Background The subtalar joint is commonly affected in many rheumatic and musculoskeletal diseases; however, subtalar joint involvement is often neglected or missed during clinical examination due to the fact that the joint is difficult to examine and most clinicians have a limited understanding of its anatomy.

Objectives To provide a detailed anatomical and US description of the subtalar joint, a single joint that, anatomically, is divided into two separate compartments: the anterior subtalar joint (ASTJ) and the posterior subtalar joint (PSTJ).

Methods Cadaver specimens of the ankle and foot were examined in detail by ultrasound (US) by rheumatologist experts in musculoskeletal US. The ASTJ of all the specimens were injected with colored latex while the PSTJs were respectively injected with 1ml of latex of a contrasting color under US guidance. Following the injections, the joints were frozen and cut into 2 centimeter sections from medial to lateral in the sagittal plane. Sections were examined independently by three authors for the presence of latex within the ASTJ, PSTJ and, also, to look for the presence of extra-articular latex extravasation or the spread of latex to adjacent articulations by means of a communication with the ankle joint or the adjacent portion of the subtalar joint.

Results Six cryopreserved intact ankle-foot specimens from three male and three female cadavers (two right and four left), with a mean age of 74 years (range, 66–80 years) were studied. A recommended list of standardized scanning technique which allows sonographers to evaluate both compartments of the subtalar joint (ASTJ and PSTJ) from the medial, lateral and posterior aspect were developed. All of the specimens (6/6, 100%) contained the appropriate colored latex in the appropriate subtalar joint compartment with minimal leakage into the surrounding soft tissues (Figure 1). Of note, five of the six (5/6, 83%) specimens revealed communication between the PSTJ and the posterior tibiotalar joint. There was no communication between the ASTJ and the tibiotalar joint, nor was there evidence of spread of the latex, i.e. communication, from one subtalar joint compartment to the other.

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Conclusions Lack of communication between the ASTJ and the PSTJ and the communication between the PSTJ and the posterior recess of the tibiotalar joint are compatible with other studies. Our study highlights the importance of employing cadaver specimens in musculoskeletal ultrasound, and presents a number of options for US imaging of both the ASTJ and PSTJ that also provide access to these distinct compartments for the purpose of aspiration and injection.

Disclosure of Interest None declared