The saphenous nerve, the terminal branch of the femoral nerve, is the femoral nerve's longest branch. It is a pure sensory nerve that is made up of fibers from the L3 and L4 spinal segments. Because of its long course, it can become entrapped in multiple locations, from the thigh to the leg. It branches from the femoral nerve just distal to the inguinal ligament and courses with the superficial femoral artery to enter the adductor (Hunter's) canal in the distal third of the thigh. This canal extends proximally from the apex of the femoral triangle to the inferomedial aspect of the thigh in the adductor magnus tendon, just proximal to the femoral condyle. The canal is somewhat triangular and lies between the vastus medialis laterally and the adductor magnus and longus muscles medially.
The roof of the canal is a dense bridge of connective tissue extending between these muscle groups. The saphenous nerve exits the canal by piercing the roof, and becomes subcutaneous about 10 cm proximal to the medial epicondyle of the femur. The nerve may also pierce the sartorius muscle. Once it becomes subcutaneous, the nerve branches to form the infrapatellar plexus, while the main branch continues along the medial leg and foot.
The saphenous nerve can become entrapped where it pierces the roof of the adductor canal. Inflammation results from a sharp angulation of the nerve at its exit and from the dynamic forces of the muscles in this region, which cause contraction and relaxation of the fibrous tissue that impinges on the nerve. The nerve can also be injured as a result of an improperly protected knee or leg support during surgery. It may be injured by a neurilemoma, entrapment by femoral vessels, direct trauma, pes anserine bursitis, varicose vein operations, and medial knee arthrotomies and meniscal repair.
Symptoms of entrapment may include a deep aching sensation in the thigh, knee pain, and paresthesias in the cutaneous distribution of the nerve in the leg and foot. The infrapatellar branch may also become entrapped on its own. This is because it passes through a separate foramen in the sartorius muscle tendon. It may also be exposed to trauma where courses horizontally across the prominence of the medial femoral epicondyle. Patients report paresthesias and numbness about the infrapatellar region that is worse with flexion of the knee or compression from garments and braces.
Saphenous nerve entrapment is a frequently overlooked cause of persistent medial knee pain in patients who experience trauma or direct blows to the medial aspect of the knee. As this is a purely sensory nerve, weakness should not be noted with an isolated injury of this nerve. If weakness is present, look for an injury of the femoral nerve or possibly an upper lumbar radiculopathy, particularly if thigh adduction is present (obturator nerve).
Deep palpation proximal to the medial epicondyle of the femur may reproduce the pain and complaints. Some weakness may be present because of guarding or disuse atrophy from pain, but no direct weakness will result from the nerve impingement. Sensory loss in the saphenous distribution may be present on examination. No weakness should be present in the quadriceps muscles or in the hip adductors.
The diagnosis may be made on the basis of injection of local anesthetic along the course of the nerve and proximal to the proposed site of entrapment. Nerve conduction techniques are available to assess neural conduction in the main branch of the saphenous nerve or in the terminal branches. The routine tests may be disappointing in persons with subcutaneous adipose tissue or swelling. A side-to-side comparison of the nerve should be made, and must demonstrate a lesion consistent with the patient's complaints. A somatosensory evoked potential (SSEP) test can also be performed and the results compared with those of the contralateral side for diagnosis, although this test may be cumbersome and time-consuming.
No findings should be present on needle examination of the muscle during electromyography. Needle examination should include the quadriceps muscle and the adductor longus to assess for femoral and obturator nerve injury. If findings are present in both of these muscles, then paraspinal muscles definitely should be examined to rule out radiculopathy.
Entrapment in the adductor canal usually is treated conservatively with an injection of anesthetic (with or without corticosteroid) at the point of maximal tenderness (usually 10 cm proximal to the medial femoral condyle). The injection may need to be repeated periodically. Avoiding aggravating activities and using proper body mechanics will also be helpful. If this approach fails, surgical decompression may be needed. In patients who have had a direct blow to the medial knee who have persistent medial knee pain despite conservative trials for treatment, a neurectomy or neurolysis of the infrapatellar branch may be helpful.
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