Human hip joint. The hip bone or innominate bone, is formed by the fusion of three bones; the ilium (top), the ischium (bottom) & the pubis (middle, truncated). It articulates with the femur (thigh bone) at the acetabulum, the deep socket into which the head of the femur fits, foming a ball & socket joint. Between the pubis & the ischium (bottom) is a large open space called the obturator foramen, only partly visible as an arc on the right of the image. The right & left hip bones form part of the pelvis.