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.
In this article, we shall look at the anatomy of the hip bones – their composition, bony landmarks, and clinical relevance.
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.
We shall now look at the individual parts of the hip bone, and their respective bony landmarks.
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.
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 centre of 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 is a feature of various hip disorders, as well as being a potential complication of hip joint replacement (arthroplasty).
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, its articulates with the opposite pubic body at the pubic symphysis.
- 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.
Clinical Relevance: Pelvic Fractures
There are two broad groups of pelvic fractures:
- Low energy injuries:
- For example, a simple fall from standing height in an osteoporotic patient resulting in pubic rami fracture.
- These are usually ‘stable’ injuries, not requiring surgery.
- High energy injuries with direct or transmitted trauma:
- For example, after a high speed road traffic accident. These result in more extensive fractures which may include the acetabulum and sacroiliac joint.
- These can be ‘unstable’ injuries and may require urgent surgery.
- Higher energy injuries can be associated with soft tissue and vascular injury. In particular, the bladder and urethra are at high risk of damage. Vascular injury can result in life threatening haemorrhage.