What is the etiology of Morton's neuroma?

Etiology of Morton's Neuroma

Morton's neuroma is primarily a mechanically-induced degenerative neuropathy caused by repetitive trauma, compression, or irritation of the interdigital nerve, rather than a true neuroma. 1, 2

Pathophysiology

• Morton's neuroma represents perineural fibrosis and inflammatory changes of the interdigital nerve, not an actual nerve tumor despite its name 3
• The condition involves fibrous enlargement of the interdigital nerve branches, most commonly affecting the third common digital nerve where the lateral and medial plantar nerves often join 4
• The underlying pathological process includes:
  • Chronic repetitive trauma to the nerve 3
  • Mechanical compression of the nerve 1
  • Ischemic changes in the nerve 3
  • Entrapment of the nerve between metatarsal heads 3
  • Association with intermetatarsal bursitis 3

Risk Factors and Anatomical Considerations

• Most commonly affects middle-aged women who frequently wear tight, high-heeled shoes that don't accommodate normal foot physiology 5, 1
• Predominantly occurs in the third web space (between the third and fourth toes), followed by the second web space 2
• The anatomical location between metatarsal heads makes the nerve vulnerable to compression during weight-bearing activities 1
• Footwear that causes forefoot crowding contributes significantly to the development of symptoms 1

Clinical Manifestations

• Patients typically present with:
  • Forefoot pain while walking 1
  • Dull or sharp pain in the affected web space 4
  • Numbness and/or tingling in the affected toes 4
  • Burning sensation and cramping 4
  • Sensation of "walking on a stone" around the metatarsal heads 4

Diagnostic Considerations

• Clinical examination remains the cornerstone of diagnosis, with specific tests including:
  • Compression test of the affected web space 5
  • Mulder's click test 4
  • Foot squeeze test 4
• Imaging studies recommended by the American College of Radiology include:
  • Ultrasound (90% sensitivity) - allows for dynamic examination and direct clinical correlation 6
  • MRI (93% sensitivity) - useful for differential diagnosis 6
  • Radiographs are insensitive for diagnosing Morton's neuroma but help exclude other causes of webspace pain such as osteoarthritis, Freiberg's infraction, and stress fractures 7

Understanding the mechanical and degenerative nature of Morton's neuroma is essential for appropriate management, which may include conservative approaches (footwear modifications, orthotic insoles, corticosteroid injections) or surgical intervention when conservative measures fail 1.