What Is Sinus Tarsi Syndrome? Causes of Outer Ankle Pain

Sinus tarsi syndrome is a persistent pain felt on the outer side of the ankle, just in front of and below the bony bump of the ankle bone. It develops most often after an ankle sprain and accounts for roughly one in five cases of lingering lateral ankle pain after such injuries. The condition centers on a small, cone-shaped space between the ankle bone and the heel bone, and what makes it tricky is that it involves not just structural damage but disruption of the foot’s sensory wiring, which helps explain why the pain can feel disproportionate to what an MRI shows.

The Sinus Tarsi Itself

The sinus tarsi is a funnel-shaped opening on the outer side of the foot, sitting between the talus (the bone that connects the foot to the leg) and the calcaneus (the heel bone). Think of it as a small tunnel that widens as it reaches the outside of the ankle. Inside this space sit ligaments, a fat pad, blood vessels, and an unusually rich supply of nerve endings. An anatomical study identified two main ligaments within this space, the interosseous talocalcaneal ligament and the anterior capsular ligament, along with three components of a structure called the inferior extensor retinaculum that anchors over the area.¹ These ligaments are the main stabilizers of the subtalar joint, the joint that lets your foot roll inward and outward on uneven ground.

What sets the sinus tarsi apart from many other joint spaces is its density of nerve tissue. Researchers studying the area’s nerve supply found abundant free nerve endings alongside several types of specialized sensory receptors near ligament attachment points.² Some of these receptors detect pressure and joint position (helping your brain know where your foot is in space), while the free nerve endings are pain fibers. A cadaver study found that free nerve endings were by far the most common type, and that they were concentrated within individual fat cells in the sinus tarsi fat pad, suggesting the fat pad itself acts as both a proprioceptive sensor and a potential pain generator.³ This helps explain a hallmark of sinus tarsi syndrome: the feeling of ankle instability or “giving way” even when the ligaments themselves have healed. The injury disrupts not just structure but sensation.

How It Develops

The most common path to sinus tarsi syndrome starts with a lateral ankle sprain, the classic inversion injury where the foot rolls inward. A retrospective analysis of 391 patients with lateral ankle sprains found that the incidence of sinus tarsi syndrome was about 22.5%.⁴ That means roughly one in four or five people who badly roll an ankle go on to develop this specific pattern of pain.

The mechanism involves excessive or repetitive motion at the subtalar joint. When the ligaments inside the sinus tarsi are stretched or torn, the subtalar joint becomes less stable. That instability leads to ongoing irritation: the joint lining becomes inflamed (synovitis), and over time, scar-like fibrotic tissue infiltrates the sinus tarsi space.⁵ The fibrotic tissue and swelling crowd the nerve-rich fat pad and ligaments already described, producing pain and interfering with the proprioceptive signals that normally help you balance.

Anatomy itself can predispose someone to the condition. A recent radiographic study found that people with a steeper angle to their sinus tarsi were more likely to develop the syndrome, probably because the steeper shape changes how tension distributes across the ligaments and increases pressure inside the space.⁶ In other words, some people may be structurally more vulnerable before they ever roll an ankle.

Who Is at Risk

Anyone who sprains their ankle can develop sinus tarsi syndrome, but certain groups show up more often in the clinical literature. The condition has been reported in dancers, basketball and volleyball players, people with obesity, and those with flatfoot deformity.⁷ The common thread is repetitive stress on the subtalar joint: dancers and court-sport athletes land and pivot constantly, while flatfoot and excess body weight increase the rolling forces that strain the sinus tarsi ligaments with every step.

There is also an underappreciated structural risk factor. Tarsal coalition, a condition where two bones in the foot are abnormally connected by bone, cartilage, or fibrous tissue, significantly raises the odds. An MRI analysis found that the presence of a tarsal coalition increased the probability of developing sinus tarsi syndrome by about 3.7 times compared to feet without coalition.⁸ Coalition restricts normal subtalar motion, forcing surrounding tissues to absorb abnormal stresses, and the sinus tarsi bears much of that burden. This matters because tarsal coalitions are sometimes present from birth and discovered only when the resulting pain leads to imaging.

What It Feels Like

The signature symptom is pain focused over the outer ankle, specifically in the soft depression you can feel just in front of and below the lateral malleolus (the bony bump on the outside of your ankle). Pressing a finger into that hollow typically reproduces the pain. The discomfort tends to worsen with weight-bearing activities, walking on uneven surfaces, or any movement that forces the foot inward, like stepping off a curb at an angle.

Many people also report a persistent sense that the ankle is unstable or about to give way, which makes sense given the disruption to the nerve sensors described earlier. This subjective instability is a distinguishing feature: it can be present even when standard ligament-stability tests show the ankle is mechanically intact. Some patients describe stiffness in the morning or after sitting, with the outer ankle feeling “blocked” until they get moving. Swelling may be present but is often subtle, nothing like the dramatic puffiness of an acute sprain.

