One of my peers came to me the other day at the hospital and asked me about a pain she was having in the back of the ankle. She stated that she had had plain films taken that were unremarkable. She also had an MRI that failed to show any bone edema, arthritis or stress fracture. She had the site injected with cortisone with no change in symptoms. She asked, “With no signs on testing, what’s causing the pain in the back of my ankle?”
Her question has really stumped me. First I had to run through the differential diagnosis in my head that may be present with posterior ankle pain prior to the MRI. Possibilities included:
- Posterior facet arthritis of the subtalar joint
- Fracture of the posterior process of the talus (Steida’s process)
- Tendonitis of flexor hallucis tendon or flexor digitorum tendon
- Stress fracture of the posterior talus or calcaneus
- Retrocalcaneal bursitis
And then she had her MRI which would have identified inflammatory presence in all of the above conditions. So what was left? One of two things –
- Focal nerve pain
- Referred nerve pain
Why nerve pain? First, neuropathy (nerve pain) doesn’t show up on an MRI. Secondly, she described her pain as transient but consistent. She said that it didn’t hurt when she first stood but increased in pain after a period of time on her feet.
The innervation of the ankle is described in the literature as all nerves passing the ankle. These nerves include the tibial, sural, deep peroneal and saphenous. But which of these nerves supplies the posterior ankle? Although this isn't clearly defined in the literature, the tibial is likely the primary source of innervations with the sural as secondary.
What other nerve entrapments do we see associated with the tibial nerve? First and foremost is tarsal tunnel syndrome. The symptoms of tarsal tunnel syndrome include a vice like sensation in the midfoot and forefoot. And then there’s Baxter’s nerve entrapment. Baxter’s nerve entrapment is specific to the first branch of the tibial nerve that supplies sensation to the plantar heel. Each of the conditions describe problems distal to the trifurcation of the tibial nerve in the tarsal canal. But what about deep to the trifurcation of the tibial nerve? Is there another branch that supplies Kajer’s triangle and the posterior ankle? And could this ‘fourth branch’ be the source of posterior ankle pain?
Usually in my blog posts I try to shed light on a condition that has a definitive cause and effect relationship. But in this case, I can’t say that I can explain this problem.
Stayed tuned, I’ll let you know how it progresses.