The trapezoid form of the trochlea tali.

The wedge-shaped superior surface of the trochlea tali may be seen in a proximal view as a trapezium. The angle of this trapezium was calculated by measuring the length, and the smallest and largest widths of the trochlea, and was found to be 16.04 degrees for the left talus and 12.48 degrees for the right talus, respectively. The difference between these angles is highly significant. Due to biomechanical features of the talocrural joint an increase in the angle of the trochlea tali also intensifies the incongruency of this joint in plantar-flexion. Thus, this incongruency is larger on the left side. With this diminution of the joint-surface contact, increased internal rotation or "wobbling" movements are possible. An explanation for these facts might be found in footedness, similar to handedness. When constructing a prosthesis one has to consider that the trochlea tali cannot be exactly mirrored from one side to the other but may have to be calculated separately for each side. The fact that footedness seems to correlate with the angle of the trochlea tali can be also helpful.