Intermetatarsal Bursitis, a condition commonly mistaken for Morton’s neuroma

Morton's Neuroma & intermetatarsal bursitis

---

If your 2020 new years’ resolution of, ‘taking up running’ has been thwarted by pain, numbness, and tingling under the forefoot during running, you might be forgiven for consulting Dr. Google and concluding that you have a Morton’s neuroma.

However, don’t throw away your running gear quite just yet, because Dr. Google, like many podiatrists, can’t easily distinguish between Morton’s neuroma and a similar condition called intermetatarsal bursitis; one that can be easily treated, and one that can’t be so easily treated with traditional methods. This inability to separate out bursitis from Morton’s neuroma is because both Morton’s neuroma and bursitis generate very similar sensations in the forefoot, and both can generate a positive ‘Mulder’s click’. 

---

 

Mulder's Click.

---

A ‘Mulder’s click’, regarded by many for decades, as the definitive test for the presence of Morton’s neuroma, can be felt by the examiner, and sometimes the patient too. The click occurs when the metatarsal bones are squeezed together whilst simultaneously applying an upward force with the thumb to the underside the forefoot, as seen below.  The clicking sensation is generated by the ‘subluxation’ or upward displacement of the enlarged nerve against surrounding structures.

For many years Morton’s neuroma were, and still are by many, diagnosed purely on the basis of a positive squeeze test. The problem with this way of diagnosing Morton’s neuroma is that the Mulder’s squeeze test also detects bursitis. This is because bursa will frequently click if squeezed. This ‘false positive’ has been known about now for a long time, and discussed in the medical literature since at least 1985 (Bosley and others 1985).

So, as you can see, an examiner can’t really tell the patient “you have a Morton’s neuroma because you have a positive Mulder’s click” at best, all they could say is, “your positive Mulder’s click means you may have bursitis, or a Morton’s neuroma, or even a combination of both”.

---

 

Running and Bursitis.

---

Returning now to your new year’s resolution and running. If you have only felt discomfort, numbness and tingling during running and at no other time, especially if you run a lot uphill, the most likely culprit is not Morton’s neuroma, but intermetatarsal bursitis.

Intermetatarsal bursa (singular) or bursae (plural) are sac-like structures that normally contain a small amount of ‘lubricating’ synovial fluid. Sometimes but not always, they are connected to nearby joints. Their function is to reduce friction between structures that might otherwise become inflamed (in this case the interdigital nerve), if you like, they are a kind of internal buffering mechanism. There are 160 bursae found all over the body (Lohr, KM. 2016).

These bursae can become problematic when inflamed, because they fill up dramatically with synovial fluid, the walls of the sac can thicken and harden and sometimes calcify, overall, a process called bursitis. Over time, the bursa press-on, and ultimately compress the nerves in the forefoot, causing pain and sometimes Morton’s neuroma. It is also worth remembering that bursae occur quite normally and harmlessly between 1st - 2nd metatarsals and the 4th - 5th metatarsal.

In those locations they are described ‘anatomical’ because they are a normal anatomic feature, a feature that we are all born with. Bursa can also develop over time in places where we weren’t born with them. These are known as ‘adventitious’. Adventitious bursa, develop at mechanical irritation ‘hot spots’ and are particularly prone to inflammation in the 2nd - 3th and 3rd - 4th interspaces, especially if someone is running in shoes too tight in the toe box, that squeeze the contents of the forefoot together, or if the soles of the shoes are too thin, causing irritation from tree roots and stones and pavement potholes from below. 

---

In runners, as the bursa become inflamed they press on the interdigital nerves causing a sensation of fullness in the forefoot, accompanied by numbness and tingling. Usually these sensations typically wear off shortly after the run stops.

For many runners, myself included, the symptoms from bursitis can often be addressed by changing running shoes. Simply changing from a shoe with a tapered toe box (seen below on the left, with my foot resting on a traditional Ascis shoe with a tapered toe box) to a shoe with a more ‘squared off’ toe box, will resolve the bursitis (seen on the right, with my own Altra paradigm 4.5s).