The popliteal fossa is defined as the space between the skin, the femur anteriorly and the biceps femoris muscle laterally, the semitendinosus and semimembranosus muscles medially, and inferiorly by both heads of the gastrocnemius (Figs 30.1 and 30.2). The space is mostly filled with fat and contains in its anterolateral aspect the popliteal vessels and nerves. The sciatic nerve is formed from roots L4 to S2 (and occasionally S3) and consists of two distinct divisions – the tibial and common peroneal nerves – which share a common epineural sheath from their origin to the popliteal fossa. The two nerves innervate the entire leg below the knee except for the anteromedial leg and foot, which are innervated by the saphenous nerve (L2, 3, 4).

Figure 30.1 Cadaver structures illustrating anatomy relevant to the posterior popliteal block technique. 1: biceps femoris; 2: semimembranosus; 3: semitendinosus; 4: gastrocnemius; 5: tibial nerve; 6: common peroneal nerve; 7: popliteal vessels.

Figure 30.2 Cadaver structures illustrating anatomy relevant to the lateral popliteal block technique. 1: biceps femoris; 2: vastus lateralis; 3: intermuscular groove; 4: common peroneal nerve.

The sciatic nerve branches in the region of the distal thigh (Fig. 30.3), anywhere between 4 and 13 cm above the popliteal crease, although occasionally they can run as two separate nerves from the sciatic foramen. The tibial nerve is the larger of the two branches and runs parallel and slightly lateral to the midline. Inferiorly, it passes between the heads of the gastrocnemius muscle. The common peroneal nerve runs laterally along the medial aspect of the biceps femoris muscle. After bifurcation, the tibial nerve immediately gives off the sural nerve, which innervates the lateral aspect of the foot. The common peroneal nerve also gives off a sural communicating nerve (Fig. 30.4), and once it is below the head of the fibula it divides into superficial and deep peroneal nerves. The nerves lie more superficial and lateral to the popliteal vessels in the popliteal fossa and about midway between the skin and the posterior aspect of the femur.

Figure 30.3 Axial T1-weighted MR image of thigh 5 cm superior to popliteal crease. Note separate tibial and common peroneal components of sciatic nerve lateral to midline. 1: Femur; 2: profunda femoris artery; 3: tibial nerve; 4: common peroneal nerve; 5: sural communicating nerve; 6: biceps femoris muscle; 7: gracilis muscle; 8: semimembranosus muscle; 9: semitendinosus muscle.

Figure 30.4 Axial T1-weighted MR image showing anatomy at the level of the superior border of the patella. Note relations of the common peroneal 10. and tibial nerves 9. to popliteal vessels. The common peroneal and tibial components are 7 mm apart in this patient. The sural communicating nerve is marked. 1: quadriceps tendon: 2: patella; 3: vastus medialis muscle; 4: popliteal artery; 5: popliteal vein; 6: sartorius muscle; 7: semimembranosus muscle; 8: gracilis muscle.

Surface anatomy

The main landmarks for the lateral popliteal block include the superior aspect of the patella and a muscular groove between the biceps femoris and vastus lateralis muscles (Fig. 30.5). Identification of this groove is difficult in obese patients and should be mastered prior to procedure. This is easily felt when patients flex their leg against resistance.

Figure 30.5 Landmarks for the lateral popliteal block. A vertical line is drawn from the superior aspect of the patella. A second line is drawn that outlines the groove between vastus lateralis and biceps femoris. Where both lines intersect is the needle insertion point.

The main landmarks for the posterior popliteal block include skin crease of the knee joint and biceps femoris muscle laterally, and semimembranosus muscle medially (Fig. 30.6). The patient can be asked to flex the leg to identify the margins of the popliteal fossa. Absence of bony landmarks with the posterior approach increases the learning curve with this approach.

Figure 30.6 Landmarks for the posterior popliteal block. A line is drawn that outlines the knee crease. Muscular boundaries formed by semimembranosus and semitendinosus medially and biceps femoris laterally are outlined. At the midpoint of the knee crease, a perpendicular line is drawn into the thigh. The needle insertion point is 5 cm along this line and 1 cm laterally.

Sononatomy

Using a transverse transducer orientation, perform a systematic anatomical survey of structures from superficial (skin) to deep and from medial to lateral in the popliteal fossa (Fig. 30.7). First identify the femur, which is deep and casts a bony shadow. Next, identify the pulsatile popliteal artery that is superficial to the femur (Fig. 30.8). If it is not visible, scan distally towards the popliteal crease where the popliteal artery is more superficial. The popliteal vein may or may not be visible (collapsed by transducer pressure). Note the muscle groups medially (semitendinosus and semimembranosus muscles) and laterally (biceps femoris muscle; Fig. 30.8). The hyperechoic sciatic nerve in this location is always superficial to the femur and lateral to the popliteal artery (Fig. 30.8). If the sciatic nerve is not easily visible, angle the transducer and aim the beam caudally towards the foot (Fig. 30.9). This will bring the nerve into view once the angle of incidence is approximately 90° to the nerve. Scan the region proximally and distally to assess nerve anatomy. Position the transducer in a location where the sciatic nerve is clearly visualized as a single nerve before its bifurcation. Nerve visualization is significantly improved once local anesthetic is injected, due to enhanced contrast between the hyperechoic nerve and the hypoechoic fluid collection.