Lower Extremity: Saphenous Nerve Block

Saphenous nerve entrapment or neuropathy is a relatively rare clinical entity, a paucity of data exists regarding its natural history and treatment outcome, although there are a few case reports in the literature. Saphenous nerve neuropathy is an important consideration in the differential diagnosis of lumbar radiculopathy, medial knee pain, and lower extremity vascular insufficiency. Saphenous nerve block can be an effective treatment for saphenous neuropathy or pain in the distribution of the saphenous nerve for this condition.

Anatomy of the Saphenous Nerve

The saphenous nerve is the largest cutaneous branch of the femoral nerve. Sensory fibers from L3 and L4 levels contribute to this pure sensory nerve. The saphenous nerve branches off the posterior division of the femoral nerve and descends anteroinferiorly through the femoral triangle, lateral to the femoral sheath, anterior to the femoral artery and posterior to the aponeurotic covering of the adductor canal (also known as Hunter canal, subsartorial canal). It then exits from the adductor canal, descends under the sartorius muscle, and then courses around the posterior edge of the sartorius muscle at its tendon portion. In the medial aspect of the knee, the saphenous nerve descends vertically behind the sartorius muscle, pierces the fascia lata and becomes subcutaneous. At this level, the infrapatellar branch pierces the sartorius muscle and courses inferiorly to the infrapatellar region and innervates the skin in front of the patella. The infrapatellar branch communicates with the anterior cutaneous branches of the femoral nerve above the knee, and with other branches of saphenous nerve below the knee, as well as branches of the lateral femoral cutaneous nerve on the lateral side of the knee, forming the plexus patellae.

The remaining course of the saphenous nerve then passes down along the medial aspect of the tibia, accompanied by the great saphenous vein and, divides into two branches at the lower third of the leg. One of the branches of the descending portion of the saphenous nerve courses along the medial border of the tibia and ends at the ankle, whereas the other branch passes anteriorly to the ankle and is distributed to the medial aspect of the foot, reaching as far as the metatarsophalangeal joint of the great toe and communicating with the medial branch of the superficial peroneal nerve ( Fig. 27-1 ). The saphenous nerve supplies sensory innervations to the anterior medial aspect of the leg, including the medial malleolus ( Fig. 27-2 ).

Figure 27-1

Anatomy of saphenous nerve.

(From Williams PL, Warwick R: Gray’s Anatomy, 36th ed. Philadelphia, WB Saunders; 1980.)

Figure 27-2

Saphenous nerve innervation area.

(From Williams PL, Warwick R: Gray’s Anatomy, 36th ed. Philadelphia, WB Saunders; 1980.)

The adductor canal, the entrapment site for the saphenous nerve, is located approximately 10 centimeters proximal to the medial femoral condyle. It is an aponeurotic tunnel in the middle third of the thigh, extending from the apex of the femoral triangle to the opening in the adductor magnus. It courses between the anterior compartment of the thigh and the medial compartment of the thigh. The vastus medialis muscle lies anteriorly and laterally, whereas the adductor longus lies posteriorly. The content in the canal includes the femoral artery, femoral vein, and branches of the femoral nerve, the saphenous nerve, and the nerve to the vastus medialis muscle). It is covered by a strong aponeurosis that extends from the vastus medialis, across the femoral vessels to the adductor longus and magnus. The sartorius muscle lies on the aponeurosis.

The adductor canal can be located by palpating along the anteromedial aspect of the vastus medialis muscle and then sliding posteriorly until the edge of the sartorius muscle is felt. The adductor canal is located directly beneath this point.

Pathophysiology of Saphenous Nerve Entrapment and Symptoms

The saphenous nerve can become entrapped in multiple locations along its long descending course from the thigh to the leg. Common sites of entrapment include the site where the nerve penetrates the roof of the adductor canal, or at the infrapatellar branch during knee surgery or varicose vein stripping surgery. Compression of the saphenous nerve against the medial femoral condyle may be caused by stirrups during lithotomy position for surgery.

The pathophysiology of saphenous nerve neuropathy involves two different mechanisms: acute compression with high pressure and chronic intermittent compression by contraction and relaxation of the fibrous tissue that impinges the nerve. Demyelination of the involved nerve seems to be a common pathologic finding.

Saphenous nerve entrapment typically manifests as burning pain after prolonged walking or standing in the medial knee and leg. Pain may also be present at rest. Stair climbing may aggravate pain. Sensory symptoms typically involve paresthesia, hypoesthesia, with variations due to the length of nerve compression at the entrapment site. Hyperalgesia, allodynia, and numbness may also be present in the distribution of the saphenous nerve innervation area. The pain associated with this neuropathy may be referred to the lower medial aspect of the knee, or it may radiate down the medial aspect of the lower extremity to the ankle and foot. There is no motor compromise because the saphenous nerve is a purely sensory nerve. This is important in differentiating this syndrome from radiculopathy in the L4 dermatome distribution.

Clinical criteria for the diagnosis of saphenous nerve entrapment neuropathy include pain in the distribution of the saphenous nerve, normal motor function, and tenderness to palpation over the entrapment site. Entrapment site tenderness is a key feature of saphenous nerve neuropathy. Vigorous palpation at the exit point for the saphenous nerve may result in local pain and referred pain in the nerve’s distribution.

Electrodiagnostic studies are a valuable tool in diagnosing saphenous nerve entrapment. Changes in latency and amplitude of the sensory nerve action potential (SNAP) can be seen. However, in some cases SNAP of the saphenous nerve may be difficult to record, even in the unaffected leg. In this event, cortical sensory evoked potential recording may be used to help diagnose this condition.

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Apr 13, 2019 | Posted by in PHYSICAL MEDICINE & REHABILITATION | Comments Off on Lower Extremity: Saphenous Nerve Block

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