Posteromedial subtalar coalitions: prevalence and associated morphological alterations of the sustentaculum tali

A variant form of subtalar coalition isolated to the posterior sustentaculum has been previously described, though its prevalence is not known and its relationship to the middle facet has not been characterized.

To determine the prevalence and morphological alterations of isolated posteromedial subtalar coalitions.

Computed tomography (CT) images of the foot or ankle performed from 2004 to 2012 were reviewed and 97 patients (mean age: 13.3+/-2.9 years; range: 9-19 years) with subtalar coalition were identified. In 41 (42%) of these, the condition was bilateral, yielding a total of 138 subtalar coalitions. In the 33 patients where CT demonstrated isolated posteromedial subtalar coalitions, multiplanar reformats along the long axis of the sustentaculum tali were generated, from which the anteroposterior dimensions of the sustentaculum tali and middle facet were measured. Posterior sustentaculum measurements defining the posterior extension of the sustentaculum beyond the middle facet were directly measured by two radiologists. Ratios of middle facet to posterior sustentaculum measurements were calculated. Thirty-three patients undergoing CT for ankle fracture served as controls.

Ninety-seven of 138 coalitions (70.2%) affected the middle facet and 2/138 (1.4%) involved the posterior facet. There were 39 (28.2%) posteromedial subtalar coalitions in 33 patients. Mean AP measurements of the middle facet and posterior sustentaculum in patients with posteromedial subtalar coalitions were 12.6 mm and 18.2 mm, respectively, compared to 16.6 mm and 9.2 mm in controls (P<0.001). Mean middle facet/posterior sustenaculum (MF/PS) ratios were 0.74 for posteromedial subtalar coalitions vs. 1.92 for controls (P<0.001).

Posteromedial subtalar coalitions comprise more than one-quarter of subtalar coalitions, and are associated with an intact, but shorter, middle facet and longer sustentaculum tali. This observation may aid in accurate diagnosis and management of this relatively common disorder.