What Is a Syndesmosis? Anatomy and High Ankle Sprains

A syndesmosis is a type of joint where two bones are held together by a tough web of ligaments and fibrous tissue rather than by the smooth cartilage lining you find in most joints. The one that gets the most attention sits just above the ankle, where the two lower leg bones, the tibia (shinbone) and fibula (the thinner bone on the outside), meet. This distal tibiofibular syndesmosis creates a snug socket called the mortise that cradles the talus, the bone at the top of your foot, and keeps your ankle stable every time you push off, land, or pivot. When this ligament complex tears, the result is what sports medicine professionals call a high ankle sprain, an injury that heals slower and causes more trouble than the far more common lateral ankle sprain.

The Ligaments That Hold It Together

The syndesmosis is not a single band you can point to on a diagram. It is a complex of several distinct ligaments working together to bind the tibia and fibula while still allowing the small amount of give the ankle needs during movement.¹ The main players are the anterior inferior tibiofibular ligament (AITFL), which runs along the front of the joint; the posterior inferior tibiofibular ligament (PITFL), which mirrors it along the back; the interosseous tibiofibular ligament (ITFL), a thick wedge of tissue sitting between the two bones; and the interosseous membrane, a sheet of connective tissue that extends up the leg between the tibia and fibula.

Detailed anatomic studies of cadaver specimens show that the ITFL is the broadest of the group, shaped like a small pyramid and ending roughly 9 mm above the joint surface. The AITFL inserts on the fibula about 30 mm above the tip of the outer ankle bone, while the superficial fibers of the PITFL insert about 26 mm above that same landmark.² Small cartilage patches on both the tibial and fibular sides also articulate directly with each other in this space, which is one reason even slight widening of the joint after an injury can lead to abnormal wear over time.

What the Syndesmosis Actually Does

Your ankle is not a simple hinge. Every step involves the fibula rotating, translating slightly outward, and shifting downward relative to the tibia. The syndesmosis permits these micro-movements while keeping the mortise tight enough that the talus does not wobble. A stable, precise fit here is essential for normal ankle motion.³

Researchers have measured these movements in living subjects using weight-bearing imaging. When you stand on one leg and then add extra load, the syndesmosis widens its range of rotation and translation. In one study, adding weight significantly increased both the extension angle and the internal-external rotation range at the syndesmosis, confirming that the joint is not static but responds dynamically to loading.⁴ This matters clinically because it explains why an injury that loosens the syndesmosis by even a millimeter or two changes how forces travel through the ankle, setting the stage for cartilage damage and chronic instability.

How High Ankle Sprains Happen

A regular lateral ankle sprain happens when you roll the foot inward and stretch or tear the ligaments on the outside of the ankle. A high ankle sprain involves a different motion entirely. The classic mechanism is external rotation of the foot, often combined with the foot being forced upward toward the shin. Both forces widen the mortise, pulling the tibia and fibula apart and tearing the syndesmotic ligaments.⁵

The picture is actually more specific than “the foot rotates outward.” A cadaver study showed that the position of the foot at the moment of external rotation determines which structure fails. When the foot was in a neutral position and rotated outward, the deltoid ligament on the inner ankle was the structure that gave way. But when the foot was everted (tilted outward) by 20 degrees before the rotation was applied, the AITFL tore instead, producing an isolated high ankle sprain.⁶ This helps explain why high ankle sprains are so common in sports where cleats anchor the foot to the ground while the body keeps rotating, and why simply “rolling the ankle” usually injures different ligaments.

These injuries can occur as isolated ligament tears or alongside ankle fractures. A fracture that involves the fibula, for instance, can disrupt the syndesmosis at the same time, which is why surgeons assess syndesmotic stability whenever they fix a broken ankle.

Who Gets High Ankle Sprains

High ankle sprains account for a smaller share of all ankle sprains than lateral sprains, but they are far from rare in certain settings. Among NCAA athletes across all sports, the overall rate is about 1 per 10,000 athletic exposures. Football, wrestling, and ice hockey top the list, with football showing the highest rate at roughly 2.4 per 10,000 exposures.⁷

In the NFL, the numbers are climbing. A study tracking high ankle sprains from 2009 through the 2019-2020 season found a steady increase, from about 1.75 per 10,000 player exposures to 2.49. Offensive players accounted for about 57% of cases.⁸ Whether the rising incidence reflects better diagnosis, changes in playing surfaces, or shifts in how the game is played is still debated.

