The acetabulum is the cup in the pelvis which receives the head of the femur. It is deep (in comparison with the shoulder joint, for instance) and the principal load on the joint (whether standing, walking or running) tends to force the two bones together.
The muscles which operate across the hip joint tend to be massive and extremely strong. Surrounding the joint as they do they make the joint extremely strong, while also limiting its degree of movement.
The size and shape of the acetabulum (socket) in relation to the head of the femur (ball) are most important for a strong joint, along with the numerous ligaments which hold the joing in place. If there is a known family history for problems in this area, young children are screened via ultra-sound for dysplasia.