Sinus Tarsi Syndrome: Understanding, Diagnosis, and Treatment
Sinus Tarsi Syndrome (STS) is a condition that affects the foot and ankle, primarily characterized by pain and a feeling of instability in the lateral hindfoot region. First described by Dr. O’Connor in 1958, STS is often associated with trauma but can also be linked to various other pathologies. While it is commonly seen in athletes, particularly in dancers and those involved in high-impact sports, it can also affect individuals with flatfoot deformities, obesity, or a history of ankle sprains. Despite its prevalence, the exact etiology of STS remains poorly understood, and the term itself is controversial. Some researchers argue that STS is not a distinct clinical entity but rather a general term used to describe pain in the tarsal sinus region with a variety of potential underlying causes.
Anatomy of the Tarsal Sinus
The tarsal sinus, also known as the sinus tarsi, is a cylindrical canal located in the hindfoot, bordered by the neck of the talus and the anterosuperior calcaneus. It is a critical region for foot stability, housing several important structures, including ligaments, arteries, adipose tissue, and nerve endings. These structures work together to stabilize the subtalar joint, preventing excessive inversion and eversion of the hindfoot. The primary stabilizing ligaments found in the tarsal sinus include the cervical ligament (CL), the interosseous talocalcaneal ligament (ITCL), and the inferior extensor retinaculum. The cervical ligament limits inversion, while the ITCL prevents excessive eversion.
The tarsal sinus is highly innervated, containing free nerve endings, Pacinian corpuscles, Ruffini endings, and Golgi-like bodies. These sensory structures are involved in nociception and proprioception, providing important feedback to the brain about joint position and movement. The presence of these nerve endings has led researchers to hypothesize that disorders of nociception and proprioception may contribute to the pain and instability associated with STS.
Pathophysiology and Etiology of STS
The etiology of STS is multifactorial and complex. Several different mechanisms have been proposed, but no single explanation has been universally accepted. Some early theories, such as those proposed by O’Connor, suggest that STS occurs as a result of ligamentous tension in the tarsal sinus following trauma. This theory posits that as ligaments heal after an injury, they become taut, leading to pain and instability. Other researchers, such as Brown, have suggested that the condition arises from soft tissue impingement, particularly from herniation of the synovial membrane into the sinus tarsi. This impingement could result in inflammation and pain in the region.
In addition to trauma, there are other potential contributing factors to the development of STS. One of the most commonly identified causes is ligament damage, particularly to the ITCL and CL, which play essential roles in stabilizing the subtalar joint. Studies have shown that inversion injuries, which are common in athletes, can lead to sprains or tears of these ligaments, contributing to the pain and instability seen in STS. Additionally, other conditions such as flatfoot deformities, osteoarthritis, synovitis, and even tumors like osteoid osteoma have been implicated in the development of STS.
An interesting finding in the literature is the suggestion that venous fibrosis in the sinus tarsi could lead to increased intrasinusal pressure, which in turn could cause pain. This vascular cause, proposed by researchers like Schwarzenbach et al., suggests that post-traumatic changes in the veins of the tarsal sinus could lead to fibrotic changes, impairing venous outflow and increasing pressure in the region.
Diagnosis of Sinus Tarsi Syndrome
Diagnosing STS can be challenging due to the nonspecific nature of the symptoms and the variety of underlying pathologies that can present with similar clinical features. The primary symptom of STS is pain located around the lateral opening of the tarsal sinus. This pain may be exacerbated by pressure applied to the sinus tarsi region, and patients often describe a sensation of instability, particularly when walking on uneven surfaces.
The diagnosis of STS is largely clinical, based on a thorough history and physical examination. A detailed medical history is crucial to identifying any prior trauma or injury that may have contributed to the condition. The physical examination typically involves palpating the sinus tarsi to assess for tenderness and performing maneuvers to reproduce the symptoms. One commonly used test is the dynamic impingement test, in which the hindfoot is everted while palpating the sinus tarsi. Pain relief following an injection of local anesthetic or corticosteroids into the sinus tarsi is considered a strong diagnostic indicator of STS.
Imaging studies are often used to complement the clinical examination. MRI is the most commonly used imaging modality for diagnosing STS, although it has limitations. MRI can reveal signs of synovitis, fibrosis, and ligament tears, but it often fails to provide a definitive diagnosis. In some cases, MRI findings may be nonspecific, and further imaging may be required. Arthrography, which involves injecting a contrast medium into the subtalar joint, can help identify abnormalities such as synovial hyperplasia, but it is not always conclusive. For more detailed visualization of the joint and its ligaments, subtalar arthroscopy is sometimes employed, which allows for both diagnosis and treatment in a single procedure.
Treatment of Sinus Tarsi Syndrome
The treatment of STS is typically conservative at first, with surgical options considered only if conservative measures fail. Conservative treatments may include rest, analgesics, physiotherapy, and the use of orthotics or bracing to offload the affected area. In many cases, corticosteroid or local anesthetic injections into the sinus tarsi provide significant pain relief and can help confirm the diagnosis.
If conservative treatment fails, more invasive procedures may be required. Subtalar arthroscopy has become a popular approach for both diagnosing and treating STS. This minimally invasive procedure allows surgeons to directly visualize the joint and any underlying pathology, such as ligament tears, synovitis, or arthrofibrosis. During arthroscopy, synovectomy (removal of inflamed tissue) and fat pad excision can be performed to alleviate symptoms. Studies have shown that subtalar arthroscopy provides excellent outcomes, with most patients experiencing significant pain relief and improvement in function.
In more severe cases or when arthroscopic treatment is not feasible, open surgical procedures may be necessary. Open sinus tarsi decompression involves removing the contents of the sinus tarsi, including the inflamed tissue and any scar tissue that may have formed. This procedure has shown good results in many patients, with some studies reporting symptom relief in up to 90% of cases.
Conclusion
Sinus Tarsi Syndrome is a complex and heterogeneous condition that involves pain and instability in the lateral hindfoot. While the etiology remains poorly understood, it is clear that the condition can result from a variety of pathologies, including traumatic ligament injury, soft tissue impingement, and inflammatory processes. The diagnosis is largely clinical, supported by imaging studies such as MRI and subtalar arthroscopy. Treatment typically begins with conservative measures, but surgical intervention may be necessary in refractory cases. Subtalar arthroscopy has become a key tool in both diagnosing and treating STS, offering good outcomes for many patients.
