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 1835 by Civinni.1,2 It is named after Thomas Morton who presented a case series of patients afflicted with this disorder in 1876.1,2 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 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.3 This condition most commonly affects the third plantar common digital nerve located in the third interspace between the third and fourth metatarsal heads. The second interspace may be affected less commonly. It is rare for neuromas to involve the first or fourth interspaces.
Morton’s neuroma disproportionally affects women between their fourth and sixth decades.4 It is especially common in those who wear high-heeled shoes, poorly fitting 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.5 Neuromas do not usually become symptomatic until their transverse diameter reaches more than 5 mm.6
ANATOMY AND PATHOPHYSIOLOGY
Neuromas form just proximal to the bifurcation of the plantar common digital nerves (Figure 222-1) and below the deep transverse intermetatarsal ligament (Figure 222-2).7 The deep transverse intermetatarsal ligament connects the metatarsal heads on the plantar aspect of the foot (Figure 222-2).7 The neuroma is made up of branches from both the medial and lateral plantar nerves (Figure 222-1). Most commonly affected is the third interdigital nerve. It is the largest of the interdigital nerves which 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. Mechanical factors combined with the impingement and stretching from a tight transverse intermetatarsal ligament results in repetitive microtrauma. Histologic evaluation reveals perineural fibroma formation consistent with compression-induced trauma.4 Injury begins with edema of the endoneurium, followed by fibrosis beneath the perineurium, axonal degeneration, ...