Morton neuromas, also known as interdigital or intermetatarsal neuromas, are focal areas of symptomatic perineural fibrosis around a plantar digital nerve of the foot. The condition is thought to be due to chronic entrapment of the nerve by the intermetatarsal ligament.
The term neuroma is a misnomer because the abnormality is non-neoplastic and does not represent a true neuroma. It may more correctly be known as Morton metatarsalgia.
It most often occurs in middle-aged individuals and is many times more common in women than men. Approximately 30% of asymptomatic middle-aged persons have the radiologic-pathologic findings of a Morton neuroma.
Patients typically present with forefoot pain which radiates from midfoot to toes 5. Symptoms are often progressive and worsened by activity. The Mulder sign is a physical sign associated with Morton neuroma, which may be elicited while the patient is in a supine position. The pain associated with the neuroma, as well as a click, can be reproduced by squeezing the two metatarsal heads together with one hand, while concomitantly putting pressure on the interdigital space with the other hand.
It is characterized by neural degeneration with epineural and endoneural vascular hyalinisation, and perineural fibrosis around a plantar digital nerve 2.
The 3rd web-space (between 3rd and 4th metatarsal heads) is the most commonly affected site. The 2nd web-space is less often involved while the remaining web-spaces are rarely involved. 10% of lesions are bilateral.
Symptomatic lesions tend to be slightly larger (mean 5.3 mm vs. 4.1 mm in one large series 1). Lesions >5 mm are very likely to be symptomatic.
Typically seen as a round to ovoid, well-defined, hypoechoic lesion in the intermetatarsal space proximal to the metatarsal head 4. A Morton neuroma is not compressible. A small proportion can have mixed echotexture 5. A sonographic Mulder sign may be elicited with the probe 10.
Dumbbell/ovoid-shaped lesion at a similar position to that described on ultrasound 1-2:
- T1: typically low-to-iso signal
- T2: typically low signal but can sometimes be intermediate in signal
- T1 C+ (Gd): tends to show intense enhancement
Treatment and prognosis
Ultrasound-guided interdigital injection of steroid and local anesthetic has been demonstrated to have a relatively high success rate 9.
Surgical excision can also be performed, also with a relatively high success rate (~80% 6).
History and etymology
It is named after Thomas George Morton (1835-1903), an American surgeon, who described a case series in 1876 4,11. However, it was first described by Civinini in 1835 12.
Other causes of metatarsalgia:
- intermetatarsal bursa/bursitis
- extruding out in between the metatarsal bones on the plantar aspect of the foot
- metatarsophalangeal joint synovitis/capsulitis
- changes secondary to a plantar plate tear/disruption 7-8
- soft tissues tumors, e.g. schwannoma 8