The fit of talus in mortise is precise, making it the most congruent of the weight-bearing joints. Both mortise and talar trochleas are narrower posteriorly. Both this diminution of width and the degree of parallelism of the malleolar auricular surfaces vary among individuals. The fit of talus in mortise remains congruent throughout the ankle's range of motion, because the joint surface is a portion of a frustum of a cone, the axis of which is the ankle's axis of rotation (Fig 7). Therefore, there is little if an change in mortise width during ankle motion (0 to 2 mm). The effect of the ankle's oblique axis is an obligatory internal and external rotation of the foot with plantar flexion and dorsiflexion, respectively (Fig 8).