From a hospital standpoint, high ankle sprains are also more serious than their lateral counterparts when someone does end up in the emergency department. Hospitalizations after high ankle sprains occur at a far higher rate than after lateral sprains, about 6% versus less than 1%.⁹

Why These Injuries Are Hard to Diagnose on the Sideline

One of the frustrating things about high ankle sprains is that no single hands-on test nails the diagnosis reliably. Several clinical tests exist, and each has trade-offs. The squeeze test, where a clinician compresses the tibia and fibula together higher up the leg to reproduce pain at the ankle, is highly specific (around 93% when compared to MRI) but only catches about a third of actual syndesmotic injuries.¹⁰ The external rotation test, where the foot is turned outward with the knee bent, reaches about 75% sensitivity when performed with the ankle pulled upward, but it lacks specificity, especially if there is a simultaneous lateral sprain muddying the picture. The fibula translation test shows similar sensitivity with better specificity (around 88%), though the movement involved is so subtle that clinicians are told to focus on whether the test provokes pain rather than whether they can feel abnormal motion.

A separate study comparing multiple clinical tests to MRI found that all of them had low sensitivity for syndesmotic injuries, ranging from about 14% to 56%. The most accurate predictor was something simpler: whether the patient had pain at rest.¹¹ That result hints at how easy it is to miss a high ankle sprain on initial evaluation, particularly in a competitive athlete who may minimize symptoms. This is a real clinical problem, because delayed diagnosis means delayed treatment and a longer road to recovery.

Imaging the Syndesmosis

Standard X-rays remain the first step. On anteroposterior and mortise views of the ankle, clinicians measure the clear space between the tibia and fibula, the amount of bony overlap, and the gap between the talus and the inner ankle bone. Widening of these measurements suggests the syndesmosis has been disrupted. But X-rays catch the obvious cases. Subtle instability, where the ligaments are partially torn but the bones have not shifted far, often looks normal on plain films.

Weight-bearing CT has emerged as a more sensitive tool for these borderline cases. Three-dimensional volume measurements of the space between the tibia and fibula up to 5 cm above the ankle joint surface showed about 96% sensitivity and 83% specificity for detecting syndesmotic instability, outperforming traditional two-dimensional measurements.¹² One practical advantage of weight-bearing CT is that the uninjured ankle can serve as a built-in comparison, since normal syndesmotic anatomy varies considerably between individuals. Comparing the injured side to the healthy side gives a personalized reference point that population-based cutoff values on an X-ray cannot match.¹³

Ultrasound is also gaining ground, particularly as a bedside or intraoperative option. In cadaver testing, portable dynamic ultrasound detected significant widening of the syndesmosis after partial ligament injury at a level of external rotation torque where standard fluoroscopy showed nothing abnormal.¹⁴ In clinical use, dynamic ultrasound has shown sensitivity around 86% and specificity of 97%, comparable to MRI for detecting syndesmotic pathology while being cheaper and faster.¹⁵ Intraoperative ultrasound stress testing is also being used during fracture surgery to decide in real time whether the syndesmosis needs to be fixed.¹⁶

Treatment Without Surgery

Most high ankle sprains, particularly those that are stable on imaging (meaning the bones have not shifted apart), are treated without surgery. The standard approach involves a period of immobilization in a walking boot or cast, restricted weight-bearing, and a progressive rehabilitation program that moves from pain management and gentle range-of-motion exercises to strengthening and sport-specific drills.¹⁷ For stable injuries in professional football players, the timeline from injury to competition is typically two to six weeks.¹⁸

A lingering question has been how these athletes fare years later. A long-term follow-up study, tracking patients for 18 years or more after nonoperative treatment of high ankle sprains without widening on imaging, found that functional outcomes were acceptable and rates of subsequent ankle injuries were low. A high proportion did show arthritis on imaging at follow-up, but most of those cases were not causing meaningful symptoms.¹⁹ That is reassuring but not a blank check: the key qualifier is “without diastasis on imaging.” A stable syndesmosis treated conservatively tends to do well over time; an unstable one that goes unfixed is a different story.

When Surgery Is Needed

Surgery enters the picture when imaging shows that the tibia and fibula have separated, or when stress testing demonstrates instability that will not hold up during healing. The two main fixation methods are syndesmotic screws (metal screws placed across both bones to clamp them together) and suture buttons (a flexible cord anchored by small metal buttons on each bone).

