The Hip Bone
The two symmetrical hip bones (also known as the innominate bones, or pelvic bones) are part of the pelvic girdle, the bony structure that attaches the axial skeleton to the lower limbs (for more information about the anatomy of the pelvic girdle, click here)
The hip bones have three articulations:
- Sacroiliac joint – articulation with sacrum.
- Pubic symphysis – articulation with the corresponding hip bone.
- Hip joint – articulation with the head of femur.
In this article, we shall look at the osteology of the hip bones – their constituents, bony landmarks, and applied anatomy.
Composition of the Hip Bone
The hip bone is made up of the three parts – the ilium, pubis and ischium. Prior to puberty, the triradiate cartilage separates these constituents. At the age of 15-17, the three parts begin to fuse.
Their fusion forms a cup-shaped socket known as the acetabulum, which becomes complete at 20-25 years of age. The head of the femur articulates with the acetabulum to form the hip joint.
We shall now look at the individual parts of the hip bone, and their respective bony landmarks.
The superior part of the hip bone is formed by the ilium, the widest and largest of the three parts. The body of the ilium forms the superior part of the acetabulum. Immediately above the acetabulum, the ilium expands to form the wing (or ala).
The wing of the ilium has two surfaces. The inner surface is concave, and known as the iliac fossa, providing origin to the iliacus muscle. The external surface is convex, and provides attachments to the gluteal muscles. Hence it is known as the gluteal surface.
The superior margin of the wing is thickened, forming the iliac crest. It extends from the anterior superior iliac spine to the posterior superior iliac spine.
Muscles attaching to the Ilium: –
- Gluteal muscles attach to the external surface of the Ilium at the anterior, posterior and inferior gluteal lines.
- The iliacus muscle attaches medially at the iliac fossa.
Clinical Relevance: Anterior Superior Iliac Spine
The anterior superior iliac spine (ASIS) is an important anatomical landmark:
- Mid-inguinal point – halfway between the ASIS and the pubic symphysis. The femoral artery can be palpated here.
- Mid-point of the inguinal ligament – halfway between the ASIS and the pubic tubercle.
In clinical practice, a patient’s “true” leg length is measured from the ASIS to the medial malleolus at the ankle joint. This is distinct from “apparent” leg length, which is measured from the umbilicus to the medial malleolus. True leg length discrepancy can seen in various hip disorders.
The pubis is the most anterior portion of the hip bone. It consists of a body and superior and inferior rami (branches).
The body is located medially, articulating with its opposite pubic body, at the pubic symphysis.
The superior ramus extends laterally from the body, forming part of the acetabulum. The inferior ramus projects towards, and joins the ischium. Together, the two rami enclose part of the obturator foramen, through which the obturator nerve, artery and vein pass through to reach the lower limb.
The posterioinferior part of the hip bone is formed by the ischium. Much like the pubis, it is composed of a body, an inferior and a 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.
On the posterior aspect of the ischium there is an indentation known as the greater sciatic notch, with the ischial spine at its most inferior edge.
Two important ligaments attach to the ischium:
- The 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.
- The sacrotuberous ligament runs from the sacrum to the ischial tuberosity, forming the lesser sciatic foramen.
Clinical Relevance: Fractures of the Pelvic Bones
There are two common ways of fracturing the pelvic bones:
- Direct trauma to the pelvic bones, for example from a vehicular accident.
- Forces transmitted from the lower limb, for example a heavy fall on the feet.
Fractures often occur at the weaker points of the bones. These are the pubic rami, the acetabulum or in the region of the sacroiliac joint.
A common complication of pelvic fractures is soft tissue injury. In particular, the bladder and urethra are at high risk of damage.