Tarsal tunnel syndrome develops when the posterior tibial nerve, or one of its branches, becomes compressed as it passes through the tarsal tunnel on the inside of the ankle. Similar to carpal tunnel syndrome in the wrist, the problem is not the nerve itself but the pressure placed upon it. Anything that reduces the available space within the tunnel or increases tension on the nerve may lead to irritation and pain.
The tarsal tunnel is a relatively narrow anatomical passage. It is formed by the bones of the ankle and covered by a strong fibrous band known as the flexor retinaculum. Within this confined space travel the posterior tibial nerve, arteries, veins and several tendons. Because there is very little spare room, even minor increases in pressure may produce nerve compression.
One of the most common contributing factors is abnormal foot biomechanics. Excessive pronation or flat feet may increase tension around the tarsal tunnel, stretching the nerve during walking and placing greater pressure on the surrounding structures. Not everyone with flat feet develops tarsal tunnel syndrome, but altered foot mechanics may increase susceptibility when combined with other contributing factors.
Previous injury around the ankle is another recognised cause. Fractures, severe ankle sprains and repetitive trauma may result in scar tissue, chronic swelling or altered joint mechanics that reduce the available space within the tunnel. Symptoms sometimes develop months or even years after the original injury as scar tissue gradually matures.
Space-occupying lesions may also compress the nerve. Ganglion cysts, lipomas, enlarged veins (varicosities), tendon sheath swelling and other soft tissue masses can all reduce the space available within the tarsal tunnel. Although these conditions are relatively uncommon, they are important because they may require different treatment from idiopathic nerve compression.
Inflammatory conditions may contribute as well. Rheumatoid arthritis, gout, inflammatory tenosynovitis and other inflammatory disorders can produce swelling around the tendons and soft tissues within the tunnel. As swelling increases, pressure on the nerve also increases, producing neurological symptoms.
Diabetes deserves particular consideration. While diabetic peripheral neuropathy is a separate condition, people with diabetes may also develop true tarsal tunnel syndrome. In these patients, it is important to distinguish between generalised nerve disease and focal nerve compression because the management differs.
Occupations and activities involving prolonged standing or walking may aggravate symptoms by increasing pressure within the tarsal tunnel. Long-distance runners, healthcare workers, retail staff and others who spend many hours on their feet often report worsening symptoms towards the end of the day. Repetitive loading does not necessarily cause the condition on its own, but it may expose an underlying tendency towards nerve compression.
Pregnancy can occasionally contribute because fluid retention increases swelling around many joints and soft tissues, including the ankle. Symptoms often improve after pregnancy as swelling resolves, although persistent cases should be investigated further.
In many patients, no single cause is identified. Instead, several relatively minor factors combine to produce sufficient pressure on the nerve. Mild flat feet, a previous ankle injury and prolonged standing at work may each contribute a small amount until symptoms eventually develop.
It is also important to remember that tarsal tunnel syndrome frequently coexists with other causes of heel pain. Patients may simultaneously have plantar fasciosis, Baxter's nerve entrapment or heel fat pad syndrome. Treating only one condition while overlooking another may explain why some patients fail to improve despite otherwise appropriate management.
Identifying the underlying cause is one of the most important parts of treatment. Simply confirming that the nerve is compressed is not enough. The clinician must determine why the compression has developed so that management can address the source of the problem rather than merely reducing symptoms.