Hip bone

The hip bones (a.k.a. the pelvic bones or the coxal bones) are large irregular bones that make up part of the pelvic girdle. They interconnect anteriorly through the pubic symphysis, a secondary cartilaginous joint, and posteriorly communicate with the sacrum at the sacroiliac joints, which are synovial joints. As components of the pelvic girdle, they assist in transferring the weight of the upper body to the lower limbs.

Each hip bone consists of three sub-parts: the Ilium (superiorly), ischium (posteroinferiorly), and pubis (anteroinferiorly). These components meet at the acetabulum, a cup-shaped hollow portion located on the outer aspect of the hip bone. The pubis and the ischium are separated by the obturator foramen, a large oval opening, inferiorly.

The ilium, the most superior part of the hip bone, features a rounded ridge called the arcuate line on its medial surface, which is part of the linea terminalis and pelvic brim. This line divides the ilium into upper and lower parts. The lower part (i.e. below the pelvic brim) contributes to the true pelvis, while the upper part forms a wing-like structure known as the ala, which belongs to the false pelvis. The ala has a concave inner aspect called the iliac fossa and faces the gluteal region externally.

The ilium has three distinct margins. The upper margin thickens to form the iliac crest, providing attachment for various muscles. Anteriorly and posteriorly, it is delimited by the anterior and posterior iliac spines, respectively. The anterior margin extends from the anterior superior iliac spine to the anterior inferior iliac spine, while the posterior margin extends from the posterior superior iliac spine to the posterior inferior iliac spine.

The ischium constitutes the posteroinferior part of the hip bone, consisting of a large body and a ramus. The body extends superiorly to join the ilium and the superior ramus of the pubis. Below, it features a prominent ischial tuberosity, serving as the site of attachment for the hamstring muscles. The ischial ramus projects anteriorly and connects with the inferior ramus of the pubis, forming a common ischiopubic ramus.

The pubis forms the anteroinferior portion of the hip bone and consists of a body and two rami: superior and inferior. The flat body articulates with the pubic body of the opposite side, forming the pubic symphysis, a secondary cartilaginous joint. The inferior ramus contributes to the ischiopubic ramus, while the superior ramus joins the ilium and ischium within the acetabular fossa. The upper sharp margin of the superior ramus is called the pecten pubis or pectineal line, which is part of the pelvic brim.

The hip joint is formed by the acetabulum of the hip bone articulating with the head of the femur. It is a ball and socket synovial joint that allows a wide range of movements, including flexion, extension, adduction, abduction, medial and lateral rotation, and circumduction.