- Discussion:
- slightly concave, is situated on the sustenaculum tali;
- sustenaculum forms the floor of middle facet;
- between middle & posterior articular facets lies interosseous sulcus (calcaneal groove), which opens broadly laterally & forms, w/ talar sulcus, sinus tarsi;
- Radiology:
- medial oblique axial view:
- demonstrates middle & post facets of sub-talar joint;
- since nl posterior & middle facets are in parallel planes at approx 45 deg to sole of foot, these two areas can be identified on axial (Harris, or ski jump) view (described by Korvin in 1933);
- effectiveness of harris view for visualizing subtalar joint is enhanced by measuring angle of posterior facet on lateral view & then adjusting the Harris View to this inclination;
- radiographic appearance of two facets either in same plane or at angle > 20 deg to each other suggests coalition;
- angulation of the middle facet by more than 20 deg off horizontal is consistent w/ coalition, even if joint space is open;
- posterior & middle facet of calcaneus, which articulate w/ talus, are normally in parallel planes and are usually best visualized on axial radiographs at 45 deg;
- in talocalcaneal coalition, angle of the middle facet may vary;
- anterior facet is in a different plane and cannot be seen on axial views because it is obsurred by the head of the talus