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Figure 1. Anteroposterior radiograph of the right foot.
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Figure 2. Lateral view of the foot.
A 51-year-old woman presents with a complaint of right foot pain that has been present for 2 months. There has been no known injury or precipitating event. The patient notes that the pain is worse when walking and running, and is relieved with rest, ice, and ibuprofen. Radiographs of the right foot are taken (Figures 1 and 2). A physical examination of the right foot reveals no obvious deformity. She has pain in her third web space when the third and fourth metatarsal heads are compressed together.
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An interdigital (Morton) neuroma is an entrapment neuropathy of the interdigital nerve of the foot. The interdigital nerve at the third webspace (between the third and fourth metatarsal heads) is the most commonly area affected, followed by the fourth webspace.1 The cause of the neuroma is thought to be from the compression of the nerve by the intermetatarsal ligament, metacarpal heads, and/or the tissues surrounding the metatarsophalangeal (MTP) joints.1 Walking in tight shoes compresses the intermetatarsal joint and usually worsens symptoms of pain and/or feeling like there is a pebble in the shoe.1
Symptoms of Morton neuroma can be reproduced by compressing the metatarsal heads together. A palpable click when squeezing the metatarsals together is considered a positive Mulder sign. Magnetic resonance imaging (MRI) and ultrasound (US) have equal sensitivity in diagnosing a Morton neuroma but are only used if the clinical picture is unclear.2 An interdigital injection should be performed to make the most accurate diagnosis.
Dagan Cloutier, MPAS, PA-C, practices in a multispecialty orthopedic group in the southern New Hampshire region and is editor in chief of the Journal of Orthopedics for Physician Assistants.
References
1. Pomeroy G, Wilton J, Anthony S. Entrapment neuropathy about the foot and ankle: an update. J Am Acad Orthop Surg. 2015;23(1):58-66. doi:10.5435/JAAOS-23-01-58
2. Bignotti B, Signori A, Sormani MP, Molfetta L, Martinoli C, Tagliafico A. Ultrasound versus magnetic resonance imaging for Morton neuroma: systematic review and meta-analysis. Eur Radiol. 2015;25(8):2254-2262. doi:10.1007/s00330-015-3633-3
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