The Hip Bone
The left and right hip bones (innominate bones, pelvic bones) are two irregularly shaped bones that form part of the pelvic girdle – the bony structure that attaches the axial skeleton to the lower limbs.
The hip bones have three main articulations:
- Sacroiliac joint – articulation with the sacrum.
- Pubic symphysis – articulation between the left and right hip bones.
- Hip joint – articulation with the head of femur.
Composition of the Hip Bone
The hip bone is comprised of the three parts; the ilium, pubis and ischium. Prior to puberty, the triradiate cartilage separates these parts – and fusion only begins at the age of 15-17.
Together, the ilium, pubis and ischium form a cup-shaped socket known as the acetabulum (literal meaning in Latin is ‘vinegar cup‘). The head of the femur articulates with the acetabulum to form the hip joint.
The ilium is the widest and largest of the three parts of the hip bone, and is located superiorly. The body of the ilium forms the superior part of the acetabulum (acetabular roof). Immediately above the acetabulum, the ilium expands to form the wing (or ala).
The wing of the ilium has two surfaces:
- Inner surface – has a concave shape, which produces the iliac fossa (site of origin of the iliacus muscle).
- External surface (gluteal surface) – has a convex shape and provides attachments to the gluteal muscles. The superior margin of the wing is thickened, forming the iliac crest. It extends from the anterior superior iliac spine (ASIS) to the posterior superior iliac spine (PSIS).
On the posterior aspect of the ilium there is an indentation known as the greater sciatic notch.
The pubis is the most anterior portion of the hip bone. It consists of a body, superior ramus and inferior ramus (ramus = branch).
- Pubic body – located medially, it articulates with the opposite pubic body at the pubic symphysis. Its superior aspect is marked by a rounded thickening (the pubic crest), which extends laterally as the pubic tubercle.
- Superior pubic ramus – extends laterally from the body to form part of the acetabulum.
- Inferior pubic ramus – projects towards the ischium. Together, the superior and inferior rami enclose part of the obturator foramen – through which the obturator nerve, artery and vein pass through to reach the lower limb.
The ischium forms the posteroinferior part of the hip bone. Much like the pubis, it is composed of a body, an inferior ramus and superior ramus.
The inferior ischial ramus combines with the inferior pubic ramus forming the ischiopubic ramus, which encloses part of the obturator foramen. The posterorinferior aspect of the ischium forms the ischial tuberosities and when sitting, it is these tuberosities on which our body weight falls.
Near the junction of the superior ramus and body is a posteromedial projection of bone; the ischial spine.
Two important ligaments attach to the ischium:
- Sacrospinous ligament – runs from the ischial spine to the sacrum, thus creating the greater sciatic foramen through which lower limb neurovasculature (including the sciatic nerve) transcends.
- Sacrotuberous ligament – runs from the sacrum to the ischial tuberosity, forming the lesser sciatic foramen.