One reason the condition goes underdiagnosed is that the symptoms overlap with several other ankle problems. People who have been told they have chronic ankle instability, peroneal tendon issues, or vague “arthritis” sometimes turn out to have sinus tarsi syndrome once someone specifically examines for it.

Getting the Diagnosis Right

Diagnosis starts in the clinic. A physician or physiotherapist presses directly into the sinus tarsi to see if it reproduces the pain. Passive inversion of the foot (turning the sole inward) often flares the symptoms. A diagnostic injection of local anesthetic into the sinus tarsi is sometimes used: if the pain temporarily disappears, the diagnosis is strongly supported.

MRI is the most informative imaging tool. In a study of patients with confirmed sinus tarsi syndrome, MRI revealed tears of the interosseous talocalcaneal ligament and cervical ligament in the majority, along with occasional ganglia and other soft-tissue lesions.⁹ MRI can also show fluid within the sinus tarsi, scarring, and subtle bone-marrow changes that suggest chronic irritation. It is particularly useful for ruling out other causes, like fractures or tendon tears.

Ultrasound is a less expensive alternative that can be performed during a clinic visit. A study using a standard 12-MHz ultrasound probe showed that the structures within the sinus tarsi, including the ligaments and fat pad, could be adequately examined with ordinary equipment.¹⁰ Ultrasound also has the advantage of being dynamic: the clinician can watch the structures move in real time while manipulating the foot. It does, however, lack MRI’s ability to detect bone-marrow edema or deep ligament tears clearly, so a normal ultrasound does not fully rule out the syndrome.

Plain X-rays are generally unremarkable in sinus tarsi syndrome, which is part of why the condition gets missed. X-rays are still worth obtaining to rule out fractures, arthritis, or coalition, but they will not show the soft-tissue damage that defines the problem.

Conditions That Mimic Sinus Tarsi Syndrome

Several ankle conditions produce pain in roughly the same location, which is why a thorough workup matters. Peroneal tendinopathy involves the tendons that run just behind the outer ankle bone and can cause pain that radiates forward into the sinus tarsi area. Anterolateral ankle impingement, where scar tissue or a bony spur pinches tissue at the front-outer corner of the ankle joint, can produce nearly identical symptoms. Subtalar arthritis causes deep lateral ankle pain and stiffness that overlaps with sinus tarsi syndrome, but imaging usually reveals joint-space narrowing or bone spurs.

Tarsal coalition deserves special mention here because it is both a mimic and a cause. As noted earlier, people with coalition are nearly four times more likely to have sinus tarsi syndrome, so the two conditions frequently coexist.¹¹ Treating the sinus tarsi syndrome without addressing an underlying coalition can lead to incomplete relief.

Stress fractures of the talus or calcaneus are less common but important to exclude, especially in athletes or military recruits with a sudden increase in training load. MRI picks these up reliably. Nerve entrapment at the tarsal tunnel (on the inner ankle) can occasionally refer pain laterally, though this is uncommon. The key to sorting through the differential is a careful physical exam combined with MRI when the diagnosis is uncertain.

Non-Surgical Treatment

Most people with sinus tarsi syndrome improve without surgery. The first-line approach combines activity modification, targeted physical therapy, and sometimes an injection.

Physical therapy focuses on restoring subtalar joint stability and overall lower-extremity function.¹² In practical terms, that means balance training, strengthening of the muscles that control foot and ankle position (particularly the peroneals and intrinsic foot muscles), and exercises on unstable surfaces like wobble boards. Because the sinus tarsi’s nerve supply is so involved in proprioception, restoring that sensory feedback is just as important as building strength. Taping or bracing the ankle can reduce symptoms during the rehabilitation phase by limiting the excessive subtalar motion that provokes pain.

When pain persists despite rehab, injections into the sinus tarsi are a well-established next step. A trial comparing corticosteroid, platelet-rich plasma (PRP), and ozone injections in patients with sinus tarsi syndrome found that all three produced meaningful improvements in pain and function that lasted at least six months.¹³ Corticosteroid worked fastest, producing the largest improvement at one month, but by six months the groups were statistically comparable on pain scores and functional measures. This is useful to know because some clinicians and patients hesitate to use corticosteroid near tendons due to concerns about tissue weakening; PRP or ozone appear to offer similar longer-term relief without that worry.

A separate study specifically examining corticosteroid injections after lateral ankle sprain found them to be effective in the 88 patients who developed sinus tarsi syndrome from that population.¹⁴ The injection is typically guided by palpation or ultrasound and delivered directly into the sinus tarsi opening. One or two injections are common. Repeated steroid injections are generally avoided because of diminishing returns and potential local tissue effects.

