In understanding this apparent paradox, a closer look must be given to the structure of the trochlea and the type of movement the talus undergoes during ankle dorsiflexion. If both sides of the trochlea are examined it is evident that the lateral articular surface, which articulates with the fibular malleolus,is longer in its anteroposterior dimension than the medial articular surface. The reason for this is that the lateral malleolus moves over a greater excursion, with respect to the talus, during plantar flexion–dorsiflexion than does the medial malleolus. This is partly because the axis of motion is farther from the superior trochlear articular surface laterally than medially. The corollary to this (and this is true of essentially all joints with sellar surfaces) is that the relatively track-bound movement that the talus undergoes on plantar flexion–dorsiflexion at the ankle is not a pure swing, but rather an impure swing; it involves an element of spin, or rotation, that results in a helical movement. Another way of conceptualizing this movement is to consider the talus as a section of a cone whose apex is situated medially rotating within the mortise about its own long axis, rather than a truly cylindrical body undergoing a simple rolling movement within the mortise (Fig. 16-27). As a result of this, the intermalleolar lines projected onto the superior trochlear articular surface at various positions of plantar flexion and dorsiflexion are not parallel lines. Therefore, the degree of wedging of the trochlea does not reflect the relative intermalleolar distances in dorsiflexion and plantar flexion of the ankle. The true intermalleolar distances are represented by the length of these nonparallel lines projected onto the superior trochlear surface. The projected line with the foot in plantar flexion is only slightly shorter, if at all, than that for dorsiflexion, and the necessary separation of the malleoli during dorsiflexion is minimal.