Morton’s neuroma typically occurs in females and is a non-neoplastic perineural fibrosis around the plantar digital nerve, in the 3rd intermetarsal space which appears as a hypoechoic mass that may be displaced during elicitation of the Mulder’s sign.
Description
Morton neuroma is a benign compressive neuropathy involving fibrotic enlargement of the interdigital nerve of the foot, most commonly the nerve between the third and fourth metatarsal heads. It results in a painful mass in the forefoot, often associated with footwear pressure.
Morton neuroma is a misnomer as it does not represent a true neuroma. Other terms used in the literature include Morton metatarsalgia, interdigital neuritis, Morton entrapment, interdigital neuralgia, interdigital nerve compression syndrome.
Pathogenesis
The exact pathogenesis is not entirely clear, but it is believed to involve repetitive trauma to the ball of the foot, leading to perineural fibrosis and nerve degeneration. Neuropathy may be due to compression and constant irritation at the plantar aspect of the transverse intermetatarsal ligament.
Epidemiology, Risk Factors & Associations
- Marked female predominance M:F 18:1
- Association with wearing high-heeled or narrow-toed shoes may account for gender predilection.
- Activities that involve repetitive trauma to the ball of the foot (e.g., running).
Clinical Features
- Metarsalgia characterised as localised sharp, burning pain in the forefoot, aggravated by walking or palpation.
- Pain often radiates to the affected toes.
- May experience a sensation of walking on a marble or pebble.
Complications
- Chronic pain leading to alterations in gait and posture.
- Development of metatarsalgia or other foot deformities due to altered gait.
Pathological Features
Histopathology
- Macroscopic: Fusiform enlargement of nerve in the intermetatarsal space, near the bifurcation of the plantar interdigital nerve with thickening of adjacent tenosynovial tissues
- Microscopic: Perineural fibrosis and nerve degeneration. The common plantar digital artery displays disruption of the arterial wall, thrombosis, and incomplete recanalisation.
Radiological Features
General Features
- Tear-drop or dumbbell- shaped soft tissue mass between the metatarsal heads projecting inferiorly into the plantar subcutaneous fat and located plantar to the intermetatarsal ligament
- 3rd intermetarsal space (between 3rd and 4th metatarsal heads) is the most common location, followed by the 2nd intermetarsal space. It rarely occurs in the 4th intermetatarsal space.
- Vast majority are unifocal and unilateral. Multifocal Morton neuromas are usually unilateral.
- Most measure more than the normal interdigital nerve diameter.
MRI
- T1: Isointense to the surrounding tissue.
- T2: Low signal intensity lesion in the intermetatarsal space.
- T1 Gad+: May show contrast enhancement.
- Short-axis non-fat-suppressed MR sequences most helpful
- MRI in prone position may assist with visualisation
US
- B-mode: Shows a non-compressible well-defined hypoechoic nodule in the intermetatarsal space.
- Dynamic: During dynamic ultrasound evaluation, compression of the intermetatarsal space to elicit the clinical Mulder’s sign may demonstrate displacement of the hypoechoic mass between the adjacent metatarsal heads towards the plantar direction. A clicking or snapping phenomenon may also be observed.
- Colour Doppler: May show increased blood flow around the lesion.
Grading and Staging
No formal grading or staging system; assessment is based on size and symptom severity.
Diagnosis
- Based on clinical symptoms and physical examination.
- Ultrasound and MRI are the preferred imaging modalities for confirmation.
Differential Diagnosis
- Metatarsalgia: Diffuse forefoot pain without a discrete mass.
- Intermetatarsal bursitis: Inflammation of the bursa which demonstrates compressibility (vs. non-compressible neuroma).
- Stress fractures: Localised pain with a history of trauma and positive findings on X-ray or MRI.
Management
- Conservative treatment includes footwear modifications, orthotics, and corticosteroid injections.
- Alcohol sclerosing injections via ultrasound guidance as a minimally invasive option1. Alcohol ablation induces chemical neurolysis by dehydration and necrosis.
- Surgical excision in cases resistant to conservative management.
References