Morton’s neuroma

Morton’s neuroma is a painful condition of the plantar interdigital nerve, most often located between the heads of the 3rd and 4th metatarsals. The 2nd space may also be involved, whereas involvement of the 1st or 4th space is unusual and should prompt a search for another diagnosis.

Despite its name, it is not a true tumour of the nerve. The term actually refers to a compressive and irritative neuropathy: the common plantar digital nerve gradually thickens and becomes fibrotic as a result of repeated mechanical compression between the metatarsal heads. This produces perineural fibrosis, nerve thickening, degenerative changes and often an associated intermetatarsal bursitis — hence the terms Morton’s metatarsalgia or interdigital neuropathy that are sometimes used.

The typical clinical result is a burning or electric pain in the forefoot, sometimes accompanied by tingling or numbness in the toes, triggered by weight-bearing, narrow footwear or running.

In each intermetatarsal space, the common plantar digital nerve runs between the metatarsal heads before dividing into two proper digital nerves supplying sensation to two adjacent toes. The 3rd intermetatarsal space, between the 3rd and 4th metatarsals, is particularly exposed to mechanical stress: the nerve runs there through a narrow anatomical environment, in contact with the deep transverse metatarsal ligament, which links the metatarsal heads together, as well as with the plantar and interosseous structures.

A small intermetatarsal bursa is also present in this space. Bursitis of this bursa can cause pain very similar to that of the neuroma, be associated with it, or even be its true origin — which explains the value of ultrasound in distinguishing between the two mechanisms.

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Why is the 3rd space most often affected?

Several anatomical features explain this predominance: the high mechanical stress on the forefoot at this point, the proximity of the metatarsal heads, their relative mobility, the particular course of the interdigital nerve at this level and the stresses exerted by the surrounding ligamentous structures. The 2nd space may also be involved, but involvement of the 1st or 4th space is unusual and should suggest another diagnosis.

Forefoot biomechanics in running

The problem can be summed up simply: the nerve tolerates repeated mechanical stress poorly when it exceeds its capacity to adapt. During running, the forefoot is subjected to compressive and shear forces, increased pressure under the metatarsal heads, dorsiflexion of the metatarsophalangeal joints and tensioning of the plantar structures. These stresses increase with narrow or heeled footwear, a recent increase in mileage or in the proportion of fast sessions, hills, trail running or jumps. The neuroma can therefore appear after a change in volume, speed, terrain or footwear.

Morton’s neuroma is a common cause of forefoot pain (metatarsalgia). It mainly affects adults, more often women, as well as people exposed to tight footwear or activities that place heavy demands on the forefoot — runners, people who walk a lot, and sports involving repeated push-offs.

It can be unilateral or bilateral, and several spaces can be affected in the same person. An important point to know: the presence of a neuroma on imaging does not necessarily mean that it is responsible for the symptoms — the diagnosis must always be correlated with the clinical findings.

Footwear

This is one of the most important factors: shoes that are narrow or pointed at the front, high-heeled shoes, stiff shoes that limit the natural mobility of the forefoot, and direct compression of the metatarsals.

Running

A sudden increase in mileage, training frequency, interval sessions, sprints, hills or trail running, a change of shoes or too rapid a transition to minimalist shoes can trigger or perpetuate the symptoms.

Morphology and biomechanics

A cavus foot (a major factor, owing to a very high arch), a flat foot with overpronation, a wide forefoot, a forefoot deformity (hallux valgus), limited ankle dorsiflexion (gastrocnemius tightness), weakness of the intrinsic foot muscles or impaired push-off all promote mechanical overload of the forefoot.

The characteristic symptom is forefoot pain, described as burning, an electric shock, a current-like sensation or tingling radiating to the toes. Some patients report a very suggestive sensation: “it feels like I have a stone in my shoe” or “as if something were stuck under my foot”.

In runners, symptoms tend to appear after several kilometres, when accelerating, running uphill or changing direction, sometimes forcing them to stop, loosen the shoe or massage the forefoot — a very characteristic sign of the condition.

Clinical examination

The diagnosis is primarily clinical. Palpation of the affected intermetatarsal space usually reproduces the usual pain. Transverse compression of the forefoot can provoke the pain and its radiation to the toes, sometimes accompanied by a Mulder’s sign: a palpable or audible click during compression, which is highly suggestive of the diagnosis — although its absence does not rule it out. The thumb-index finger compression test of the intermetatarsal space is also useful: when negative, it makes it reasonable to rule out the diagnosis in an appropriate clinical context.

Surgery is reserved for persistently disabling forms with a reliable clinical diagnosis that are resistant to well-conducted conservative treatment (appropriate footwear, load adjustment, rehabilitation, injections). It should not be decided solely on the basis of the size of the neuroma on imaging: a small neuroma can be very symptomatic, and vice versa.

There are two main strategies. Neurolysis (decompression) aims to free the nerve without removing it, sometimes combined with a transposition; it carries a lower risk of postoperative neurological symptoms. Neurectomy involves removing the diseased portion of the nerve: it results in permanent loss of sensation in the corresponding digital territory, but often brings a clear improvement in pain. However, the quality of studies comparing these two approaches remains limited.

The operation can be performed via a dorsal approach (the most common, making access to the intermetatarsal space easier) or via a plantar approach, used in some revision surgeries but carrying a greater risk of a painful scar under the weight-bearing area.

After straightforward surgery, walking is generally resumed gradually according to the surgeon’s instructions, in three main phases: pain control and wound healing, then prolonged walking, strengthening and balance work, then a gradual return to impact activities and running.

The return to sport should be guided by symptoms and wound healing rather than by a fixed date, progressing from walking to continuous running and then to accelerations and interval training. Some residual tenderness in the forefoot may persist for several weeks or even several months, without compromising the overall result.

Prevention relies mainly on reducing the mechanical stress placed on the forefoot. The choice of footwear is essential: favour a wide toe box, no lateral compression and a sole suited to the shape of the foot.

In terms of training, sudden increases in mileage, speed, elevation gain or sessions that place heavy demands on the forefoot should be avoided. Regular strengthening of the intrinsic foot muscles and the triceps surae, together with maintaining single-leg control, ankle mobility and push-off capacity, complete this prevention.