From the Guidelines
Assessing the subtalar joint on X-ray requires lateral, Harris-Beath, and oblique views to evaluate alignment and joint space, with CT scanning providing superior visualization if X-rays are inconclusive, as noted in the evaluation of chronic foot pain 1. When evaluating the subtalar joint on X-ray, it is essential to consider the limitations of radiographs in detecting subtalar coalitions, which may be overlooked due to overlapping structures, but secondary signs on the lateral view could suggest a subtalar coalition, with an overall sensitivity of 100% and a specificity of 88% reported for radiographs in the diagnosis of talocalcaneal coalitions 1. Key projections for assessing the subtalar joint include:
- Lateral view: evaluate the alignment between the talus and calcaneus, looking for any step-offs or abnormal angulation
- Harris-Beath view: provides excellent visualization of the posterior facet of the subtalar joint and allows assessment of varus/valgus alignment
- Oblique views, such as Broden's views: help visualize the posterior facet from different angles When interpreting these images, look for:
- Joint space narrowing, indicating arthritis
- Subchondral sclerosis
- Osteophytes
- Fractures involving the joint surfaces
- Tarsal coalition, a congenital fusion between bones, which may be visible as bridging between the talus and calcaneus CT scanning remains the most reliable method for diagnosing subtalar coalitions, especially if X-rays are inconclusive, as it provides superior visualization of the subtalar joint and surrounding structures 1.
From the Research
Assessing Subtalar Joint on X-ray
To assess the subtalar joint on X-ray, several factors need to be considered, including the position of the ankle and the angulation of the X-ray beam.
- The study by 2 found that changes in the projection angle can significantly influence the assessment of the subtalar joint, with internal rotation of the foot and caudal angulation of the X-ray beam affecting the subtalar inclination angle (SIA) and calcaneal slope (CS).
- The same study recommended an internal rotation of the foot of 20° to assess the posterior aspect of the posterior facet, and a combined 20° internal rotation of the foot and 40° caudal angulation of the X-ray beam to assess the anterior aspect of the posterior facet.
- Another study by 3 evaluated the posterior facet of the subtalar joint on eight standard radiographic views of the foot and ankle, and found that different views can visualize different parts of the posterior facet.
- The study by 3 also found that the AP view of the ankle joint visualizes the posterior part of the posterior facet, while the mortise view visualizes a slightly more anterior part, and the subtalar view visualizes the anterior part of the posterior facet.
Standard Radiographic Views
The following standard radiographic views can be used to assess the subtalar joint:
- Antero-posterior (AP) view of the ankle joint
- Mortise view of the ankle joint
- Subtalar view
- Broden views
Limitations of Conventional Radiographs
Conventional radiographs have limitations in evaluating the subtalar joint, as noted by 4, and computed tomography (CT) and MRI are often needed to better delineate the joint anatomy and detect joint pathology.
- The study by 5 used low-dose weightbearing computed tomography (WBCT) to provide reliable, quantitative 3D measures of subtalar joint space width (JSW) following intra-articular calcaneal fractures, and found that WBCT-based methods can be used to objectively assess subtalar posttraumatic osteoarthritis.