Interdigital (Morton’s) Neuroma

Historical

  • 1876 coined Morton’s Neuroma by Morton
  • Betts 1940 (Adelaide) described lesion of the digital n in 1/19 consecutive dissections of the third and fourth interspace

Other data since that time includes the following four points

  1. Pain stems from the terminal part of the common plantar digital n near point of division into two proper digital n in 3/4 cleft normally but occasionally the 2/3 cleft
  2. Entrapment of the nerve
  3. Cardinal sign is acute tenderness on moderate pressure applied upwards and backwards in appropriate web space
  4. Histology of lesions reveals three constant features
    • Atrophic changes in nerve itself
    • Fibrous proliferation
    • Obliterative changes in the contiguous artery

Pathogenesis / Pathology

Entrapment of the nerve

Histology of lesions reveals three constant features

  • Atrophic changes in nerve itself
  • Fibrous proliferation
  • Obliterative changes in the contiguous artery

Pathogenesis

  • The nerve to 3/4 interspace formed from medial and lateral plantar nerves
  • Passes either side of the FDB
  • Contraction of FDB caused nerve to be stretched around the distal margin of the deep transverse ligament during toe extension
  • Does NOT explain lesion in the 2/3 cleft
  • Other theories include
    • Involvement of swollen interphalangeal component of the intermetatarsophalangeal bursa
    • During walking swollen bursa causes traction injury to the artery and the nerves

Fits with picture of entrapment neuropathy but exact aetiology unknown

Pathology

  • Degenerative change in plantar nerve
  • Just proximal to bifurcation
  • Near distal part of Transverse Intermetatarsal Ligament
  • Nerve develops fusiform swelling
  • Not true neuroma
  • Contains degenerating & regenerating neurones
  • Most likely due to ischaemia & pressure
  • Occurs 3rd webspace as confluence of rays

History

  • Female > Male – 5:1
  • Age 20-50
  • Bilateral in 15%
  • Neuralgic pain in the second, third and fourth toes
  • Most often the fourth
  • Referred pain can extend to the knee
  • The pain often not present in AM or with bare foot
  • Worse with shoe wear – particularly with high heels
  • May disturb sleep and can occur at rest

Examination

  • Cardinal sign is acute tenderness on moderate pressure applied upwards and backwards in appropriate web space
  • Pain with upwards compression in the affected web space
  • Mulders Click (1951)
    • Painful click on laterally squeezing foot
    • With plantar pressure on neuroma
    • Attempt to “trap” between metatarsals
    • Compressing metatarsal heads together
  • Sensory changes in the space unusual

Investigations

  • USS for Neuroma

Treatment

Non-operative

  • Appropriate shoes
  • Wide toe box
  • Pre-metatarsal dome
  • Steroid injection

Operative

  • Plantar or Dorsal incision
    • Dorsal incision
      • Primary procedure
      • Divide intermetatarsal ligament
      • Neuroma identified and resected proximal to metatarsal heads to allow retraction and prevent stump neuroma
      • The two distal digital nerves then divided
      • Advantages
        • lack of painful scar
      • Disadvantages
        • lack of exposure and inadequate resection
  • Plantar incision
    • Best for revision surgery
    • Better exposure
    • Protect digital vessels
  • Resect nerve sufficiently proximally to prevent development of stump neuroma
  • If nerve OK, leave alone & inspect other web-space
  • Worst complication is amputation neuroma
  • Differential diagnosis is early RA metatarsal bursitis

Prognosis

Outcome of Surgery

  • 80% of patients subjectively improved
  • Only 25% could wear shoes they desired
  • 75% limited with high heel shoes
  • Some patients found area of plantar numbness annoying

Complications

  • Neuroma formation at stump
  • Painful scar in plantar approach