Soft tissue

• Not fully understood
• Hypotheses, probably multifactorial
  • Repetitive microtraumas: mechanical stress of walking causes pinching of the interdigital nerve by the metatarsal heads and the metatarsophalangeal joints (Foot Ankle Int 2007;28:1007, Clin Radiol 2021;76:235.e15)
  • Intermittent neural ischemia: degenerative changes in the common plantar digital artery precede the nerve fibrosis (J Bone Joint Surg Br 1948;30B:84)
  • Intermetatarsal bursitis: causing compression of the nerve, secondary inflammation and subsequent neural fibrosis (J Bone Joint Surg Br 1951;33-B:94, J Bone Joint Surg Br 1980;62-B:184)
  • Entrapment: nerve entrapped between the deep transverse metatarsal ligament and the plantar aspect of the foot (Clin Orthop Relat Res 1979;142:90)
  • Unlike the other interdigital nerves, the third nerve has a double origin from the medial and lateral plantar nerves, producing a sling over the belly of the flexor digitorum brevis, thus tethering the nerve and rendering it vulnerable to traction upon dorsiflexion (Med J Aust 1940;1:514, Foot Ankle 1983;3:238)

Contributed by Rola H. Ali, M.D.

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• Fusiform swelling of the nerve particularly at its bifurcation with adherence to fibrofatty tissue (Foot Ankle Int 2004;25:79)

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Contributed by Rola H. Ali, M.D.

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• Traumatic neuroma:
  
  • M = F, history of trauma or surgery
  • Location: may occur anywhere
  • Histology: numerous small nerve twigs
• Localized neurofibroma:
  
  
  • M = F
  • Solitary lesions, mostly sporadic; multiple may be in NF1
  • Location: may occur anywhere, deep and superficial
  • Gross: when arising from a large nerve may look fusiform similar to Morton neuroma while cutaneous examples are not
  • Histology: mixed population of Schwann cells, fibroblasts, perineurial cells and occasional mast cells along with scattered axons and shredded collagen usually in a myxoid stroma
• Plexiform neurofibroma:
  
  
  • M = F; arises early in life
  • Strongly associated with NF1
  • Location: may occur anywhere
  • Gross: tortuous nerve with a bag of worms appearance
  • Histology: expanded nerve branches with cytological features of conventional neurofibroma
• Pacinian corpuscles and neuroma:
  • Corpuscles often encountered in tissue sections of Morton neuroma
  • Neuroma typically occurs on fingertips
  • Histology: hyperplasia of Pacinian corpuscles usually following trauma
• Clinically, the differential may include intermetatarsal bursitis, rheumatoid arthritis, stress fracture and metatarsal / soft tissue tumors (Radiol Case Rep 2023;18:2416, Cureus 2022;14:e25305, Acta Biomed 2019;90:214)

A patient has been experiencing bouts of burning pain and numbness localized to the plantar aspect of the left forefoot at the level of the metatarsal heads. The pain is exacerbated by walking and relieved by rest and removal of the shoe. Excision shows the histology above. What is the diagnosis?

1. Morton neuroma
2. Pacinian neuroma
3. Perineurioma
4. Plexiform neurofibroma
5. Traumatic neuroma

A. Morton neuroma. The clinical presentation is typical. The histology image shows degenerative neuropathy with marked fibrosis and loss of nerve architecture. In contrast, all of the other answers are proliferative lesions. Answer B is incorrect because although a Pacinian corpuscle is seen on the right, there is no corpuscle proliferation. Answer C is incorrect because perineurioma of soft tissue is characterized by a proliferation of slender spindle cells with whorling. Answer D is incorrect because plexiform neurofibroma is composed of nerve bundles that are tortuous and expanded (as opposed to shrunken and fibrotic) by spindle cell proliferation. Answer E is incorrect because traumatic neuroma shows a haphazard proliferation of numerous nerve twigs.

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Reference: Morton neuroma

A 45 year old woman has been experiencing bouts of pain in her left foot and has undergone a surgical procedure to remove the lesion. The histology of the lesion is shown above. Where did the lesion most likely arise from?

1. First common plantar digital nerve
2. Lateral plantar nerve
3. Medial plantar nerve
4. Proper plantar digital nerve
5. Third common plantar digital nerve

E. Third common plantar digital nerve. The picture shows a Morton neuroma, which classically affects the third common plantar digital nerve supplying the webspace between the third and fourth metatarsal heads. Answers A - D are incorrect because these nerves are not usually affected.

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Reference: Morton neuroma