eMedicine Specialties > Radiology > Musculoskeletal

Morton Neuroma

Author: Steven Needell, MD, Director of Musculoskeletal Imaging, Department of Radiology, Boca Raton Community Hospital
Coauthor(s): Jonathan Cutler, DPM, Consulting Staff, Western Communities Foot and Ankle Center
Contributor Information and Disclosures

Updated: Dec 29, 2008

Introduction

The etiology remains controversial, and treatment varies among practitioners. Diagnosing the condition involves clinical judgment, and in difficult cases, imaging may be useful in determining the precise diagnosis.

Related eMedicine topics:

Morton Neuroma (Orthopedic Surgery)

Morton Neuroma (Physical Medicine and Rehabilitation)

Traumatic Peripheral Nerve Lesions

Nerve Entrapment Syndromes

Acute Nerve Injury

Anatomy

The medial and lateral plantar nerves, which are terminal branches of the tibial nerve, supply most of the innervation for the digits of the foot. The medial plantar nerve divides at the base of each metatarsal into 3 nerves that supply the branches to digits 1-3. The lateral plantar nerve divides into superficial and deep branches. The superficial branch supplies cutaneous innervation to toes 4 and 5 and also sends a communicating branch to the medial plantar nerve to innervate the third interspace.¹

Pathophysiology

The etiology of Morton neuroma is controversial. Histologic findings include neural degeneration, epineural and endovascular hyalinization, and perineural fibrosis.² Morton neuroma represents perineural fibrosis along the plantar nerve. Morton neuroma is not a nerve tumor, and it does not contain inflammatory cells or cystic components. In 1876, Morton believed that the etiology was related to compression of the nerve.³ He found that the plantar nerve stretches under the pulley of the transverse metatarsal ligament as the toes dorsiflex and as the belly of the flexor digitorum brevis muscle contracts.

Nissen⁴ believed that an ischemic component existed and that it arose from compression from the transverse metatarsal ligament. Bossley and Cairney⁵ believed the etiology was secondary to an intermetatarsal bursa distal to the transverse metatarsal ligament, undergoing inflammatory changes that caused fibrosis around the nerve. Mulder agreed with Bossley and Cairney and determined that this enlarged bursa causes a click when the metatarsals are squeezed. This finding is called the Mulder sign.³

Diagnosis

Patients may report numbing, burning, and shooting pain that courses between the metatarsals into the digits. The symptoms usually are relieved when the affected areas are massaged and appropriate shoes are worn. Such shoes are wide in the toe box and have adequate cushioning, with heels no higher than 0.5-1 in. Many asymptomatic patients have clinically silent neuromas; therefore, careful assessment of clinical findings is essential in evaluating the significance of a lesion that is evident on magnetic resonance imaging (MRI) scans.

Objective findings include tenderness dorsal to the metatarsal heads that reproduces an underlying pain upon compression of the nerve. The Mulder click can be distinguished by squeezing the metatarsal heads together (1-5 in the transverse plane) and by massaging the interspace in the vertical plane. A click (representing an enlarged nerve or inflamed bursa) can be felt and heard.

Differential Diagnosis

The differential diagnosis of Morton neuroma includes a stress fracture, a tendon sheath ganglion, a foreign-body reaction, a nerve-sheath tumor, strain of the plantar capsule, and capsulitis or bursitis at the level of the plantar metatarsal-phalangeal joint (MPJ).^2,6 Occasionally, the diagnosis can be challenging because, in addition to inflammation of the capsule or bursa, inflammation of the adjacent nerve also may be present, causing the neuritic sensation of a Morton neuroma.

Peripheral neuropathy also can cause symptoms such as a burning or neuritic sensation, but these tend to occur throughout the toes and on both feet. Additional differential diagnoses with similar symptoms involve neoplasms and fibromas. Imaging studies, particularly MRIs, are a valuable tool in differentiating these conditions.

Imaging

Although a Morton neuroma can often be confidently diagnosed on the basis of the clinical findings alone, imaging is useful in the preoperative setting or in cases with atypical symptoms.² Morton neuromas have been imaged successfully by using MRI, CT scanning, and ultrasonography.^6,7 MRI has been the preferred imaging modality for demonstrating Morton neuroma largely because of MRI's superior contrast resolution and reproducibility.^8,9

Ultrasonography has the advantage of being less expensive and more portable than MRI, but because of its high learning curve and dependence on the operator's skill, whether ultrasonography will replace MRI as the preferred imaging modality for this condition remains to be seen. Relatively few radiologists and technologists in the United States are experienced in using ultrasonography to diagnose Morton neuroma. The employment of ultrasonography in this diagnosis, although described in the literature, most commonly occurs in subspecialty academic settings and outside the United States.¹⁰

Not uncommonly, Morton neuromas are incidental findings on MRIs; they are detected best on short-axis (transverse) T1-weighted MRIs through the metatarsal heads. Morton neuroma is typically seen as a bulbous mass arising between the metatarsal heads. Because neuromas have a highly cellular and/or fibrous nature, lesions demonstrate low signal intensity on both T1- and T2-weighted images (see Image below and Images 1-2 in Multimedia). The lesions are highly vascular and are typically enhancing after the intravenous administration of a gadolinium-based contrast agent.¹¹  Contrast enhancement is best visualized on T1-weighted fat-saturated images.

