"Absent middle facet": a sign on unenhanced radiography of subtalar joint coalition

We compared the accuracy of the previously unevaluated "absent middle facet" sign with the more commonly used "talar beak" sign and C sign for the detection of coalition of the subtalar joint middle facets on lateral foot radiographs obtained with the patient standing.

Three musculoskeletal radiologists graded the visibility of these three signs on standing lateral radiographs of 32 feet that had been proven to have coalitions of the middle facets of the subtalar joint and 62 feet that had normal subtalar joints. The reviewers were unaware of the presence or absence of subtalar coalitions in these feet.

The absent middle facet sign had a sensitivity, specificity, and accuracy of 75%, 98%, and 90%, respectively, for the diagnosis of subtalar joint coalition in this patient population, whereas these values were 56%, 100%, and 85% for the C sign and 53%, 90%, and 78% for the talar beak sign, respectively. A finding of either a positive absent middle facet sign or a positive C sign resulted in a sensitivity, specificity, and accuracy of 84%, 98%, and 94%, respectively.

In this study population, the absent middle facet sign was more sensitive than and nearly as specific as the talar beak sign and C sign for diagnosing subtalar coalition on standing lateral foot radiographs. The highest accuracy was obtained when a finding of either a completely absent middle facet or a complete C sign was considered as a positive indicator of a subtalar coalition.