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.
A number of other clinical tests for Morton neuroma have been described, all of which have a high specificity and aim to either reproduce symptoms of pain or paraesthesia. These include 15:
- thumb-index finger squeeze: squeeze the symptomatic intermetatarsal space between the index (dorsal) and thumb (plantar) surfaces
- foot squeeze test: compression of metatarsal heads between fingers and thumb
- plantar and dorsal percussion tests: percussion of the dorsal and plantar intermetatarsal spaces with a finger
- light touch sensory test: stroking the tip of the affected toe produces sensation different to the adjacent toes
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.
Occasionally specific terms are used when occurring certain spaces 13
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 1,2,14:
- T1: typically low-to-iso signal
- T2: typically low signal but can sometimes be intermediate in signal
- T1 C+ (Gd): variable 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:
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- 2. Zanetti M, Strehle JK, Kundert HP et-al. Morton neuroma: effect of MR imaging findings on diagnostic thinking and therapeutic decisions. Radiology. 1999;213 (2): 583-8. Radiology (full text) - Pubmed citation
- 3. Delfaut EM, Demondion X, Bieganski A et-al. Imaging of foot and ankle nerve entrapment syndromes: from well-demonstrated to unfamiliar sites. Radiographics. 23 (3): 613-23. doi:10.1148/rg.233025053 - Pubmed citation
- 4. Murphey MD, Smith WS, Smith SE et-al. From the archives of the AFIP. Imaging of musculoskeletal neurogenic tumors: radiologic-pathologic correlation. Radiographics. 19 (5): 1253-80. Radiographics (full text) - Pubmed citation
- 5. Quinn TJ, Jacobson JA, Craig JG et-al. Sonography of Morton's neuromas. AJR Am J Roentgenol. 2000;174 (6): 1723-8. AJR Am J Roentgenol (full text) - Pubmed citation
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- 7. Yao L, Cracchiolo A, Farahani K et-al. Magnetic resonance imaging of plantar plate rupture. Foot Ankle Int. 1996;17 (1): 33-6. - Pubmed citation
- 8. Ashman CJ, Klecker RJ, Yu JS. Forefoot pain involving the metatarsal region: differential diagnosis with MR imaging. Radiographics. 21 (6): 1425-40. Radiographics (full text) - Pubmed citation
- 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
- 11. A Peculiar and Painful Affection of the Fourth Metatarso-Phalangeal Articulation. (1876) The American Journal of the Medical Sciences. 71 (141): 37. doi:10.1097/00000441-187601000-00002
- 12. Honorio Benzon, Srinivasa N. Raja, Scott M Fishman, Spencer S Liu, Steven P Cohen. Essentials of Pain Medicine E-Book. (2017) ISBN: 9780323445412
- 13. Walter R. Frontera, Julie K. Silver. Essentials of Physical Medicine and Rehabilitation. (2018) ISBN: 9780323549479
- 14. Barbara N. Weissman. Imaging of Arthritis and Metabolic Bone Disease. (2019) ISBN: 9780323041775
- 15. Mahadevan D, Venkatesan M, Bhatt R, Bhatia M. Diagnostic Accuracy of Clinical Tests for Morton's Neuroma Compared With Ultrasonography. (2015) The Journal of foot and ankle surgery : official publication of the American College of Foot and Ankle Surgeons. 54 (4): 549-53. doi:10.1053/j.jfas.2014.09.021 - Pubmed