Metatarsalgia/Morton’s neuroma/Intermetatarsal Neuropathy
Morton’s Neuroma – Intermetatarsal Neuropathy
There is a painful condition of the forefoot that feels like an electrical shock.
This feeling of walking with a stone under the ball of your foot will quickly limit your activity level. There could be many reasons that pain to the ball of the foot could happen. Your doctor will need to perform a thorough examination and only your doctor can diagnose your condition. This section explains the mechanical reason for nerve disease between the metatarsal heads. Let’s explore why this is such a painful issue and why historic treatments don’t make a lot of sense.
What is a Morton’s Neuroma?
There is a nerve located between the heads of the 3rd and 4th metatarsal bones that develops a nerve callus. There is a swelling of the nerve that gets bigger and bigger. This condition is referred to as a Morton’s neuroma after Dr. Thomas George Morton who published an article in 1876 describing this condition. It was previously published by Filippo Civinini in 1835 but news back in those days did not spread as far and wide as it does now.
This is a specific condition that is found between the heads of the metatarsal bones of the foot.
The most common location is the 4th interspace between the 3rd and 4th toes but they are commonly found in other intermetatarsal spaces as well. The nerve within this space develops swelling of specialized nerve tissue. Neuroma means “nerve tumor” but that is somewhat of a mischaracterization of what is actually happening. It is more of a nerve callus, a thickening of tissue as a result of mechanical trauma. Just like when you get a callus on your toe. It occurs due to excessive, repeated micro-trauma. After decades of millions and millions of steps, this nerve callus forms as a protective mechanism. Just like that callus on the toe, this nerve callus will eventually reach a point when it gets very painful. The big difference between a callus on your skin and one on the nerve inside your foot is that the callus on your toe can get reduced with a pumice stone. The poor nerve within your foot does not have that luxury. Meanwhile, every step is a reminder that the nerve callus is there.
When a nerve is subjected to repeated microtrauma, it will eventually react as a form of self-protection like that callus on the bottom of a foot is trying to protect the skin.
This is only a temporary solution because, just like that callus, it will continue to grow since the underlying cause is still there causing more micro-trauma. Eventually, the callus gets so big that it is no longer part of the solution, it becomes part of the problem. Just like a callus, it can get thick enough that it causes a blister or even worse an ulcer.
The micro-trauma on the nerve happens during the walking cycle. There is a specific point when the heel lifts off the ground which causes compression of the nerve. Damage occurs when walking; no walking – no damage. Nerves can heal from a reasonable amount of strain/compression. Eventually, that micro-trauma takes its toll and a point is reached when the nerve is no longer able to repair during the night when you are sleeping and off of your feet.
Once the nerve reaches a certain point in the disease process it will give you a signal – PAIN! That is a warning signal that something is wrong. Both pain and numbness can be experienced. Nerve disease is called neuropathy. Once this has occurred attention needs to be directed immediately because failure to eliminate the trauma will result in permanent nerve damage.
How do we know this is the case? When surgeons remove the nerve callus from someone’s foot, they send it for analysis. The typical findings are:
- Fibrosis around and within the nerve
- Schwann cell and fibroblast proliferation
- Loss of myelinated nerve fibers
What that means is there is scar tissue surrounding and inside the nerve.
Scar tissue develops after trauma. The Schwann cell and fibroblast proliferation are signs that the nerve is trying to repair itself from the trauma but at the same time there is also more scar tissue thickening. Finally, the loss of myelin means that the important nerve fibers are no longer connected. It’s like the wiring to the light switch has worn away – that means loss of nerve function.
Why do people get nerve-calluses?
An online search for the cause of nerve callus formation is pretty generic. They claim it’s from wearing tight shoes, running, high arched feet, and low arched feet. There is an underlying condition that is seen in the majority of people with nerve calluses which has to do with ankle bone instability. A stable ankle bone on the heel bone means a stable foot structure. A partially dislocating ankle bone on the heel bone leads to excessive forefoot motion. Let’s take a closer look. The bones of the foot are divided into 2 columns, the inner/medial and outer/lateral. The inner column of bones is allowed to have a slight amount of motion. The outer column of bones should have little to no motion. That slight amount of motion between the inner and outer columns allows the foot to handle an uneven ground surface. Ankle bone instability, the partial dislocation of the ankle bone in the heel bones leads to excessive inner column motion. So, what does all of this have to do with forming a nerve callus? Everything. Where do we see the most common location of the nerve callus? Between the 3rd and 4th metatarsal bone. This is between the inner and outer columns of foot bones.