Parts of the Hip Bone
During skeletal development, the hip bone (os coxae) is formed from three independent parts that fuse into a single solid bone in adults.
- Ilium (os ilium). Occupies the upper section. It consists of a thickened body (corpus ossis ilii) and a broad ala (ala ossis ilii). The upper free margin of the ala forms the iliac crest (crista iliaca), which separates the external and internal surfaces and has lips for the attachment of abdominal muscles. Paired projections stand out anteriorly and posteriorly on the ala — the anterior and posterior iliac spines (superior and inferior). The inner side features the iliac fossa (fossa iliaca) and the auricular surface (facies auricularis) for articulation with the sacrum. The gluteal lines (lineae gluteae), to which the corresponding muscles attach, are located externally.
- 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.
- 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 meet, enclosing a large obturator foramen (foramen obturatum).
Acetabulum
The site of fusion of the bodies of the ilium, pubis, and ischium appears as a deep articular fossa. This is the acetabulum (acetabulum), which serves for articulation with the femoral head.
At the periphery, the acetabulum is bounded by a prominent margin (limbus acetabuli). The articular surface proper is the smooth lunate surface (facies lunata). In the very center of the depression lies 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
Two hip bones, articulating with the sacrum and coccyx, form the pelvis (pelvis), inside of which is the pelvic cavity (cavitas pelvis). Anatomically, the pelvis is divided into two parts:
- Greater Pelvis (pelvis major). Framed posteriorly by the lower lumbar vertebrae and laterally by the flared wings of the iliac bones. It lacks an anterior bony wall.
- Lesser Pelvis (pelvis minor). Bounded posteriorly by the sacrum and coccyx, laterally by the inner surfaces of the ischial bones, and anteriorly by the pubic bones.
The boundary between them is the pelvic brim (linea terminalis). It passes through 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 ischial tuberosities.
Obstetrical Dimensions and Sex Differences
The lesser pelvis is of immense practical importance in obstetrics as it forms the rigid bony framework of the birth canal. To assess its parameters, a number of distances are measured:
- True Conjugate (conjugata vera): the anteroposterior diameter of the pelvic inlet. Normally 11–11.5 cm.
- Diagonal Conjugate (conjugata recta): 9–11.5 cm.
- Oblique Diameter (diameter obliqua): 12–12.5 cm.
- Transverse Diameter of the Outlet: about 11 cm.
The pelvic bones exhibit marked sexual dimorphism. In women, the inferior rami of the pubic bones converge at an obtuse angle, forming a wide pubic arch (arcus pubicus) measuring 90–100°. In men, the analogous junction forms an acute subpubic angle (angulus subpubicus), which is approximately 70°.
Briefly: answers to topical questions
What anatomical structures form the pelvic brim (linea terminalis)?
The pelvic brim (linea terminalis) is formed by the sacral promontory, the superior margin of the pubic symphysis, and the arcuate line of the ilium. This line is an important anatomical landmark as it serves as the boundary 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 bony and ligamentous structures:
- Pubic Symphysis (symphysis pubica) — located anteriorly along with the inferior margin of the pubic arch.
- Inferior Rami of the Ischium and Pubis — bound the outlet anteriorly and laterally.
- Ischial Tuberosities (tuber ischiadicum) — form the lateral boundaries together with the sacrotuberous ligaments.
- Coccyx (os coccygis) — bounds the aperture posteriorly with its apex.
Which ligaments convert the greater and lesser sciatic notches into the respective sciatic foramina?
The greater and lesser sciatic notches are converted into the respective foramina by two strong ligaments extending from the sacrum:
- Sacrospinous ligament (lig. sacrospinale) — runs toward the ischial spine.
- Sacrotuberous ligament (lig. sacrotuberale) — attaches to the ischial tuberosity.
These structures bridge 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 the sexual differences between the male and female pelvis?
Sexual differences of the pelvis are determined by the shape, dimensions, and structure of the bones. The female pelvis is wider and shorter; the iliac wings in females are more flared and positioned more vertically. The pelvic inlet in women is larger and narrows less inferiorly. The male pelvis is characterized by a wedge-shaped pelvic inlet that narrows toward the pubic symphysis.
What anatomical landmarks are located on the superior and inferior rami of the pubic bone?
The following anatomical landmarks of the pubic bone are described in sources: iliopubic eminence (eminentia iliopubica), symphyseal surface (facies symphysialis), pubic tubercle (tuberculum pubicum), pubic crest (pecten ossis pubis), obturator crest (crista obturatoria), obturator groove (sulcus obturatorius), and anterior and posterior obturator tubercles (tuberculum obturatorium anterius et posterius).
Which vessels and nerves pass through the obturator canal (canalis obturatorius)?
The obturator canal (canalis obturatorius) transmits the obturator neurovascular bundle of the same name. It comprises:
- Obturator artery (a. obturatoria) — a branch of the internal iliac artery.
- Obturator vein (v. obturatoria) — accompanies the artery.
- Obturator nerve (n. obturatorius) — innervates the adductor muscles.
These structures emerge onto the medial surface of the thigh, providing blood supply and innervation to the adductors, gracilis muscle, and skin of the medial thigh.
To easily remember the three bones forming the hip bone (Ilium, Pubis, Ischium), one can visualize their spatial arrangement: the Ilium acts as the "roof" on top, the Pubis protects anteriorly, and we sit on the Ischium (inferiorly and posteriorly). All three meet right in the middle at the acetabulum.
Class and exam questions
Which 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 external surface of the hip bone, at the site of fusion of the bodies of the ilium, pubis, and ischium.
How does the false pelvis differ from the true pelvis?
The false pelvis is wider, formed by the iliac wings, and lacks an anterior bony wall. The true pelvis lies inferior to the pelvic brim and forms a closed bony ring.
What is the true conjugate?
This is a crucial obstetrical conjugate (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.
What else to study on this topic
- Detailed relief of the iliac crest: outer and inner lips, intermediate line
- Gluteal lines of the ilium and muscle attachment sites
- Topography and functional significance of the greater and lesser sciatic notches
- Obturator foramen: obturator groove and obturator tubercles
- Pelvimetry in obstetrics: methods for determining the diagonal and anatomical conjugates
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