The antitrochanter is a uniquely avian osteological feature of the pelvis that is located lateral to the postero-dorsal rim of the acetabulum. This feature makes the avian hip joint unique among all vertebrates, living and fossil, in that a significant portion of the femoral-pelvic articulation is located outside of the acetabulum. This additional acetabular articulation occurs between the neck of the femur and the antitrochanter, and operates as a hinge joint or ginglymus. It is complementary to the articulation of the head of the femur with the acetabulum, which is a pivot joint or trochoides. The size, location, and spatial orientation of the antitrochanter were determined for 77 species of birds representing a variety of hindlimb functions (e.g., highly cursorial, vertical clinging, foot-propelled diving) and spanning a wide range of body sizes (swifts to rheas). The area of the antitrochanter is a good predictor of body mass in birds; its position and orientation are reasonably consistent within hindlimb morphofunctional groups, but not among all birds. The antitrochanter serves as a brace to prevent abduction of the hindlimb and to absorb stresses that would otherwise be placed on the head of the femur during bipedal locomotion. The drum-in-trough-like form of the antitrochanter-femur articulation tends to assist in the transfer of long-axis rotational movements of the femur to the pelvis. The avian antitrochanter is a derived feature of birds that evolved as an aid in maintaining balance during bipedal terrestrial locomotion.

El Antitrocánter en las Aves: Forma y Función en el Equilibrio