Custom orthotics or insoles can help people whose foot structure contributes to the problem, particularly those with flatfoot. By supporting the arch and controlling excessive pronation, orthotics reduce the repetitive subtalar motion that keeps the sinus tarsi inflamed. They are a long-term strategy rather than a quick fix.

When Surgery Becomes an Option

Surgery is reserved for patients who have failed a reasonable course of conservative treatment, typically at least three to six months of physical therapy and one or more injections. The most common surgical approach is subtalar arthroscopy, a minimally invasive procedure in which a small camera and instruments are inserted into the subtalar joint through two or three tiny incisions. The surgeon can directly inspect the sinus tarsi, remove inflamed tissue and scar tissue, and address ligament tears or ganglia.

Outcomes from this procedure are generally encouraging. In a series of 33 patients who underwent subtalar arthroscopy for sinus tarsi syndrome, pain scores dropped from an average of about 7 out of 10 before surgery to roughly 3 out of 10 afterward. Functional scores nearly doubled, and about half of patients achieved an excellent result with the remainder rated good or fair.¹⁵ A separate series of 29 cases reported a mean return to full activity at four months and similar functional scores.¹⁶

For athletes, a study tracking long-term outcomes found that six out of ten patients returned to their pre-injury type of sport after subtalar arthroscopy, with favorable long-term scores on patient-reported outcome measures and acceptable safety profiles.¹⁷ The four out of ten who did not return to their exact pre-injury sport often returned to a modified or lower level of activity rather than stopping altogether. These numbers are realistic rather than spectacular, and they underscore that surgery improves things for the majority but is not a guaranteed return to 100%.

Open surgery, where a larger incision is made to directly access and clean out the sinus tarsi, is less common today but still performed when arthroscopy does not provide adequate visualization or when a larger reconstruction is needed. If underlying subtalar instability is severe, a ligament reconstruction may be combined with the debridement. Recovery from open procedures is typically longer, on the order of several months of protected weight-bearing followed by a graduated return to activity.

Why It Gets Missed

Sinus tarsi syndrome has a reputation as an underdiagnosed condition, and several factors contribute to that. The biggest is probably the shadow of the lateral ankle sprain. When someone sprains their ankle and the pain does not fully resolve, the default assumption is often “chronic ankle instability” or “it just hasn’t healed yet.” Clinicians may focus on the lateral ligaments (the anterior talofibular and calcaneofibular ligaments) while overlooking the deeper subtalar structures. Since X-rays look normal and even MRI findings can be subtle, the diagnosis sometimes does not occur to anyone until months of ineffective treatment have passed.

The lack of universally accepted diagnostic criteria is another barrier. A critical review noted that despite decades of case reports and small studies, no consensus exists on the specific clinical or radiographic criteria needed to diagnose sinus tarsi syndrome.¹⁸ Different studies define it differently, and without a standardized checklist, diagnosis relies heavily on the individual clinician’s awareness and experience. This is one area where the evidence genuinely lags behind clinical need.

The proprioceptive dimension compounds the problem. When patients describe their ankle as feeling unstable or “off” even though it tests stable on physical exam, their complaints can be dismissed or attributed to anxiety. Understanding that the sinus tarsi is packed with sensory nerve endings that can be disrupted by inflammation and scarring makes this complaint entirely plausible from a biological standpoint.¹⁹ The ankle may be structurally intact, but its internal GPS is offline.

Foot Structure and Long-Term Management

One of the more practical takeaways from the research is that sinus tarsi syndrome is not always a one-time injury that heals and stays healed. People with predisposing foot structures, whether flatfoot, a steep sinus tarsi angle, or an undiagnosed tarsal coalition, may be prone to recurrence unless the structural factors are addressed. For flatfoot, that might mean lifelong use of supportive footwear or orthotics. For tarsal coalition, treatment of the coalition itself (sometimes involving surgical resection of the bony or fibrous bar) may be necessary to prevent repeated episodes.

Athletes returning to sport after treatment should expect a period where proprioceptive retraining is just as important as regaining strength. Balance drills, single-leg exercises, and sport-specific agility work gradually rebuild the sensory map that the sinus tarsi damage disrupted. Ankle bracing during the transition back to full activity is reasonable, especially for sports that involve cutting, pivoting, or landing from jumps. The goal is not permanent dependence on a brace but bridging the gap while the neurological side of recovery catches up with the structural side.

For people with chronic or recurrent symptoms who have exhausted conservative measures, the arthroscopic evidence suggests surgery is a viable path with good odds of meaningful improvement, though outcomes vary. If you are in that position, a clinician experienced with subtalar pathology, whether an orthopedic foot-and-ankle surgeon or a podiatric surgeon, is likely to reach the diagnosis faster and offer the most targeted treatment options. The condition is well understood anatomically; the challenge is simply getting it on the diagnostic radar in the first place.