Subtalar instability refers to an unstable talocalcaneal or subtalar joint and is difficult to diagnose.
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Epidemiology
Subtalar instability occurs in patients with lateral ankle instability in up to 10-25% 1.
Associations
Clinical conditions associated with subtalar instability include the following 1-3:
Clinical presentation
Patients usually present with symptoms similar to a lateral ankle sprain or ankle instability. They might frequently report a history of previous inversion injury as well as a recurrent feeling of instability and ‘giving way’. Rearfoot pain, stiffness and swelling worse during sports activity or walking on uneven ground are other complaints 2,3.
Physical exam might reveal ecchymosis, swelling and tenderness in the area of the sinus tarsi. Medial translation of the calcaneus on the talus can be assessed with the subtalar glide test.
However, clinical differentiation of isolated subtalar instability from its combination with tibiotalar instability remains difficult 2.
Complications
If left untreated subtalar instability can lead to the following conditions 2:
Pathology
Subtalar instability usually involves varus tilting of the subtalar joint together with anterior and medial subluxation of the calcaneus in relation to the talus 3.
Etiology
Subtalar can arise from injury of the following ligamentous structures 2,3:
Concomitant extensor retinaculum or anterior capsular ligament injury may add to instability.
Radiographic features
Subtalar instability can be assessed on stress radiographs by measuring the subtalar tilt angle 2.
Plain radiograph
Anteroposterior, lateral and mortise views of the ankle are a first-line imaging modality and should be performed in a standing position. In addition, a long axial or hindfoot alignment view should be performed to assess for hindfoot varus or valgus 2.
Stress radiographs can be considered for direct assessment of instability but might be rather limited for the chronic condition due to aggravation of pain in acute injury 2:
- separation of the posterior talar and calcaneal facet >7 mm indicate chronic subtalar instability
- anterior displacement of the calcaneus vs. the talus was shown after anterior heel traction 4
Ultrasound
Ultrasound might be helpful in the assessment of the lateral ankle ligaments. In the case of subtalar instability, it might detect calcaneofibular ligament injury depicted as an anechoic defect or undulating, irregular fibers morphology 3.
However, in the evaluation of the intrinsic stabilizers of the subtalar ligaments, the role of ultrasound is limited 3.
CT
CT might be helpful in the assessment of hindfoot alignment and in an acute situation to rule out fractures 2.
MRI
MRI can be performed for the assessment of the sinus tarsi and its ligaments including the intrinsic and extrinsic stabilizers of the subtalar joint such as the interosseous talocalcaneal ligament, the cervical ligament and the extensor retinaculum as well as the calcaneofibular ligament and tibiocalcaneal part of the deltoid ligament and the detection of injury to these structures 2.
Radiology report
The radiological report should include a description of the following:
- posterior facet separation
- malalignment
MRI
The MRI report should include a description of the following:
- abnormal signal of the sinus tarsi indicating sinus tarsi syndrome
- ligamentous injuries including calcaneofibular ligament and/or deltoid ligament injury
Treatment and prognosis
Similar to chronic ankle instability an optimized conservative non-operative therapy is essential and the mainstay of the management and includes physical therapy and functional rehabilitation aiming for the surrounding soft tissues as well as the dynamic stabilizers of the ankle as well as adequate stabilizing orthotics 2.
If conservative treatment fails surgical management might be considered 2.
Reconstruction or augmentation of the extensor retinaculum has been shown effective in chronic instability with intact interosseous talocalcaneal and calcaneofibular ligaments 2.
Subtalar instability due to calcaneofibular ligament insufficiency can be treated surgically with proximal advancement of the calcaneofibular ligament or reconstruction. The modified Gould procedure has been advocated to address this issue 2.
History and etymology
Subtalar instability as a potential cause of foot and ankle instability was first described by Rubin and Whitten in 1962 2.
Differential diagnosis
Conditions that can mimic the presentation and/or the appearance of subtalar instability include 1-3:
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