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Hip Bone and Pelvis

Os coxae et Pelvis

For medical students3 min readUpdated 2026-10-10

The hip bone is a large, paired bone that, together with the sacrum and coccyx, forms the pelvic girdle. These bony structures create a strong ring—the pelvis—which serves as a connective link between the axial skeleton and the lower limbs, as well as a protective cavity for internal organs.

Bone FusionIn adults, the hip bone is formed by the complete fusion of three separate bones.
Common CenterThe bodies of all three bones converge at a single point, forming the acetabulum.
Two DivisionsThe pelvic cavity is anatomically divided by the pelvic brim into the greater and lesser pelvis.
ObstetricsThe true conjugate (conjugate vera) of the pelvic inlet is normally 11–11.5 cm.

Parts of the Hip Bone

During skeletal development, the hip bone (os coxae) develops from three separate parts that fuse into a single solid bone in adults.

  1. Ilium (os ilium). Occupies the superior portion. It consists of a thickened body (corpus ossis ilii) and a broad ala or wing (ala ossis ilii). The superior free margin of the wing forms the iliac crest (crista iliaca), which separates the external and internal surfaces and has lips for abdominal muscle attachment. Anterior and posterior projections—the anterior and posterior iliac spines (superior and inferior)—stand out on the wing. The internal surface features the iliac fossa (fossa iliaca) and the auricular surface (facies auricularis) for articulation with the sacrum. Externally, gluteal lines (lineae gluteae) provide attachment sites for the corresponding gluteal muscles.
  2. Pubis (os pubis). Located anteriorly. It consists of a body, as well as superior and inferior rami. Key landmarks include the symphysial surface (facies symphysialis) for the formation of the pubic symphysis, the pubic tubercle (tuberculum pubicum), the pubic crest, and the iliopubic eminence.
  3. Ischium (os ischii). Forms the posteroinferior part. It includes a body and a ramus. The most massive structure here is the ischial tuberosity (tuber ischiadicum). Superior to it is the ischial spine (spina ischiadica), which separates the greater and lesser sciatic notches.

The rami of the pubis and ischium enclose a large opening called the obturator foramen (foramen obturatum).

Acetabulum

The site of fusion of the bodies of the ilium, pubis, and ischium appears as a deep articular socket. This is the acetabulum, which serves to articulate with the head of the femur.

At its periphery, the socket is bounded by the acetabular rim (limbus acetabuli). The actual articular surface is the smooth lunate surface (facies lunata). At the very center of the depression is the rough acetabular fossa (fossa acetabuli), and the inferior margin of the ring is interrupted by the acetabular notch (incisura acetabuli).

The Pelvis as a Whole: Structure and Boundaries

The two hip bones, connecting with the sacrum and coccyx, form the pelvis, inside of which lies the pelvic cavity (cavitas pelvis). Anatomically, the pelvis is divided into two regions:

The boundary between them is the pelvic brim (linea terminalis). It runs along the sacral promontory, the upper margin of the pubic symphysis, and the arcuate lines of the iliac bones. This same line forms the pelvic inlet (superior aperture). The pelvic outlet (inferior aperture) is bounded by the coccyx and the ischial tuberosities.

Obstetric Diameters and Sexual Dimorphism

The lesser pelvis is of immense practical importance in obstetrics as it forms the rigid bony framework of the birth canal. Several distances are measured to assess its dimensions:

The pelvic bones exhibit pronounced sexual dimorphism. In females, the inferior rami of the pubic bones converge at an obtuse angle, forming a wide subpubic angle or pubic arch (arcus pubicus) measuring 90–100°. In males, the corresponding junction forms an acute subpubic angle (angulus subpubicus) of approximately 70°.

Mnemonic

To easily remember the three bones forming the hip bone (Ilium, Pubis, Ischium), picture their spatial arrangement: the Ilium acts as a 'roof' on top, the Pubis protects anteriorly, and we sit on the Ischium (bottom and back). All three meet right in the middle at the acetabulum.

Frequently asked questions

What anatomical landmarks form the pelvic brim (linea terminalis)?

The pelvic brim (linea terminalis) is formed by the sacral promontory, the upper margin of the pubic symphysis, and the arcuate line of the ilium. This line serves as an important anatomical landmark dividing the pelvic cavity into two parts: the greater pelvis (pelvis major) and the lesser pelvis (pelvis minor). The pelvic inlet is also bounded by this line.

What bony and ligamentous structures bound the pelvic outlet (inferior aperture of the lesser pelvis)?

The pelvic outlet (apertura pelvis inferior) is bounded by the following structures:

  • Pubic symphysis (symphysis pubica) — located anteriorly along with the inferior border of the pubic arch.
  • Inferior rami of the ischium and pubis — limit the outlet anterolaterally.
  • Ischial tuberosities (tuber ischiadicum) — form the lateral boundaries along with the sacrotuberous ligaments.
  • Coccyx (os coccygis) — limits the aperture posteriorly by its tip.
Which ligaments convert the greater and lesser sciatic notches into the respective sciatic foramina?

Two strong ligaments extending from the sacrum convert the greater and lesser sciatic notches into foramina:

  • Sacrospinous ligament (lig. sacrospinale) — runs to the ischial spine.
  • Sacrotuberous ligament (lig. sacrotuberale) — attaches to the ischial tuberosity.

These structures close off the notches of the hip bone, forming the greater and lesser sciatic foramina, through which important muscles and neurovascular bundles pass.

What anatomical features (aside from the subpubic angle) determine sexual differences between the male and female pelvis?

Pelvic sexual differences are determined by shape, size, and bone structure. The female pelvis is broader and shorter; the iliac wings in females are more flared and positioned more vertically. The pelvic inlet in females is larger and narrows less toward the outlet. The male pelvis is characterized by a wedge-shaped pelvic inlet that narrows anteriorly toward the pubic symphysis.

What anatomical features are located on the superior and inferior rami of the pubic bone?

The pubic bone features the iliopubic eminence (eminentia iliopubica), the symphysial surface (facies symphysialis), the pubic tubercle (tuberculum pubicum), the pubic crest (pecten ossis pubis), the obturator crest (crista obturatoria), the obturator groove (sulcus obturatorius), and the anterior and posterior obturator tubercles (tuberculum obturatorium anterius et posterius).

What vessels and nerves pass through the obturator canal (canalis obturatorius)?

The obturator neurovascular bundle passes through the obturator canal (canalis obturatorius). It includes:

  • Obturator artery (a. obturatoria) — a branch of the internal iliac artery.
  • Obturator vein (v. obturatoria) — accompanies the artery.
  • Obturator nerve (n. obturatorius) — supplies the adductor muscles.

These structures emerge onto the medial surface of the thigh, providing blood supply and innervation to the adductor muscles, gracilis, and medial thigh skin.

What bones make up the hip bone?

It is formed by the fusion of three bones: the ilium, pubis, and ischium.

Where is the acetabulum located?

The acetabulum is located on the lateral surface of the hip bone, at the site of fusion of the bodies of the ilium, pubis, and ischium.

How does the greater pelvis differ from the lesser pelvis?

The greater pelvis is wider, formed by the wings of the iliac bones, and lacks an anterior bony wall. The lesser pelvis lies below the pelvic brim and forms a closed bony ring.

What is the true conjugate?

It is a crucial obstetric diameter (11–11.5 cm) representing the shortest distance from the sacral promontory to the most prominent point on the posterior surface of the pubic symphysis.

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