Subtalar instability

Abstract

Subtalar instability is a confusing yet important condition in patients with lateral ankle instability. The author will explore subtalar kinematics, and how they are closely related to the joint stability of the subtalar joint, both with respect to its intrinsic ligaments and its extrinsic ligaments. As subtalar instability is difficult to diagnose, this article will provide readers with a better understanding of its clinical presentation. Discussions will also include useful radiographic modalities and the most recent evidence regarding their accuracy. The last section discusses surgical options and what the readers need to know in order to make a decision.

Introduction

Subtalar instability occurs when there are chronic symptoms of hindfoot instability due to abnormally increased subtalar joint motion. ¹ It is a functional problem involving the talocalcaneal joint, characterized by a combination of anterior movement, medialization, and varus tilt of calcaneus. ² This is a distinct entity from lateral ankle instability (tibiotalar instability). However, the problem is commonly associated with lateral ankle instability. Combined ligamentous injuries of lateral ankle and subtalar joint happen in about 25% of cases. ³ Therefore, all patients with ankle instability should be evaluated for subtalar instability.

However, subtalar joint motion is complex and is difficult to assess both clinically and radiologically. A study by Pearce and Buckley ⁴ compared differences in subtalar motion measured externally with measurement by computed tomography (CT). They found subtalar motion measured externally overestimated CT measurement by three-fold. This finding is explained by soft tissue motion and ankle joint motion that were also captured externally.

Due to difficulty in the assessment of the subtalar joint, subtalar instability can be missed out in the initial assessment and therefore, present as a cause of failure after surgery addressing just lateral ankle ligament instability. If the surgery only addressed the anterior talofibular ligament, patients may still present with recurrent pain and sensation of instability at the subtalar joint. Thus, revision surgery to address the subtalar ligaments is helpful to resolve symptoms.

Anatomy

It is important to understand the bony anatomy of the subtalar joint and how it contributes to joint kinematics and ligamentous stability. Subtalar joint consists of 3 facets, namely posterior, middle, and anterior facets on the superior calcaneum and inferior talus. The posterior facet is separated from the middle and anterior facets by the tarsal canal.

The subtalar joint together with the talocalcaneonavicular joint (acetabulum pedis) contribute to not just inversion and eversion movement, but also plantarflexion and dorsiflexion of the hindfoot. ⁵ Movements of this nature are created by the rotation of the calcaneum underneath the talus. The calcaneum rotates from dorsolateral to medioplantar during the motion from inversion to eversion. ⁶ The stability of this complex 3D motion is provided by several subtalar ligaments.

Subtalar ligaments are grouped into intrinsic and extrinsic ligaments. Extrinsic ligaments consist of calcaneofibular ligament (CFL) and lateral talocalcaneal ligament (LTCL) shown in Figure 1. Intrinsic ligaments are located within the tarsal canal and sinus tarsi. Intrinsic ligaments are arranged from medial to lateral-interosseous talocalcaneal ligament (ITCL), anterior capsular ligament (ACL), and cervical ligament (CL) shown in Figure 2. There are also insertions of the inferior extensor retinaculum (IER) in the tarsal canal and sinus tarsi. Due to their attachments elsewhere in the ankle, these structures are not strictly intrinsic ligaments despite contributing to subtalar instability.

Figure 1.

Subtalar extrinsic ligaments.

Figure 2.

Subtalar intrinsic ligaments.

The importance of each ligament to subtalar stability remains controversial. Pellegrini et al. ⁷ reported a cadaveric study with sequential sectioning of CFL, CL, and ITCL, where they found a significant increase in motion only after sectioning of CFL. In another recent study by Michels et al. ⁸ who investigated the stiffness of different ligaments, it was found that CL and CFL are biomechanically more compliant (lower stiffness) compared to ITCL and ACL complex, and therefore contribute more to subtalar instability. These studies were conducted on cadaveric specimens, so dynamic muscle contractions were not taken into account.

Clinical diagnosis

The symptoms of subtalar instability are similar to those of lateral ankle instability. Patients often present with inversion injury, with lateral ankle swelling and tenderness. In the acute setting, it is often difficult to differentiate subtalar instability from lateral ankle instability. After the resolution of swelling and pain, there are several subtle differences in symptoms and signs that one can elicit between the two entities.

