The hip bone (also known as the pelvic bone, coxal bone, or innominate bone Latin: os coxae) is a bony structure composed of the ilium, ischium, and pubis. The left and right hip bones join at the pubic symphysis.
Three bones - the ilium, ischium, and pubis - compose the hip bone. Before puberty, these bones are separated by cartilage. During puberty around the age of 15 to 17, the three parts fuse and ossify around 25.
The ilium or iliac bone is a paired bone forming the uppermost and most substantial part of the hip bone. The bone consists of a body, which is the central component, and a wing - an extended portion of the bone.
The ischium is a paired bone forming the lower and back part of the hip bone, as well as the posterior and inferior boundary of the obturator foramen. It has two main components: a body and a ramus.
The pubis or pubic bone is a paired bone that forms the anterior part of the hip bone and the anterior and inferior boundary of the obturator foramen. It is composed of three main parts: a body and two rami - a superior ramus and an inferior ramus. The left and right pubic bones join at the pubic symphysis.
The three parts of the hip bone create several anatomical landmarks in between them, such as the acetabulum and obturator foramen.
The acetabulum is a deep, cup-shaped cavity in the hip bone, where the head of the thigh bone (femur) fits, forming the hip joint. All three bones that form the hip bone contribute to creating the acetabulum. The ischium forms the lower and the side borders of the acetabulum, while the ilium forms the upper boundary. The rest is formed by the pubis, near the midline.
The acetabulum features the following landmarks:
- lunate surface,
- acetabular fossa,
- acetabular notch,
- iliopubic eminence.
The lunate surface of the acetabulum is the curved articular surface surrounding the acetabular fossa. It articulates with the head of the femur.
The acetabular fossa is a circular non-articular depression located at the center of the acetabulum of the hip bone, which is occupied by the ligament of the head of the femur. Inferiorly, the acetabular fossa is continuous with the acetabular notch.
The acetabular notch is a deep indentation in the acetabulum of the hip bone, which is continuous with the acetabular fossa below it. The edges of this notch serve for the attachment of the ligament of the head of the femur. The transverse acetabular ligament transforms the acetabular notch into a foramen, through which nerves end blood vessels enter the hip joint.
The iliopubic eminence (or iliopectineal eminence) is a flat prominence at the proximal part of the pubis marking the point of union between the ilium and pubis bones. The iliopubic eminence is the medial boundary of the groove, over which the iliac and psoas major muscles pass.
The obturator foramen is a large opening on each side of the pelvic skeleton formed by the ischium and pubis of the hip bone. The obturator artery, vein, and nerve pass out of the pelvis through the obturator foramen.
This opening has a thin, uneven margin, which presents with a deep groove called the obturator groove. This groove runs from the pelvis obliquely in a medial and inferior direction. The obturator groove transforms into the obturator canal with the help of a ligamentous band, a specialized part of the obturator membrane. This ligamentous band attaches to two tubercles:
- posterior obturator tubercle - located on the medial border of the ischium, right in front of the acetabular notch;
- anterior obturator tubercle - found on the obturator crest of the superior ramus of the pubis.
The obturator canal serves for the passage of the obturator artery, obturator vein, and obturator nerve, exiting the pelvis.