Morton’s Neuroma (Interdigital Neuroma)
Presentation

The most common presentation of an interdigital (Morton’s) neuroma is pain located between the third and the fourth metatarsal heads (in the third interspace) (Fig. 5–79) that radiates into the third and fourth toes. Patients often describe this as a burning pain that intermittently “moves around.” Usually, the pain is exacerbated by tight-fitting and/or high-heeled shoes or increased activity on the foot. The pain is often relieved by removing the shoe and rubbing the forefoot. Occasionally, these symptoms occur in the second interspace with radiation into the second and third toes. Seldom do neuromas occur in both interspaces simultaneously.

Table 5–11 presents a list of the preoperative symptoms given by patients (percentage) with an interdigital neuroma in Mann’s series (1997).

Anatomy and Pathophysiology
The “classic” Morton neuroma is a lesion of the common digital nerve that supplies the third and fourth toes (see Fig. 5–79). This is not a true neuroma, but rather an irritated perineural fibrosis where the nerve passes plantar to the transverse metatarsal ligament (Fig. 5–80).

It has been speculated that because the common digital nerve to the third interspace has branches from the medial and lateral plantar nerves (and thus increased thickness) that this accounts for the third interspace being the one most commonly involved. The occasional involvement of the second interspace may be a result of anatomic variation in the distribution of the common digital nerves.

The incidence of interdigital neuromas is 8 to 10 times more common in females.
The mechanism is probably chronic hyperextension of the MTP joints (in high heels) with tethering and irritation of the nerve across the transverse metatarsal ligament. This results in an entrapment neuropathy.

Diagnosis
The diagnosis of a Morton neuroma is clinical. There are no useful radiographic or electrodiagnostic tests. Serial examinations may be necessary to establish the correct diagnosis.

Examination
Direct palpation and palpation with a stripping motion (Fig. 5–81) of the interspace will usually reproduce the patient’s pain. This maneuver, called “Mulder’s sign,” often reproduces a clicking or popping sensation and pain in the third (or second) interspace. The examiner places the index finger and thumb proximal to the metatarsal heads in the interspace, and while pushing firmly into the interspace, “strips” distally to the end of the interspace, often feeling a click or pop that elicits pain (Mulder’s click).

Widening of the involved (third and fourth) toes may be noted on occasion as a result of the neuroma mass in the interspace. Subjective numbness of the involved toes is often noted, but sensory examination may reveal partial, complete, or no sensory deficit in the nerve’s distribution (see Fig. 5–79).

The patient with a Morton neuroma does not have pain over the metatarsal heads.
Occasionally, the examination will be positive only after a vigorous workout or tight shoewear. Often, the patient’s physical examination is inconclusive and requires several serial examinations and a ruling out of related pathology.

Differential Diagnosis
Morton’s neuroma may be mimicked by a number of other conditions. The following differential diagnoses should be considered to rule out an incorrect diagnosis of a Morton neuroma.

1.       Neurogenic pain, tingling, or numbness