Also known as Morton's metatarsalgia or Morton's neuroma, although Morton described neither (Morton thought this was a problem in the 4th MTP joint and Betts described the "neuroma" 70 years later).
The "neuroma" consists of degenerative and fibrotic changes in the common digital nerve near its bifurcation. However, there may be similar changes in adjacent unaffected nerves and it is not known why one becomes symptomatic. A number of causative factors have been suggested:
entrapment by the deep transverse metatarsal ligament
tethering of the 3rd space nerve by the anastomotic branch between medial and lateral plantar nerves
traction on the nerve by hindfoot valgus, interdigital bursitis or forced toe dosiflexion in high-heeled shoes

Wheeless' Textbook of Orthopaedics
Morton's Neuroma: Interdigital Perineural Fibrosis
it is not a neuroma but a perineural fibrosis and it was not first accurately described by Morton but by Durlacher, a chiropodist in 1845; both Thomas G. Morton (1876) and Thomas K. Morton (1892) mistook it for a painful affection of the fourth MTP articulation
- it is a type of nerve compression syndrome which involves the common digital nerves of the lesser toes: most often 3rd (80-85 %) and less often the 2nd (15-20 %) interspace;
- interdigital neuromas do not occur in the 1st and 4th web space: consider other etiologies;
- the occurance of two interdigital neuromas is also very rare;
- occurs most often in middle aged women (78 % are women);