Working Diagnosis:
saphenous nerve impingement at the adductor canal.

Treatment:
After an ultrasound of the thigh, the patient underwent a saphenous nerve block with ultrasound guidance with 1 mL of 1% lidocaine, 1 mL of 0.5% bupivacaine, and 1 ml of 6mg/ml betamethasone. Case Photo #2 Two weeks after his injection, the patient was seen and noted that he had 48-72 hrs of complete relief of symptoms with the return of his pain about 72 hours after as he ramped up his physical activity.

Outcome:
At the follow-up visit two weeks after the initial injection, he had the injection repeated with 3 mL of 0.5% bupivacaine and again had relief of his symptoms for 72 hours. After discussing treatment options with pain medicine, orthopedics, physical therapy, and the patient, he decided to undergo an open saphenous nerve release and cooled radio-frequency ablation. The patient then had improved pain for about 2-3 weeks before he developed worsening pain. He then received a repeat bupivacaine block which brought him back to pre-intervention pain levels, followed by a phenol block plus pregabalin 150 mg two times a day. The final regimen gave him 70% pain relief allowing him to ambulate without a cane.

Author's Comments:
The saphenous nerve is a sensory nerve that arises from the L3-L4 nerve roots. The nerve travels through the adductor canal (along with the femoral artery and vein) and exits before the adductor hiatus.1 Impingement of the saphenous nerve can occur in the adductor canal which can lead to pain in the medial leg, however this patient also had pain in the medial thigh, indicating possible irritation to the anterior cutaneous branches as well. Saphenous nerve block is frequently used in both the emergency department and peri-operative settings for procedural anesthesia and post-procedure pain control, but should also be considered as a treatment option for saphenous nerve impingement. This case highlights the unique presentation of pathology affecting the saphenous nerve which can classically present with medial leg pain at the knee but can also present with a component of medial thigh pain.

Editor's Comments:
The saphenous nerve is a branch of the femoral nerve that is purely sensory and is most vulnerable at the knee. Entrapment or irritation typically presents with pain and numbness in the affected area. Saphenous neuritis affecting the knee can be caused by entrapment of the nerve at the adductor canal, from pes anserine bursitis, as a complication to dislocation of the patella, iatrogenically from a post surgical knee or ankle, and can be a complication from a knee injection. Since it is a sensory nerve, there are no motor deficits associated with saphenous nerve entrapment.

References:
The Femoral Nerve - Course - Motor - Sensory - TeachMeAnatomy. Accessed April 17, 2024. https://teachmeanatomy.info/lower-limb/nerves/femoral-nerve/

Peck E, Finoff J, Smith J. Neuropathies in Runners. Clinics in Sports Medicine. 2010 July; 29 (3) 437-457.

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