A meta-analysis of randomized controlled trials found that suture button fixation had significantly fewer complications and fewer unplanned repeat operations compared to screws, along with modestly better functional scores.²⁰ However, a large umbrella review that looked across many such meta-analyses added an important caveat: while the statistical advantages for suture buttons kept appearing, the actual size of the functional difference was generally too small to be clinically meaningful for most patients. Most of the score differences fell below the threshold where a patient would actually feel better in daily life.²¹

One practical advantage of suture buttons is that they do not usually need to be removed, while screws sometimes break or need a second surgery for removal. A randomized trial found that removing syndesmotic screws routinely offered no functional advantage over leaving them in and only removing them if they caused problems. Given the complication rate of screw-removal surgery itself, on-demand removal (taking the screw out only if it bothers the patient) is now favored as standard practice.²²

Long-Term Complications to Watch For

Beyond the arthritis noted in long-term follow-up of conservatively treated injuries, a distinct complication can develop: heterotopic ossification, where bone forms within the interosseous membrane between the tibia and fibula. In extreme cases, this progresses to a full synostosis, a bony bridge fusing the two bones together and restricting the normal micro-movement the syndesmosis exists to allow. Case reports in professional athletes have shown that this restriction of fibular motion can cause persistent ankle problems, though some athletes with the finding on imaging are not disabled by it.²³ When the ossification does cause symptoms, surgical removal is usually needed.²⁴

Teenagers Are a Special Case

In adults, the same external rotation force that tears the syndesmotic ligaments produces a high ankle sprain. In teenagers whose growth plates have not yet fully closed, that same force can instead fracture the bone at the growth plate, pulling off a fragment of the tibia. This fracture pattern, called a Tillaux fracture, occurs because the partially closed growth plate is weaker than the ligaments it sits next to. The growth plate in this area closes from the center outward, so there is a window of vulnerability in adolescence when the outer portion is still open and can be avulsed.²⁵

The catch is that Tillaux fractures can easily be mistaken for simple ankle sprains on initial evaluation, since the teenager presents with ankle pain and swelling after a twisting injury, and the fracture fragment can be subtle on standard X-rays. Missed Tillaux fractures have been reported to cause persistent pain and nonunion.²⁶ CT scanning can clarify the true displacement of the fragment, which is important because fractures displaced more than 2 mm generally need surgical fixation.

There is also an in-between population of older teenagers whose growth plates are in the process of closing. A Tillaux fracture in this group might seem like it should protect the syndesmosis, since the fracture theoretically absorbs the energy that would otherwise tear the ligaments. But clinical experience shows this is not always true. One published case of a 15-year-old with a Tillaux fracture demonstrated persistent syndesmotic instability on intraoperative stress testing even after the fracture itself was fixed, requiring separate suture button fixation of the syndesmosis.²⁷ The lesson is that ankle stability should be assessed in adolescents with these injuries regardless of the fracture pattern.

An Evolutionary Footnote

The ankle syndesmosis is not just an injury-prone nuisance. It reflects an evolutionary compromise. The tighter and more constrained the mortise, the more stable the ankle becomes for upright walking and running on flat surfaces, which is exactly what modern humans need. But earlier hominins and some fossil relatives appear to have had a more mobile fibula and a syndesmosis that permitted a greater range of ankle motion. Analysis of fibular fragments from Homo floresiensis, a small-bodied hominin from Indonesia, found features adapted for obligate bipedalism alongside traits shared with great apes and earlier hominins that suggest a more versatile ankle joint with enhanced load-bearing capabilities of the fibula.²⁸ Our relatively tight, stiff syndesmosis is well suited for pavement and playing fields, but it comes with the trade-off of being vulnerable to the kinds of rotational forces that sports regularly impose on it.

Bracing and Taping for Prevention

Given how long high ankle sprains sideline athletes, prevention gets a lot of attention. External ankle support, whether in the form of athletic tape or a lace-up brace, is widely used. A prospective trial comparing prophylactic bracing to taping in high school athletes found no difference in sprain rates between the two approaches over an entire season. The practical difference was cost and time: taping took about 67 seconds per ankle per session and added up to more expense over a season than a reusable brace.²⁹ Both methods restrict ankle motion to some degree, which may help limit the extreme positions that tear the syndesmosis. Neither is a guarantee, but for athletes with a history of ankle injuries or those in high-risk sports, some form of external support is a reasonable and low-risk measure.