The syndesmosis is the strong group of ligaments that binds the two lower leg bones — the shin bone (tibia) and the outer bone (fibula) — together just above the ankle joint. It holds the “mortise” of the ankle perfectly lined up so that your body weight passes cleanly through the joint with every step.

An injury here is often called a “high ankle sprain”, because the pain and damage sit above the ankle rather than on the outside, where a common sprain occurs. Four ligaments make up the syndesmosis: the anterior inferior tibiofibular ligament (AITFL), the posterior inferior tibiofibular ligament (PITFL), the interosseous ligament (IOL) and the interosseous membrane (IOM).

Chronic syndesmotic instability (CSI) develops when these ligaments do not heal properly after injury, leaving the two leg bones loose. The ankle no longer holds its shape when you put weight through it. If left untreated, this instability can lead to ongoing pain, stiffness, the ankle giving way again and again and, over time, wear of the joint cartilage (post-traumatic ankle arthritis).

Because the early injury can be subtle and easily mistaken for an ordinary sprain, high ankle instability is often missed at the time of injury — which is exactly why some patients present months or years later with a “high ankle sprain that never got better.” Specialist assessment by a Consultant Foot & Ankle Surgeon — such as Mr Matthew Welck in London — gives you the widest range of treatment options.

Long-term instability is almost always the result of a high ankle injury that did not heal in a stable position. Things that contribute to this include:

• A missed or under-treated high ankle sprain — the most common cause; the first injury is mistaken for an ordinary sprain on the outside of the ankle.
• Broken ankles that also damage the syndesmosis — especially if it was not put back into the right position or held firmly enough, or if a piece of bone at the back of the ankle (the posterior malleolus) was broken.
• Twisting injuries with the foot turned outwards and the toes pulled up — the classic way this happens, common in football, rugby, skiing and other twisting sports.
• Healing that is not strong enough after treatment without surgery — when an unstable injury was treated as though it were stable.
• Damage to the deltoid ligament on the inner side of the ankle as well — together this makes the ankle bone (talus) more unstable and long-term problems more likely.

Some things make instability or poor healing more likely, including being very overweight, diabetes that has affected the nerves (neuropathy), and breaks in the bone at the back of the ankle. Spotting these early means a stronger surgical plan can be made.

No single test is perfect, so diagnosing long-term high ankle instability reliably usually needs a combination of examination and scans:

• Examination in clinic — checking for tenderness over the syndesmosis, plus tests that move or squeeze the leg bones (the fibular translation, squeeze and external rotation tests) to bring on the instability.
• Standing X-rays — the first scans we use, showing how well the ankle joint is lined up and how far apart the two leg bones sit. Comparing with your other ankle is often helpful.
• Weight-bearing CT scan (WBCT) — a modern, low-dose 3D scan taken while standing that picks up small shifts of the outer leg bone out of its groove much better than a plain X-ray. WBCT is a particular area of expertise for Mr Welck and is used routinely in his practice.
• MRI scan — very accurate at confirming which ligaments are torn and at picking up any damage to the cartilage, the inner (deltoid) ligament or the tendons.
• Arthroscopy (keyhole camera) — widely regarded as the most reliable test, letting the surgeon look at and test the syndesmosis directly and treat any other damage at the same time.

Together these confirm the instability, show how severe it is and pick up any other problems — the basis for deciding on treatment together.

Surgery is the main treatment once long-term instability is confirmed. The right operation is different for everyone: it depends on how much time has passed, which ligaments are involved, how healthy the remaining tissue is, how the ankle is lined up, how active you are and whether the joint is already showing signs of wear. Published success rates across the main operations are broadly good — more than 78% of patients do well when studies are combined.

For competitive athletes and people with physically demanding lives, a combination of the two types of fixation — sometimes with ligament reconstruction — may be recommended to protect the repair while letting you start moving again early. Every plan is discussed and agreed with you.