Patients with subtalar instability present with pain more over sinus tarsi and lateral hindfoot. Physical examination often reveals increased inversion of the hindfoot. A good clinical test should be able to isolate the subtalar joint and reproduce the direction of instability. Anterolateral drawer test had been described for subtalar instability. ⁹ The test first involves locking the tibiotalar joint by dorsiflexion (Figure 3), followed by inverting the foot with the tibiotalar joint in a neutral position (Figure 4). A positive test will reveal increased varus tilt and medial shift of calcaneum underneath the talus. One must keep in mind this test is very operator-dependent and is only reliable in the hands of experienced examiners.

Figure 3.

Locking the tibiotalar joint by dorsiflexion.

Figure 4.

Inverting the foot with tibiotalar joint in neutral position.

It is also crucial to check for other important features such as hindfoot deformity particularly varus deformity, the quality of ATFL ligament by anterior drawer test, and the range of motion of the subtalar and ankle joints. These factors are important especially when considering surgical interventions and the need for other concomitant procedures in the same setting.

Radiological diagnosis

Radiological investigations are crucial in the evaluation of subtalar instability and can be categorized into plain radiographs, stress radiographs, and cross-sectional imaging. Plain radiographs may pick up bony avulsion of lateral ligaments, but these findings are often not critical in the diagnosis of subtalar instability.

Stress radiograph with Broden’s view is commonly described. ¹⁰ Manual stress with varus force is applied (Figure 5). Broden’s view is then taken focusing on the sinus tarsi region. Positive findings include medial displacement of the calcaneum and subtalar joint tilting (Figure 6). However, the validity of stress radiograph in the diagnosis of subtalar instability remains questionable. Michels et al. in a systematic review in 2020 found conflicting results and concluded that there was insufficient evidence for the use of stress radiographs. One of the main drawbacks of this technique was difficulty evaluating 3D movement with plain radiographs and difficulty defining abnormality in angulation and translation.

Figure 5.

Stress Broden’s view.

Figure 6.

Subtalar joint tilting.

Subtalar instability had been evaluated using stress ultrasound. ¹¹ Fibula-trochlear angle between the fibula and trochlea peronealis (peroneal tubercle) was measured. A ratio between the fibula-trochlear angle measured in a neutral position and a stress position was calculated. A ratio of more than 1.6 correlated with an unstable subtalar joint. This technique, however, is highly operator-dependent, and the results have not been replicated in more recent studies.

MRI is useful in the evaluation of different subtalar ligaments. Anterior capsular ligament (ACL) (Figure 7) condition had been found to be related to subtalar instability in a recent study. ¹² It was found that thin or narrow ACL, absent or complete tear of ACL (Figure 8) suggest the presence of subtalar instability. MRI is also useful to evaluate for concomitant ATFL injury and other conditions that may mimic subtalar instability. These conditions include osteochondral lesions of the subtalar joint, the presence of loose bodies, and calcaneo-navicular coalition. If there is a suspicion of intraarticular pathology, subtalar arthroscopy is useful in both the diagnosis and treatment of the problem.

Figure 7.

Intact anterior capsular ligament (blue arrow).

Figure 8.

Absent anterior capsular ligament (green arrow).

There are several problems with the use of MRI. Firstly, high-resolution MRI and special sequences are required to evaluate subtalar ligaments. ¹³ There is also a problem of lack of awareness amongst radiologists on reporting of subtalar ligaments. These factors make it difficult to use MRI as the gold standard in subtalar joint evaluation.

Weight-bearing computed tomography (WBCT) is a promising and useful entity. It is both a 3D and dynamic investigation with the ease of different loading directions. Burssens et al. ¹⁴ investigated WBCT in a cadaveric study and found displacement at the subtalar joint (Figure 9) in specimens with sectioned subtalar ligaments, compared to that of intact ligaments. However, WBCT is still not widely available and clinical studies with recruitment of patients with subtalar instability are needed. This can be challenging when there is no gold standard for the diagnosis of subtalar instability.

Figure 9.

