Foot Ultrasound
In the foot, most of the structures are located superficially under the skin and are therefore easily amenable to ultrasound evaluation.
Arthritis
Ultrasound is a very good test to look for arthritis of the toes. Whereas X-rays only show the bony structures, ultrasound can also assess the soft tissues around the joints. It is very sensitive in detecting acute inflammation in suspected rheumatoid arthritis or other inflammatory joint disorders. In a patient with suspected rheumatoid arthritis, the amount of fluid in the joint, the thickness of the joint capsule, and the blood flow are assessed and graded [1].
Morton’s Neuroma
Morton’s neuroma is a very common cause of forefoot pain (metatarsalgia) and toe pain. A Morton’s neuroma is thickening and scarring of the tissues around the nerve in web spaces between the toes. This is a reaction to increased friction and compression of tissues in this narrow space. It occurs therefore more common in patients who do a lot of running. The spaces between the second and third, and between the third and fourth toes are commonly affected. A Morton’s neuroma can be confirmed on ultrasound as a soft tissue nodule, often associated with accumulation of fluid (i.e. bursa formation).
Intermetatarsal bursitis is an inflammation of the bursa between the toes and can mimic Morton’s neuroma. However, symptoms and ultrasound appearance and the treatment may be similar. The initial treatment consists of activity modification, changes in shoe wear and special insoles and toe spacers.
Ultrasound can be used to guide steroid injection directly into the Morton’s neuroma. Whilst this can often help with the symptoms, a small portion of patients ultimately undergo surgical removal [2].
Plantar plate injuries
The plantar plates are tough fibrous bands along the sole of the toe joints, that help to stabilise the joints. An important cause of dysfunction and pain around the toes and forefoot (metatarsalgia) is a plantar plate injury. Injuries can occur with acute trauma, or they gradually over time as part of degeneration. Once the plantar plate is torn, it can lead to deformities of the toe, such as claw or hammer toe deformity or sideways deviation.
During the ultrasound examination, the plantar plate can be assessed while the toes are moved. This provides a dynamic assessment and may visualise tears otherwise difficult to appreciate. Over time, scar tissue can form around a torn plantar plate, mimicking a Morton’s neuroma [3].
Plantar Fibromatosis
A plantar fibroma is a benign nodular growth along the plantar fascia at the sole. They are located along the inner side of the arch of the foot and cause discomfort and pain. Plantar fibromatosis is when there are multiple of those nodules [4].
On ultrasound, these nodules can be seen as focal thickening of the plantar fascia. They have characteristic features on imaging, and in most cases, a more sinister lesion can be ruled out on ultrasound. Ultrasound can also help to look for other causes of pain along the sole, such as plantar fasciitis.
References
[1] M. A. D’Agostino et al., “Scoring ultrasound synovitis in rheumatoid arthritis: a EULAR-OMERACT ultrasound taskforce – Part 1: definition and development of a standardised, consensus-based scoring system,” RMD Open, vol. 3, no. 1, Jul. 2017, doi: 10.1136/RMDOPEN-2016-000428.
[2] J. Y. Choi, H. il Lee, W. H. Hong, J. S. Suh, and J. W. Hur, “Corticosteroid Injection for Morton’s Interdigital Neuroma: A Systematic Review,” Clinics in Orthopedic Surgery, vol. 13, no. 2, p. 266, 2021, doi: 10.4055/CIOS20256.
[3] C. L. McCarthy and G. v. Thompson, “Ultrasound findings of plantar plate tears of the lesser metatarsophalangeal joints,” Skeletal Radiol, vol. 50, no. 8, pp. 1513–1525, Aug. 2021, doi: 10.1007/S00256-020-03708-1.
[4] P. Carroll, R. M. Henshaw, C. Garwood, K. Raspovic, and D. Kumar, “Plantar Fibromatosis: Pathophysiology, Surgical and Nonsurgical Therapies: An Evidence-Based Review,” Foot Ankle Spec, vol. 11, no. 2, pp. 168–176, Apr. 2018, doi: 10.1177/1938640017751184.