The hip bone (sometimes, archaically, referred to as the innominate or "nameless" bone) is a large, irregularly shaped flat bone. Together with the sacrum, the two hip bones form the pelvic girdle.
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The hip bone (sometimes, archaically, referred to as the innominate or "nameless" bone) is a large, irregularly shaped flat bone. Together with the sacrum, the two hip bones form the pelvic girdle. |
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Each hip bone is initially made up of three embryonic elements - the ilium, ischium and pubis - which meet at a Y-shaped junction. With age these elements fuse but, prior to fusion, the elements are separated from one another by cartilage. |
The ilium (= flank-bone) is the upper component and is a wide, flat rectangular plate. The superior border of the plate possesses a thickened crest (the iliac crest), chiefly for the attachment of muscles of the abdominal wall. There are small protuberances at the corners of the plate (the anterior superior, posterior superior, anterior inferior and posterior inferior iliac spines).
The anterior superior iliac spine gives attachment to the sartorius muscle
and to the inguinal ligament, the anterior inferior one to the capsule of
the hip joint (especially the strong ilio-femoral ligament) and to rectus
femoris (one of the heads of the quadriceps muscle). The outer surface of
the ilium gives origin to the three large gluteal muscles (gluteus maximus,
medius and minimus) and small ridges - gluteal lines -demarcate their attachments.
The inner surface of the ilium forms the origin of iliacus, one of the two
major hip flexors. The ilium has an auricular (= "ear-shaped") surface which
articulates with the sacrum at the stable and immobile sacro-iliac joint.
The ischium is the lowest and most posterior component. It possesses a large downward protuberance - the ischial tuberosity - which is the part of the hip bone that you sit on; in addition, the tuberosity is the origin of the hamstring muscles. The ischium also has a small, backwards-projecting ischial spine. Strong ligaments run from the sacrum to both the ischial tuberosity (sacrotuberous ligament) and the ischial spine (sacrospinous ligament). These ligaments help to counter possible instabilities of the pelvic girdle as a whole, including a) downwards displacement of the sacrum due to the weight of the vertebral column and b) backwards rotation of the sacrum due to the curvatures of the lower end of the vertebral column. A forward projection, the ischial ramus, makes contact with the pubis, which is the anterior part of the hip bone and meets the pubis of the opposite side at the mid-line pubic symphysis.
The midline component of the pubis is the body, from which an upper and a lower spur (the superior and inferior pubic rami) project laterally and backwards. The adductor muscles of the hip joint are attached all around the outer surface of the pubis. The body has an upper protrusion - the pubic tubercle - to which the inguinal ligament is attached.
The posterior surfaces of the ilium and ischium form boundaries of the greater sciatic foramen, through which structures leave the pelvis (including the small hip muscle piriformis, the sciatic nerve, the superior and inferior gluteal nerves and vessels and the posterior cutaneous nerve of the thigh).
The outer surface of the hip bone possesses a large cavity, the acetabulum,
into which the head of the femur fits to form the hip joint. The acetabulum
is also the point at which the three original component parts of the hip
bone meet.
The pubis and ischium also surround a large hole in the hip bone - the
obturator foramen. In life this is almost entirely filled in with fibrous
tissue (the obturator membrane) which provides additional surface for muscle
attachment except for a small deficiency at the top of the foramen which
transmits the obturator nerve and vessels.