Direction of motion in subtalar instability with weight-bearing CT scan (Eversion, plantar and posterior displacement of talus).

Due to the complexity of reaching a diagnosis for instability involving the subtalar joint, five diagnostic criteria were suggested. ¹² Diagnosis is made if four out of five criteria are met, namely recurrent ankle sprain, sinus tarsi pain and tenderness, hindfoot looseness or giving way, hindfoot instability on physical examination, and stress radiographs. However, hindfoot looseness and hindfoot instability are not well defined, and the criteria are not validated. Consequently, subtalar instability must be diagnosed with caution using these criteria.

Treatments

Surgical treatments should be considered if the patient fails a trial of nonoperative management. Surgical treatments should include concomitant procedures such as osteotomy for deformity correction and subtalar/ankle arthroscopy for articular lesions. It is also crucial to address lateral ankle instability in the same setting with the repair or reconstruction of ATFL.

Surgery addressing subtalar ligaments is broadly divided into surgery addressing extrinsic ligaments and surgery addressing intrinsic ligaments. Surgery for extrinsic ligaments is commonly performed and technically less demanding compared to surgery for intrinsic ligaments. There is no evidence that one group is superior to the other. The levels of evidence for studies investigating these surgeries are mostly level 4 and level 5. ³ Additionally, there is no clear indication for reconstructing intrinsic ligaments. Michels et al. ¹⁵ described in their paper an intraoperative test to confirm subtalar instability and an indication of cervical ligament reconstruction. The subtalar joint was assessed to be unstable if a small spatula (1.5 mm thickness) could be inserted and rotated 60° in the joint. Cervical ligament reconstruction was indicated if there was an absent or abnormal cervical ligament under direct examination.

The restoration of CFL is important and well-described in several studies. Traditionally several open techniques were described to address CFL ligament. ¹⁶ Techniques included shortening and refixation of CFL, periosteum augmentation, extensor retinaculum augmentation, and peroneus tendon augmentation. ¹⁷ Several authors had also described arthroscopic techniques to address the 2 ligaments. ¹⁸ A recent arthroscopic technique was described to reconstruct ATFL, LTCL, and CFL using the gracilis tendon. ¹⁹ The technique is useful to provide further stability to the subtalar joint and can salvage damage to LTCL during arthroscopic surgery. In view of the attachment of IER in the tarsal canal and IER contribution to subtalar stability, Gould modification to the Bostrom procedure had also been shown to provide stability after CFL, ITCL, and CL were sectioned. ²⁰

There are few recent articles describing techniques for intrinsic ligament reconstruction. Jung et al. in 2012 ²¹ described a technique to reconstruct anterior and posterior limbs of subtalar ligaments, with the anterior limb aimed to replace ITCL and CL, while the posterior limb aimed to reconstruct CFL using an allograft. In their subsequent study ²² reporting the outcome of the technique, they reported overall improvement of the functional score and no complications arising from the surgical technique. A percutaneous technique to reconstruct ITCL was recently described in a cadaveric study. ²³ Using fluoroscopy guidance, the technique was found to accurately restore the footprints of ITCL and avoided injury to important anatomic structures.

Traditionally there were several non-anatomical reconstruction procedures described. Techniques such as Chrisman-Snook and Elmslie procedures utilized part of the peroneus tendon to reconstruct subtalar ligaments. These techniques could reduce subtalar motion, but they do not restore the anatomical bony origins of ligaments and therefore unable to recreate the kinematics of the subtalar joint. As a result, there is a concern about the future risk of osteoarthritis and subtalar joint stiffness. ²⁴

Summary

It is important to identify the potential presence of subtalar instability in all patients with lateral ankle instability. Subtalar joint kinematics is closely associated with joint stability from both extrinsic and intrinsic ligaments. Understanding these c aspects help us in the diagnosis and treatment of this condition. Clinical diagnosis of subtalar instability is not straightforward, and the use of an anterolateral drawer test in an experienced hand will help in localizing the problem. MRI and weight-bearing CT scans are promising in further evaluation while stress views are likely not reliable. Surgery addressing extrinsic ligaments is technically less demanding, and there is still no well-established evidence supporting the role of surgery addressing intrinsic ligaments.