Interdigital (Morton) neuroma is a benign perineural fibrosis of a plantar digital nerve that can cause pain, which may be nonspecific, burning, or lancinating, or a foreign body sensation. An accepted misnomer, these neuromas are not neoplastic in nature without nerve tumor growth. Diagnosis is usually clinical. Treatment may involve correction of footwear, local injection, or sometimes surgical excision.
(See also Overview of Foot and Ankle Disorders.)
Interdigital neuroma is a common cause of metatarsalgia. The interdigital nerves of the foot travel beneath and between the metatarsals, extending distally to innervate the toes. Neuroma of the interdigital nerve along its distal innervation near the ball of the foot develops primarily as a result of improper or constrictive footwear or, less commonly, nerve traction resulting from abnormal foot structure (eg, splayfoot deformity). As a result of chronic repetitive trauma, a benign thickening or fibrosis of the nerve develops (Morton neuroma).
Symptoms and Signs of Interdigital Neuroma
Interdigital neuroma is characterized by pain around the metatarsal heads or the toes, most frequently at the third intermetatarsal space, but it can occur in other (ie, first, second, or fourth) or multiple intermetatarsal spaces.
Early interdigital neuroma is associated with an acute inflammatory process and often causes an occasional mild ache or discomfort in the ball of the foot, usually when wearing a specific shoe, such as those that are too narrow at the front. Neuralgia is usually unilateral, but it can sometimes be bilateral.
As the condition progresses, perineural fibrosis occurs. The pain becomes worse, often with a burning or lancinating quality or paresthesias. In time, patients are unable to wear most closed-toe shoes. While walking, patients often falsely sense a pebble in their shoes, which they take off for relief.
Diagnosis of Interdigital Neuroma
Clinical evaluation
The symptoms of interdigital neuroma are often specific, and the diagnosis is confirmed by tenderness on plantar palpation of the interdigital space and by reproduction of the radiating burning pain, often accompanied by a notable click, by squeezing the space (Mulder sign). Although MRI does not usually confirm neuroma, it may be useful to rule out other interspace lesions or arthritis causing similar symptoms.
Treatment of Interdigital Neuroma
Modification of footwear and injection
Neuroma of recent onset may resolve quickly with properly fitting shoes and insoles or with local anesthetic injection. Using a metatarsal pad placed proximally to the metatarsal heads of the affected interspace may also help reduce symptoms.
Persistent symptoms may require one or more perineural infiltrations of long-acting corticosteroids with a local anesthetic. Injection is at a 45° angle to the foot, into the interspace at the level of the dorsal aspect of the metatarsophalangeal joints (see Considerations for Using Corticosteroid Injections). Orthotics with neuroma pads, rest, cold packs, and properly fitting shoes often relieve symptoms. Nerve ablation techniques, such as injecting 20% alcohol with a local anesthetic directly into the nerve with ultrasonographic guidance, radiofrequency ablation (1), or cryogenic freezing of the nerve may help relieve symptoms but alcohol sclerosing injections risk increasing perineural fibrosis.
If other treatments are ineffective, excision often brings complete relief. A true neuroma occasionally develops at the site of nerve excision (amputation or stump neuroma), which may require additional surgery.
Treatment reference
1. Connors JC, Boike AM, Rao N, Kingsley JD: Radiofrequency ablation for the treatment of painful neuroma. J Foot Ankle Surg 59(3):457-461, 2020. doi:10.1053/j.jfas.2019.09.003
Key Points
Metatarsal pain can result from irritation or benign thickening of the interdigital nerves.
Initially, mild pain caused by wearing narrow shoes can worsen and become lancinating, sometimes with paresthesias and/or a foreign body sensation.
Diagnose the disorder by clinical findings, including tenderness and reproduction of symptoms with palpation of the interdigital space.
Treat by modifying footwear, giving local anesthetic injections and sometimes corticosteroid injections, doing nerve ablation techniques, or doing surgery.
