Hello all,
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I was dabbling in the lab today with a cut down foot. All tendons and muscle were removed. Other ligaments remained intact. When I placed the foot in the position of an inversion sprain, the lateral side of the STJ tried to open….. no issue here. It did not merely open like a hinge, but rotated as well. That is, the talus externally rotated on the calcaneus during extreme inversion. The truly interesting observation was seeing this same mechanism occurring when I severely pronated the foot. That is, I freed ligaments enough to allow “extreme” motion and then created a simulated ground force reaction through the forefoot until the talonavicular joint reached its endpoint. At this point, the talus was driven posterior lateral into the same position as with supination. As a thought experiment, this seems quite logical, but I expected the joint surfaces to exceed their limits. The joints remained in contact – articular cartilage to articular cartilage – throughout the entire range. More interesting was the action of the lateral cuneiform. When the calcaneus plantarflexed it appeared that the joint angle between the lat. cuneiform and navicular increased such that the joint lost stability. This seemed to allow the lateral and medial columns to separate slightly. That is, the medial column continued to dorsiflex while the lateral column remained motionless. In other words, as soon as the STJ lost stability (in the posterior lateral direction) the midfoot created a new axis of motion for itself.
Any thoughts???
Kevin M
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