors to this important genicular anastomosis are the:

  • Descending genicular artery, a branch of the femoral artery, superomedially.
  • Descending branch of the lateral femoral circumflex artery, superolaterally.
  • Anterior tibial recurrent artery, a branch of the anterior tibial artery, inferolaterally.

Muscular branches of the popliteal artery supply the hamstring, gastrocnemius, soleus, and plantaris muscles. The superior muscular branches of the popliteal artery have clinically important anastomoses with the terminal part of the profunda femoris and gluteal arteries.

The popliteal vein begins at the distal border of the popliteus as a continuation of the posterior tibial vein (Fig. 5.51). Throughout its course, the vein lies close to the popliteal artery, lying superficial to it and in the same fibrous sheath. The popliteal vein is initially posteromedial to the artery and lateral to the tibial nerve. More superiorly, the popliteal vein lies posterior to the artery, between this vessel and the overlying tibial nerve. Superiorly, the popliteal vein, which has several valves, becomes the femoral vein as it traverses the adductor hiatus. The small saphenous vein passes from the posterior aspect of the lateral malleolus to the popliteal fossa, where it pierces the deep popliteal fascia and enters the popliteal vein.

Lymph Nodes in Popliteal Fossa

The superficial popliteal lymph nodes are usually small and lie in the subcutaneous tissue. A lymph node lies at the termination of the small saphenous vein and receives lymph from the lymphatic vessels that accompany this vein (see Fig. 5.15). The deep popliteal lymph nodes surround the vessels and receive lymph from the joint capsule of the knee and the lymphatic vessels that accompany the deep veins of the leg. The lymphatic vessels from the popliteal lymph nodes follow the femoral vessels to the deep inguinal lymph nodes.