Saphenous Nerve Block

Indications and Clinical Considerations

Saphenous neuralgia is caused by compression of the saphenous nerve by the sartorius muscle and the adductor longus and magnus muscles as the nerve passes through the Hunter canal or, more commonly, as the nerve passes over the medial condyle of the femur ( Fig. 169.1 ). This entrapment neuropathy manifests as pain, numbness, and dysesthesias in the distribution of the saphenous nerve. These symptoms often begin as a burning pain over the medial knee. Patients with saphenous neuralgia note that sitting or squatting often causes the symptoms of saphenous neuralgia to worsen ( Fig. 169.2 ). Although traumatic lesions to the saphenous nerve after vein stripping surgery and vein harvest surgery for coronary artery bypass surgery have been implicated in the onset of saphenous neuralgia, in most patients, no obvious antecedent trauma can be identified.

Physical findings include tenderness over the saphenous nerve just medial to the midline of the midthigh. A positive Tinel sign over the saphenous nerve as it passes over the medial femoral condyle may be present. Careful sensory examination of the medial thigh reveals a sensory deficit in the distribution of the saphenous nerve. No motor deficit should be present. Sitting or squatting, which compress the saphenous nerve, may exacerbate the symptoms of saphenous neuralgia.

Saphenous neuralgia often is misdiagnosed as lumbar radiculopathy or is attributed to primary knee disease. Radiographs of the knee and electromyography help distinguish saphenous neuralgia from radiculopathy or pain emanating from the knee. Most patients with a lumbar radiculopathy have back pain associated with reflex, motor, and sensory changes and associated with neck pain, whereas patients with saphenous neuralgia have no back pain and no motor or reflex changes. The sensory changes of saphenous neuralgia are limited to the distribution of the saphenous nerve. Lumbar radiculopathy and saphenous nerve entrapment may coexist as the so-called double crush syndrome. Occasionally, diabetic femoral neuropathy may produce anterior thigh pain, which may confuse the diagnosis.

Electromyography helps distinguish lumbar radiculopathy and diabetic femoral neuropathy from saphenous neuralgia. Plain radiographs of the back, hip, and pelvis are indicated for all patients with saphenous neuralgia to rule out occult bony disease. On the basis of the patient’s clinical presentation, additional testing may be indicated, including complete blood cell count, uric acid, sedimentation rate, and antinuclear antibody testing. Magnetic resonance imaging (MRI) of the back is indicated if herniated disk, spinal stenosis, or a space-occupying lesion is suggested ( Fig. 169.3 ). The injection technique described later serves as both a diagnostic and a therapeutic maneuver.

Clinically Relevant Anatomy

The saphenous nerve is the largest sensory branch of the femoral nerve. The saphenous nerve provides sensory innervation to the medial malleolus, medial calf, and a portion of the medial arch of the foot. It is derived primarily from the fibers of the L3 and L4 nerve roots ( Fig. 169.4 ). The nerve travels along with the femoral artery through the Hunter canal and moves superficially as it approaches the knee. It passes over the medial condyle of the femur, splitting into terminal sensory branches ( Figs. 169.5 and 169.6 ; also see Fig. 169.1 ). The saphenous nerve is subject to trauma or compression anywhere along its course. The nerve frequently is traumatized during vein harvest procedures for coronary artery bypass grafting procedures. The saphenous nerve also is subject to compression as it passes over the medial condyle of the femur ( Fig. 169.7 ).