Morton neuromas are focal areas of symptomatic perineural fibrosis around a plantar digital nerve of the foot. The abnormality is non-neoplastic and does not represent a true neuroma. It may more correctly be known as Morton metatarsalgia. The condition is thought to be due to chronic entrapment of the nerve by the intermetatarsal ligament.
It most often occurs in middle-aged individuals and is many times more common in women than men. ~30% of asymptomatic middle-aged persons have the radiologic-pathologic findings of a Morton neuroma. 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. 10% of lesions are bilateral.
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 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.
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.
It is characterised by neural degeneration with epineural and endoneural vascular hyalinisation, and perineural fibrosis around an intermetatarsal nerve 2.
Typically seen as a round to ovoid, well-defined, hypoechoic lesion in the intermetatarsal space proximal to the metatarsal head 4. 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.
- T1: typically low-to-iso signal 1-2
- 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 anaesthetic has been demonstrated to have a relatively success rate 9.
Surgical excision can also be performed, also with a relatively high success rate (~ 80% 6).
History and etymology
It was initially described by Thomas Morton in 1876 4.
US differential considerations include:
- intermetatarsal bursa
- extruding out in between the metatarsal bones on the plantar aspect of the foot
MRI differential considerations include:
- changes secondary to a plantar plate tear/disruption 7-8
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- 9. Sofka CM, Adler RS, Ciavarra GA et-al. Ultrasound-guided interdigital neuroma injections: short-term clinical outcomes after a single percutaneous injection-preliminary results. HSS J. 2007;3 (1): 44-9. doi:10.1007/s11420-006-9029-9 - Free text at pubmed - Pubmed citation
- 10. Torriani M, Kattapuram SV. Technical innovation. Dynamic sonography of the forefoot: The sonographic Mulder sign. AJR Am J Roentgenol. 2003;180 (4): 1121-3. doi:10.2214/ajr.180.4.1801121 - Pubmed citation