What causes neuromas?
The exact cause of neuroma is controversial. There are many theories that are accepted. While doctors do not know exactly which one is correct, it is likely that it is a combination of many of them. Below are the most commonly accepted theories.
What is a Morton’s neuroma?
A Morton’s neuroma is an enlargement of the common metatarsal nerve. The common metatarsal nerve is a nerve that runs between the metatarsal bones and divides into two smaller nerves that go into two adjacent toes.
The nerve’s function is to allow a person to have sensation on the inside of one toe, and also have sensation on the inside of the opposite side of the adjacent toe. They are sensory nerves only and have no motor function.
Which nerves are most likely to be affected by Morton’s neuroma?
The most common area to have a Morton’s neuroma is between the third and fourth digits of the foot. The reason for this is that the normal anatomy at the third and fourth toes is such that two nerves come together to form one common metatarsal nerve, and therefore is larger than the adjacent single common metatarsal nerves. With this said, one can have a Morton’s neuroma between the other toes as well. The second most common place to have a Morton’s neuroma is between the second and third toes.
We have seen neuromas in all four of the interspaces of the foot. Usually, neuromas in the first and fourth interspaces are not painful. We have seen these when performing surgery on other anatomy of the foot as an incidental finding. The patient did not complain of pain where we observed the neuromas to be present in the first or fourth interspaces.
Morton’s neuroma anatomy
Going forward in this description, I will describe a Morton’s neuroma in the third intermetatarsal space, or between the third and fourth toes. Adjacent to the common metatarsal nerve on the medial and lateral aspects are the metatarsal bones. Above, or dorsal to the nerve anatomically, is the deep transverse intermetatarsal ligament. This is a ligament that runs from one metatarsal to the adjacent metatarsal bone, connecting them together, and stabilizing the forefoot. And finally, under the nerve, or inferior to it, is the thick plantar skin structures.
Due to the shape of the third and fourth metatarsals, the heads of the bones are closer together than the necks. This allows for less space for the nerve to pass between them. In addition, when the nerve is enlarged it takes up more space, and therefore is more likely to be irritated by the bones. It is an unfortunate cycle such that when the nerve gets larger, it gets more irritated, which then makes it get larger, and so on.
After the common metatarsal nerve splits into two separate nerves, these nerves go to the inside one quarter of the third and fourth toes on the plantar (bottom) aspect. These nerves allow us to feel sensation in these areas only.
Forces on the metatarsal nerve that cause irritation
There are multiple reasons why there may be increased forces on the metatarsal nerve that cause it to become inflamed.
Flat feet, pronation
A person who pronates, or has an arch that flattens out, will also put pressure on the nerve. When the arch collapses, the foot does not simply go straight down. It is a 3-dimensional motion such that there is external rotation and tilting of the joints at the back of the foot and ankle. This causes the outside of the foot to press against one’s shoes. That can cause the metatarsal bones to irritate the nerve as well.
In our practice, we find a high correlation between patients who pronate, or have flattened feet, with the presence of Morton’s neuroma.
High impact activity
Another theory for increased incidence of Morton’s neuroma involves increased activities. Individuals who participate in high impact activities such as running, tennis, and skiing often have neuroma symptoms as well. Activities that cause one to make frequent turns and have lateral forces on the foot within the sneaker, cleat, or boot can increase the prevalence of Morton’s neuroma.
Tight shoe gear
The above described four boundaries create a tunnel where the common metatarsal nerve passes. One of the theories for the cause of Morton’s neuroma is the adjacent structures put pressure on the nerve and cause irritation. This causes the nerve to get enlarged and painful. This pressure is made worse when one wears tight shoes. In theory, this can cause a lateral force to push the two metatarsal bones closer together and into the nerve.
In our practice, we see a high correlation with women who wear tight shoes or high heeled shoes with Morton’s neuroma.
Traumatic neuromas
Another theory is that trauma can cause Morton’s neuroma. This can be due to a fall from a height, something falling on the foot, or an abnormal twisting motion. It can also be due to repetitive trauma such as marathon training.