Chapter 166

Morton’s neuroma, also referred to as an interdigital neuroma, is one of the most common painful disorders of the forefoot. It was first described in 1845 by Dulacher and is named after Thomas Morton, who presented, in 1876, a case series of patients afflicted with this disorder. Patients with an established Morton’s neuroma are usually cared for by a Podiatrist or an Orthopedist. They may present to the Emergency Department with a previously undiagnosed neuroma or with a painful exacerbation of known neuroma.

The term neuroma is actually a misnomer. Histologic investigation does not reveal the proliferation of axons found in true neuromas. Instead, in Morton’s neuroma, there is a progressive fibrosis and thickening of the perineural tissue with degeneration of the underlying nerve. This most commonly affects the third plantar common digital nerve located between the third and fourth metatarsal heads. It may occur between the second and third metatarsals. A neuroma is rarely seen between the first and second or the fourth and fifth metatarsals.

Morton’s neuroma is most commonly found in women in their fourth to sixth decades.1 It is especially common in those who wear high-heeled shoes or shoes that are narrow at the forefoot. It is more commonly seen in persons with pronated or pes cavus feet.2 Neuromas do not become symptomatic until their transverse diameter reaches more than 5 mm.3

Neuromas form just proximal to the bifurcation of the plantar common digital nerves (Figure 166-1) and below the deep transverse intermetatarsal ligament (Figure 166-2). The deep transverse intermetatarsal ligament connects the plantar aspects of the metatarsal heads (Figure 166-2). The neuroma is made up of branches from both the medial and lateral plantar nerves (Figure 166-1). Most commonly affected is the third interdigital nerve. It is the largest of the interdigital nerves and may explain the increased frequency of neuroma formation in this location.

###### Figure 166-1

Morton’s neuroma most commonly occurs in the third intermetatarsal space beneath the transverse metacarpal ligament.

###### Figure 166-2

Anatomy of the forefoot. Cross-section through the distal metatarsals.

Morton and others postulated that the increased mobility of the fourth and fifth metatarsal heads relative to the others results in disproportionate trauma to the third interdigital nerve. These mechanical factors, combined with the impingement and stretching from a tight transverse intermetatarsal ligament, result in repetitive microtrauma. Histologic evaluation reveals perineural fibroma formation consistent with compression-induced trauma.1 There is a progression from edema of the endoneurium, fibrosis beneath the perineurium, axonal degeneration, and neuronal necrosis.

Some authors believe that a more significant contributor to neuroma formation is enlargement of the interphalangeal component of the intermetatarsophalangeal bursa, leading to microvascular trauma....

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