Surgical Exposure of Peripheral Nerves of the Lower Extremity. II. Femoral Nerve, Saphenous Nerve, Lateral Femoral Cutaneous Nerve, and Sural Nerve

Clinical Anatomy of the Femoral Nerve

The femoral nerve originates in the retroperitoneum from the lumbar plexus. The main motor component innervates the iliacus and psoas muscles, and the quadriceps muscles (hip flexion and knee extension, respectively). The predominant sensory branch of the femoral nerve, the saphenous nerve, innervates the parts of the thigh and calf.

The femoral nerve, the largest branch of the lumbar plexus, arises from the ventral rami of L2, L3, and L4. It penetrates the psoas muscle and descends in the groove between the psoas muscle and the iliacus muscles, covered by the iliac fascia. Classically, the nerve emerges from the lateral border of the psoas major muscle, below the iliolumbar ligament, and exits the abdomen into the anterior thigh deep to the midpoint of the inguinal ligament. The nerve continues laterally in the femoral triangle, which is bounded by the inguinal ligament rostrally, the adductor longus muscle medially, and the sartorius muscle laterally. A nerve to the pectineus muscle exits medially near the inguinal ligament, passing behind the femoral sheath before entering the muscle.

The femoral nerve divides below the inguinal ligament, within the femoral triangle, splitting into ventral and dorsal divisions, divided by the lateral circumflex femoral artery. The ventral division of the femoral nerve has muscular branches to the sartorius muscle and usually arises in common with the intermediate femoral cutaneous nerve. The dorsal division supplies muscular branches to the rectus femoris, vasti, and articularis genu muscles, as well as branches to the anterior femoral cutaneous nerve. The largest branch, the saphenous nerve, also exits from the dorsal division, crossing ventral to the femoral artery, entering the adductor canal at the apex of the femoral triangle, to supply the medial thigh and the medial and ventral aspects of the calf.

The femoral nerve is associated with a variety of clinical syndromes. The most common cause of femoral nerve neuropathy is diabetic amyotrophy. Traumatic injuries, including iatrogenic injuries to the femoral nerve, particularly in the region of the femoral triangle, can be serious and these injuries can be complicated by vascular injury. Most entrapment neuropathies occur below the inguinal ligament because of the relative exposure to injury and the complex anatomical relations; however, other neuropathies or injuries can occur prior to exiting the abdomen and are associated with intra-abdominal pathology.

71.1.2 Preoperative Preparation and Operative Procedure

Surgical Exposure of the Femoral Nerve

The patient is positioned supine on the operating table with the ipsilateral knee partially flexed. The approach for the proximal portion of the femoral nerve in the thigh is taken through the femoral triangle. Classically, the anatomical vascular orientation of the femoral nerve, from lateral to medial, is femoral nerve, artery, and vein. A curvilinear incision is made on the ventral aspect of the thigh, beginning at the anterior–superior iliac spine. It is continued caudally across the sartorius muscle to the junction of the mid- to lower thigh ( ▶ Fig. 71.1). The sartorius muscle is retracted laterally, and the iliacus fascia is opened from caudal to rostral to reveal the femoral nerve and femoral vessels.

Fig. 71.1 Surgical exposure of the femoral nerve. A curvilinear incision is made, beginning in the anterior–superior iliac spine region parallel to the inguinal ligament and curving caudally in the medial thigh until it crosses the sartorius muscle.

The proximal portion of the femoral nerve and its branches can be identified lateral to the femoral vessels, outside the femoral sheath and deep to the fascia lata ( ▶ Fig. 71.2). It is followed rostrally or caudally as necessary. The caudal intrapelvic portion of the nerve can be identified and dissected by dividing the inguinal ligament and splitting the external oblique muscle, which is identified rostral to the inguinal ligament and superficial to the nerve, at the depth of this ligament.

Fig. 71.2 The femoral nerve is identified after the fascia is opened medially to the sartorius muscle. This muscle is gently retracted laterally. The femoral nerve and its branches are localized lateral to the femoral vessels. Rostrally in the exposure, the femoral nerve can be identified where it passes under the inguinal ligament.

71.2 Saphenous Nerve

71.2.1 Introduction and Patient Selection

Clinical Anatomy of the Saphenous Nerve

The saphenous nerve is the largest cutaneous branch of the femoral nerve. It innervates the medial thigh and the medial and ventral aspects of the calf. The saphenous nerve branches from the dorsal division of the femoral nerve just below the level of the inguinal ligament. It descends through the femoral triangle, lateral to the femoral sheath and vessels. It then crosses the femoral artery, from lateral to medial, before exiting the femoral triangle at its lower apex. It enters the adductor canal of Hunter with the femoral vessels. This canal extends rostrally from the apex of the femoral triangle to the caudal and medial aspect of the thigh, rostral to the medial femoral condyle. The saphenous nerve exits the adductor canal with the saphenous artery, ~ 8 to 10 cm rostral to the medial femoral condyle, and becomes subcutaneous after piercing the fascia lata proximal to the knee. The nerve descends between the tendons of the gracilis and sartorius muscles before it divides in two branches, the infrapatellar and descending branches. The infrapatellar branch supplies the ventromedial aspect of the knee. The descending branch joins the great saphenous vein, descending in a trajectory toward the medial malleolus, and innervates the ventromedial aspect of the leg.

The saphenous nerve may become entrapped at several places along its course because it is a component of the femoral nerve rostral to the inguinal ligament and caudal to the inguinal ligament it courses along the entire lower extremity. Classically, the most common point of entrapment is at the point of exit from the adductor canal, where nerve angulation and inflammation may induce and exacerbate compressive symptoms. Affected patients have severe pain along the medial aspect of the knee. This pain is worsened by activity and improved with rest and can often be treated with conservative management. Occasionally, surgical exploration is necessary.

71.2.2 Preoperative Preparation and Operative Procedure

Surgical Exposure of the Saphenous Nerve

The surgical approach of the saphenous nerve in the region of the adductor canal can be either the same incision used for femoral nerve exploration ( ▶ Fig. 71.1), or it can be an extended, curvilinear, vertical incision over the ventral midthigh ( ▶ Fig. 71.3). The latter is preferred. It is opened as previously described for the femoral nerve. The saphenous nerve is most readily identified as it enters the adductor canal with the femoral vessels ( ▶ Fig. 71.4). It is followed caudally by unroofing the adductor canal when necessary. The caudal exit from the adductor canal through the thickened fascial roof with the saphenous artery is clinically the most relevant aspect to explore in most cases because this is the most common site associated with saphenous nerve entrapment. More caudal dissection and exploration are often not warranted.

Fig. 71.3 Surgical exposure of the saphenous nerve. A curvilinear vertical incision is made in the medial-ventral thigh, beginning medial to the sartorius muscle and crossing above this muscle caudally. (a) Denotes rostral exposure for femoral triangle exploration. (b) Denotes caudal exposure for adductor canal exploration.

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