Working Diagnosis:
Saphenous neuralgia of unknown etiology.

Treatment:
The diagnosis of saphenous neuralgia was made based on sonopalpation of the saphenous nerve, though no known etiology was determined. As the athlete was an adolescent, an initial trial of a 2-week oral prednisone taper was initially prescribed in place of a corticosteroid injection of the nerve. The athlete was also prescribed gabapentin for neuropathic pain.

Outcome:
The patient followed up 2 weeks later in the high school training room. The athlete reported significant improvement in his symptoms, with only mild residual pain at the proximal medial thigh. Given his now normal gait and full range of motion of the left knee, the athlete was allowed to return to football-related activities and successfully played for the remainder of the season.

Author's Comments:
Saphenous neuralgia is implicated in fewer than 1% of adults presenting with leg pain. The saphenous nerve is a pure sensory branch of the femoral nerve that descends from the femoral triangle to the ankle and midfoot. Etiologies typically are compressive or traumatic, with surgical injury most frequently described. Diagnosis is difficult, but can be supported by squeezing the distal thigh over Hunter's canal. nerve stretch testing, nerve blocks, nerve conduction studies, MRI, and/or sonopalpation with diagnostic ultrasound of the saphenous nerve in the medial thigh [Photos 1 and 2]. Nonsurgical treatments include topical pain medications, oral medications for neuropathic pain, and/or ultrasound guided nerve block or hydrodissection. Surgical options include neurolysis, decompression, or neurectomy.

Editor's Comments:
Saphenous neuralgia is underdiagnosed as a source of knee pain in adolescents, in part, as saphenous neuralgia can present similar to other conditions that cause anterior and/or medial knee pain. Saphenous nerve entrapment can occur proximal to the knee at the distal adductor canal in the medial thigh, known as Hunter's canal, as in this case or can occur more distally with compression of the infrapatellar branch near the medial femoral condyle. As illustrated in this case, physical exam and musculoskeletal ultrasound with sonopalpation are useful in making the diagnosis. An ultrasound-guided nerve block can confirm the diagnosis as well as provide therapeutic benefit. The nerve can be visualized on musculoskeletal ultrasound approximately 7cm proximal and 10cm medial to the superior pole of the patella. Consider saphenous neuralgia as a cause of anterior and/or medial knee pain, particularly in adolescents with patellofemoral type pain that has failed to respond to a course of high quality physical therapy, as well as patients that have undergone prior knee arthroscopy or arthroplasty.

References:
1. Herman, Daniel C. MD, PhD, FACSM; Vincent, Kevin R. MD, PhD, FACSM Saphenous Neuropathy-A Masquerading Cause of Anteromedial Knee Pain, Current Sports Medicine Reports: June 2018 - Volume 17 - Issue 6 - p 177
2. Herman DC, Vincent KR. Saphenous nerve block for the assessment of knee pain refractory to conservative treatment. Curr. Sports Med. Rep. 2018; 17:146-7.

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