What keeps the tibia from slipping sideways off the talus?

The tibia is stabilized on the talus primarily by the strong ligaments surrounding the ankle joint, such as the deltoid ligament medially and the lateral collateral ligaments. These ligaments provide support and prevent excessive lateral movement. Additionally, the bony architecture of the ankle joint, including the mortise formed by the tibia and fibula, helps to keep the tibia securely positioned above the talus, preventing sideways displacement.