The stabilizing capacity of the ligaments and articular surface in the ankle was determined under defined physiologic loading conditions. The concept of primary and secondary constraints was adapted to the ankle. With physiologic loading, the articular surface ac counted for 30% and 100% of stability in rotation and version, respectively. That the articular surface was the sole source of inversion and eversion stability under the prescribed physiologic loading conditions has not been previously reported. The demonstration that the articular surface resists inversion displacement in the loaded ankle supports the conclusion of previous studies that rotation, rather than inversion, may account for a type of clinically sympto matic ankle instability. Further, ankle instability may occur during loading and unloading but not once the ankle is fully loaded. The results of our study confirm the importance of the anterior talofibular and calcaneo fibular ligaments and suggest an important role for the deltoid ligament.