Reliably differentiating Morton neuroma from a normal plantar nerve is difficult unless the lesion is 5 mm or larger.¹² An intermetatarsal space lesion that is bright on T2-weighted images is unlikely to represent a Morton neuroma. Lesions with hyperintense signal on T2-weighted images are more likely to represent a true neuroma, a synovial or rheumatoid nodule (see Image below and Image 3 in Multimedia), or bursal fluid. The administration of contrast material assists in differentiating these entities because true neuromas are intensely enhancing, and bursal fluid is not enhancing.¹¹

Treatment

Conservative treatment

Conservative treatment consists of shoe modifications as well as the administration of various types of medication. First, the patient should be advised to wear appropriate shoes. These should be wide in the toe box, have adequate cushioning, and feature heels no higher than 0.5-1 in. A narrow toe box causes the metatarsals to impinge on the nerve, which causes a neuritic-type symptom. A neuroma pad can be placed just proximal to the affected metatarsal head (second and third metatarsal heads for a second interspace neuroma). When the patient steps down, the neuroma pad is designed to separate the metatarsal heads to prevent rubbing or irritating the affected neuroma. Neuroma pads are most successful when used with an appropriate shoe, as described.

Nonsteroidal anti-inflammatory drugs play a role in the conservative treatment of the neuroma. An oral anti-inflammatory drug can reduce the inflammation around the nerve and alleviate some of the symptoms. Cortisone injections delivered into the interspace at the level of the neuroma also can help in decreasing the amount of inflammation around the nerve. The use of no more than 4 injections is advised because they can decrease the fat pad and symptoms such as metatarsalgia may develop.

Shoe modifications, neuroma pads, and cortisone injections are typically used concurrently to eliminate pressure on the nerve and to reduce inflammation around the nerve.

Surgical treatment

Surgical excision of the neuroma is indicated when conservative care does not satisfactorily alleviate the patient's symptoms. Surgery is usually performed from a dorsal approach to prevent scar formation on the plantar aspect of the foot. An incision is made over the affected interspace by means of sharp and blunt dissection. Care is taken to avoid the neurovascular structures. The deep transverse metatarsal ligament is released. The neuroma is usually situated on the plantar aspect of the interspace and is resected as proximally as possible. Branches to the 2 affected digits are cut as distally as possible. The nerve is sent for pathologic analysis to confirm the diagnosis, and the area is closed with absorbable suture material in the deep structures and with nonabsorbable suture material in the skin.¹³

Summary

Neuromas are a common ailment seen by foot and ankle specialists. The diagnosis is often straightforward because of the description of a sharp, shooting pain or numbness in the affected digits. However, because the patients' descriptions of the pain can vary or because the pain can be related to another ailment in the differential diagnosis, MRI has become a useful imaging tool in obtaining a precise diagnosis. Ultrasonography also is commonly used. It is more widely available and less costly than MRI. Despite these advantages, the operator-intensive aspects of sonography, as well as its steep learning curve, are limiting factors. Whether ultrasonography will eventually replace MRI as the primary imaging modality for this condition remains to be determined.

Multimedia

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Media file 1: Morton neuroma. Longitudinal T1-weighted (left) and short-tau inversion recovery (right) magnetic resonance images reveal a low–signal-intensity mass between the third and fourth metatarsal heads. Note how the mass is inconspicuous on the latter image.

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Media file 2: Morton neuroma. Transverse T1-weighted (top) and contrast-enhanced fat-suppressed T1-weighted (bottom) MRIs show the bulbous morphology of the perineural mass with plantar extension. The administration of contrast material reveals enhancement of the lesion.

(Enlarge Image)

Media file 3: Rheumatoid nodules. T1-weighted (top) and short-tau inversion recovery (bottom) MRIs reveal large masses at the second and third interspaces. The latter image shows high signal intensity, which is not a feature of Morton neuromas. The patient had known rheumatoid arthritis, and the signal-intensity characteristics of these lesions are consistent with the proliferative synovium seen in this condition.

Keywords

Morton neuroma, Morton's neuroma, interdigital neuroma, Morton neuralgia, interdigital nerve, intermetatarsal space, medial plantar nerve, medial and lateral plantar nerve

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