102 - Critical angle of Gissane

*In 1947, Gissane described his critical angle or crucial angle.

*He noted a distinct angular cortical platform that parallels the lateral process of the talus on lateral radiographic projection.

*This cortical density represents the dense subchondral bone lying beneath the posterior, anterior and middle facets.

*The angular measurements vary from 130 to 145 degrees, with an average of 130 degrees.

*During an axial impaction load, the lateral process of the talus is driven through the posterior facet in a wedge like manner that facilitates the primary fracture. This extends from the lateral cortical vertex of the crucial angle and exits plantarly through the neutral triangle.

*The initial fracture, as described by Essex Lopresti, is located on the anterior distal lip of the posterior facet and connects to the primary fracture line. This fracture extends through the facet, splits it into one or multiple fragments, and impacts the lateral portions into the body.

*The crucial angle reveals the angular relationship of the calcaneal facets and should appear identical when taken bilaterally.

*Unlike Bohler’s angle, which may be aberrant with displaced extraarticular and intraarticular fractures, the crucial angle is more specific for intraarticular distortion.

 *In the above second figure: Lateral radiograph of the calcaneus shows compression (light blue arrows) and traction (yellow arrows) trabeculae, with the neutral triangle (brown triangle) in between with sparse trabeculae. The thickened cortical or thalamic portion of the bone supporting the articular facets is shown (T). The critical angle of Gissane (G) and the Boehler angle (B) have also been drawn in. The Boehler angle is normally 20°–40°.