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Boyer diss 2009 1046..

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(sustentacular) facet. This sulcus leads medially to the inferior astragalar foramen (which<br />

leads to the superior astragalar foramen).<br />

Medial to the inferior astragalar foramen is the proximal extension of the<br />

sustentacular facet, the medial edge of which meets the medial tibial facet. This facet is<br />

proximally concave, where it occupies the sulcus beneath a platform holding the flexor<br />

fibularis groove. Followed distally it becomes convex as it also diverges slightly<br />

laterally, wrapping around the neck of the astragalus. Because this “strip-like” facet<br />

wraps obliquely around the “cylindrical” neck of the astragalus, it is often referred to as<br />

“helical” in its shape (Szalay and Decker, 1974). Distally it meets the facets for the<br />

spring ligament and navicular bone.<br />

In dorsal view the neck of the astragalus is oriented at an angle with respect to the<br />

body, such that it projects medially. Distal to the neck is the head, which is occupied by<br />

the distal end of the sustentacular facet, the spring ligament facets and the navicular<br />

facets mentioned above on its posterolateral, posteromedial, and distal surface,<br />

respectively. In dorsal view, the head is much wider than the neck. The medial and<br />

lateral edges of its navicular facet appear to flare dorsally and proximally with respect to<br />

the middle part of the facet. Thus, in distal view, the facet appears reniform. The lateral<br />

side of the navicular facet faces distally with respect to the proximodistal axis of the<br />

lateral tibial facet. The mediolateral long axis of this side of the facet is parallel to the<br />

plane of the lateral tibial facet of the astragalus. The medial part of the navicular facet<br />

(the other half of the “kidney bean”) faces mediodistally, and its mediolateral long axis is<br />

oriented dorsomedial to plantolateral with respect to the lateral tibial facet.<br />

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