A plantar fibroma is a benign (non-cancerous) nodule that grows in the arch of the foot and usually appears between ages 20 and 60. It usually is slow-growing and often less than one inch in size. Some can grow faster and are considered plantar fibromatosis. A plantar fibroma or fibromatosis is a disease of the fibrous tissue that grows between the skin and the underlying fascia.
The main symptom is a mass on the bottom of the foot, roughly in the middle of the arch or instep, between the heel pad and the forefoot (front of the foot) pad. The mass will cause a soft curve in the contour of the bottom of the foot that may be painful with pressure or shoe wear.
The cause is unknown but thought to have a genetic component. Plantar fibromas are commonly seen in people of white, Anglo-Saxon, Protestant backgrounds. Trauma to the foot does not seem to be a factor. Alcohol consumption may be a factor.
Plantar fibromas reside in the deep fascia of the foot between the skin and the first layers of muscle. The more aggressive condition of plantar fibromatosis may involve the skin and the muscle layers and may also wrap around the local digital nerves and arteries.
Your foot and ankle orthopedic surgeon will conduct a physical exam. If a plantar fibroma exists, they will find a mass in the plantar fascia in the arch. The mass has no swelling, increased warmth, or redness. It is firm and does not move. There are no skin changes. Advanced imaging is usually not needed but MRI will show the fibroma in the plantar fascia layer.
There are a few conditions that can cause soft-tissue masses in the foot, including cysts, swollen tendons or tendon ruptures, nerve tumors (neurilemomas), or fat tumors. Foreign body reactions from previous penetrating trauma also can cause a mass in the bottom of the foot, as can an infection. A more serious synovial cell sarcoma, a malignancy, usually will show calcification on X-ray and a more worrisome appearance on MRI. Clinical exam, X-ray, and sometimes an MRI may be needed for diagnosis. Biopsy usually is not needed.