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. It is named after Thomas Morton, who presented a case series of patients afflicted with this disorder in 1876. 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 a previously diagnosed neuroma.
The term neuroma is actually a misnomer. Histologic investigation does not reveal the typical proliferation of axons found in true neuromas. Instead, there is a fibrosis and thickening of the perineural tissue with corresponding degeneration of the underlying nerve in a Morton's neuroma.1 This most commonly affects the third plantar common digital nerve located in the third interspace, between the third and fourth metatarsal heads. It may also occur less commonly in the second interspace, between the second and third metatarsals. A neuroma is rarely seen in the first or fourth interspaces, between the first and second or the fourth and fifth metatarsals, respectively.
Morton's neuroma most commonly affects women between their fourth and sixth decades.2 It is especially common in those who wear high-heeled shoes, poor fitting shoes, worn shoes, shoes with poor or no padding, or shoes that are narrow at the forefoot. Persons with pronated or pes cavus feet are similarly at risk.3 Neuromas do not usually become symptomatic until their transverse diameter reaches more than 5 mm.4
Neuromas form just proximal to the bifurcation of the plantar common digital nerves (Figure 186-1) and below the deep transverse intermetatarsal ligament (Figure 186-2). The deep transverse intermetatarsal ligament connects the metatarsal heads on the plantar aspect of the foot (Figure 186-2). The neuroma is made up of branches from both the medial and lateral plantar nerves (Figure 186-1). 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.
Morton's neuroma most commonly occurs in the third intermetatarsal space beneath the transverse metacarpal ligament.
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 more fixed medial portion of the foot 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.2 Injury begins with edema of the endoneurium, followed by fibrosis beneath ...