Morton’s neuroma is an enlarged nerve that usually occurs in the third interspace, which is between the third and fourth toes (see image below). To understand this further, it may be helpful to look at the anatomy of the foot.
Problems often develop in this area because part of the lateral plantar nerve combines with part of the medial plantar nerve here. When the two nerves combine, they are typically larger in diameter than those going to the other toes. Also, the nerve lies in subcutaneous tissue, just above the fat pad of the foot, close to an artery and vein.
Above the nerve is a structure called the deep transverse metatarsal ligament. This ligament is very strong, holds the metatarsal bones together, and creates the ceiling of the nerve compartment. With each step, the ground pushes up on the enlarged nerve and the deep transverse metatarsal ligament pushes down. This causes compression in a confined space.
The reason the nerve enlarges has not been determined. Flatfeet can cause the nerve to be pulled toward the middle (medially) more than normal, which can cause irritation and possibly enlargement of the nerve. The syndrome is more common in women than men, possibly because women wear confining shoes more often. High heels cause more weight to be transferred to the front of the foot and tight toe boxes create lateral compression. As a result, there is more force being applied in the area and the nerve compartment is squeezed on all sides. Under such conditions, even a minimal enlargement in the nerve can elicit pain. (For more information, please see Flatfeet Risk Factors and Causes.)