Working Diagnosis:
Infrapatellar neuralgia

Treatment:
An infrapatellar nerve block relieved pain for several hours with some lasting effect. He then returned for radiofrequency ablation of his right infrapatellar nerve.

Outcome:
He was able to continue wrestling with a pad to mitigate pain levels.

Author's Comments:
Infrapatellar neuralgia can be misdiagnosed as more common musculoskeletal diagnosis. The infrapatellar nerve is most at risk of damage during knee surgery. The second most common cause is trauma. In this case, repetitive knee trauma from mat takedowns contributed to this patient's pathology. This highlights the importance of a sport-specific history in considering a differential diagnosis. This condition should be suspected in athletes with repetitive trauma, allodynia of the anteromedial knee, and those with treatment resistant patellofemoral pain. Physical exam findings, musculoskeletal ultrasound, and /or image guided diagnostic injection can help in diagnosis. Treatment includes hydrodissection with or without corticosteroid, surgical excision, and/or radiofrequency ablation.

Editor's Comments:
The saphenous nerve branches into the infrapatellar nerve branches after traveling through the adductor canal and piercing through the sartorious muscle. The superficial location of this nerve, the proximality to surgical incision sites, and the anatomic variability of the distal branches make iatrogenic injury to this nerve is a known surgical risk. Clinicians should be aware of infrapatellar neuralgia as a possible diagnosis for anterior or medial knee pain, particularly in the setting of prior surgery or trauma.

Diagnostic ultrasound can be used to identify signs of entrapment or injury to the saphenous or more distal infrapatellar nerve. High clinical suspicion is important to request the ultrasonography to evaluate the saphenous and infrapatellar nerve specifically as the infrapatellar nerve is not specifically examined with routine ultrasound studies of the knee. Body habitus and technical expertise may limit the sensitivity of this imaging modality in some patients.

While nerve conduction studies aid in the diagnosis of most nerve entrapments, nerve conductions studies are technically very difficult to evaluate the saphenous nerve and would be even less sensitive to detect infrapatellar neuralgia. There is one report using nerve conduction studies to evaluate the infrapatellar nerve, but this described using a needle electrode to stimulate the femoral nerve in the groin which may not be well tolerated and is limited to research setting. Therefore, electrodiagnosis is not currently sensitive to diagnose an infrapatellar neuralgia.

The use of ultrasound guided small volume diagnostic nerve block and/or hydrodisections can be a key diagnostic test to confirm the suspicion of infrapatellar neuritis. Radiofrequency ablation or nerve excision can be effective for patients with recurrent pain following an effective nerve block or hydrodisection. Due to the rare occurrence of this injury and the paucity of literature there is limited data on clinical outcomes for any of the these treatment options.

References:
Szwedowski D, Ambrozy J, Grabowski R,
Dallo I, Mobasheri A. Diagnosis and
treatment of the most common neuropathies following knee injuries and reconstructive surgery - A narrative review. Heliyon [Internt].
2021 Sep 22 [cited 2022 Jul 6];7(9):e08032. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8477214.
McLean BC, Nguyen CD, Newman DP. Cryoablation of the infrapatellar branch of the saphenous nerve identified by non-invasive peripheral nerve stimulator for the treatment of non-surgical anterior knee pain: a case series and review of the literature. Cureus [Internet]. [cited 2022 Jul 6];12(6):e8747. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7377036.

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