If the talus is displaced 1 mm laterally, what percent decrease occurs in tibiotalar contact?

• A. 24%
• B. 60%
• C. 42%
• D. 15%

Answer: (C)

The key idea here is how sensitive ankle joint contact is to the talus’s position inside the mortise. The tibial plafond and talar dome form a curved, congruent pair, so even a tiny shift of the talus laterally changes how much surface is actually in contact and bearing load. When the talus moves 1 mm to the outside, a large portion of the original contact area is lost because the overlapped region of the two curved surfaces shrinks nonlinearly. In this specific situation, about 42% of the tibiotalar contact area is reduced, leaving a smaller contact footprint and concentrating forces on the remaining contact zone. That’s why such a small lateral shift markedly diminishes contact area and elevates contact stress, which can contribute to cartilage wear over time.