What is the pathogenesis of interdigital nerve entrapment?

Updated: Oct 15, 2019
  • Author: Minoo Hadjari Hollis, MD; Chief Editor: Thomas M DeBerardino, MD  more...
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Answer

The most direct cause of interdigital nerve entrapment is compression of the nerve as it passes under the transverse intermetatarsal ligament. As weight is transferred to the ball of the foot when the toes are dorsiflexed during the last phase of stance, the interdigital nerve is compressed between the plantar foot and the distal edge of the intermetatarsal ligament. Many theories exist about the pathophysiology of this compression, including ischemia, inflammation, soft-tissue trauma, tumor, muscle imbalance, and fibrous ingrowth.

Edema of the endoneurium, fibrosis beneath the perineurium, axonal degeneration, and necrosis are often seen; their presence suggests that nerve damage occurs secondary to mechanical impingement. Using electron microscopy, Ha’Eri et al saw lesions consisting of a progressive fibrosis that enveloped and disrupted nerves and arteries; they did not see nerve-tissue proliferation or inflammation. [33] The authors concluded that repeated trauma leads to reactive overgrowth and scarring that disrupts the nerves and the arteries.

Typically, these changes are evident proximal to the bifurcation of the common digital nerve, immediately distal to the transverse intermetatarsal ligament. In more chronic cases, degeneration of the axons and proliferation of blood vessels may occur about the site of an enlarged nerve. Shereff and Grande described the presence of Renaut bodies, which are densely packed whorls of collagen, in the supraneural space. [34] These bodies are characteristically present only in peripheral neural entrapment.

However, Morscher et al, in a histomorphologic study, found only a diameter difference between biopsy results from patients with typical symptoms of interdigital neuritis and nerves examined from autopsies of persons without forefoot problems. [35] In addition, some have implicated the inter-MTP bursa as the main cause of interdigital neuritis.

Forefoot deformities (eg, hammertoe) can further aggravate the interdigital nerve because of dorsal subluxation of the proximal phalanx, which stretches the nerve over the intermetatarsal ligament. In addition, several external conditions are known to contribute to the development and aggravation of this compression. High-heeled shoes put the feet in chronic dorsiflexion, forcing one’s weight onto the forefoot; and tight shoes, which compress the foot further, limit the intermetatarsal space.

Multiple investigations have looked into the underlying reason for the common locations in the second and third webspace. Levitsky et al demonstrated that the relative space in the metatarsal head/transverse metatarsal ligament is smaller in the second and third webspaces, where interdigital neuritis is more common, than it is in the other webspaces, a finding that supports mechanical factors as the underlying pathophysiology. [36]

Levitsky et al also dismissed the common theory that the cause of interdigital neuritis is related to the occurrence of a lateral-medial plantar nerve combination and to the anatomic thickness of the nerve. [36] They reported third-webspace neuromas in which the plantar nerve did not arise from a combination of medial and lateral plantar nerves. The intermetatarsal head distances in the second and third interspace have been noted to be significantly smaller than those in the first and fourth intermetatarsal spaces.


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