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Pelvis

Contents

Overview

The pelvis is a bony structure that can be found in both male and female skeletons. The exception with this compound structure when compared to all other bones is that it has differences that are classified by sex, both for functional and general developmental reasons. The rest of the human skeleton differs only in size, which is genetically determined and is usually slightly larger in males than in females. The structure of the pelvis is designed to give females the ability to undergo pregnancy and childbirth, while males are able to hold larger and heavier muscles upon their frame.

Pelvis - ventral view

Therefore it is heavier in men and has more muscle attachments, a narrower pubic arch, subpubic angle and space between the ischial tuberosities which in turn makes the pelvic outlet smaller. The ilia in women are comparatively more flared than in men which makes their greater pelvis more shallow. The shape of the pelvic inlet and the obturator foramen is oval in women and heart shaped and round in men respectively. In general, the pelvis is broader in women so that there is ample space for the fetus to exit its mother's body.

Bones

The pelvis is formed by four bones which include a pair of hip bones otherwise known as innominate bones, the sacrum, which comes with the five lower sacral bones that are fused together and the coccyx which has four fused and a single individual terminal vertebra. The pelvic girdle consists of the hip bones and the sacrum and its function is to transmit the weight from the upper body to the lower limbs, while allowing the body to stay balanced. Meanwhile the ilium, the ischium and the pubis fuse together at puberty to form the innominate bones and are joined by the cartilage found in the acetabulum.

Recommended video: Bones of the pelvis
Overview of the different bones of the pelvis.

Joints

The pelvis as a compound structure contains four different joints, one of which is paired. The lumbosacral joints are compound joints that are stabilised by the iliolumbar ligaments and are made up of two posterior zygapophyseal joints and an intervertebral joint via an intervertebral disc between L4 and S1.

Lumbosacral joint - dorsal view

The sacrococcygeal joint is a secondary cartilaginous joint that connects the sacrum to the coccyx. It is reinforced via the anterior and posterior sacrococcygeal ligaments.

Sacrococcygeal symphysis - ventral view

The sacroiliac joint is an atypical synovial joint which contains fibrocartilage and has a very limited range of movement. Its articulatory surfaces are between the sacrum and the ilium. It is stabilised by the interosseous ligaments and by the anterior and posterior sacroiliac ligaments.

Sacroiliac joint - dorsal view

Lastly, the pubic symphysis is the joint that unites the pubic bones in a secondary cartilaginous articulation. It contains a fibrocartilaginous interpubic disc and is reinforced by the superior and inferior pubic ligaments.

Pubic symphysis - ventral view

Ligaments

The two main ligaments of the pelvis are the sacrotuberous and sacrospinous ligaments, which enclose the greater and lesser sciatic notches, forming the greater and lesser sciatic foramina. Due to natural forces, when in an orthostatic or upright stance, the weight of the upper body which is relayed via the spine rotates the sacrum and tips it backwards. However, this movement is prevented by the pelvic ligaments.

Sacrotuberous ligament - dorsal view

The sacrotuberous ligament extends from the dorsum of the lateral border of the sacrum as well as the posterior surface of the ilium to the ischial tuberosity and is the larger of the two ligaments. The sacrospinous ligament stretches between the lateral border of the sacrum to the ischial spine.

Sacrospinous ligament - dorsal view

Sciatic Foramina

These foramina are created by the positioning of bony notches and ligaments. The greater sciatic foramen is made up of the greater sciatic notch and the fibers of the sacrospinous and sacrotuberous ligaments. It contains:

Sciatic nerve - dorsal view

The lesser sciatic foramen is made up of the same ligaments as the greater foramina, however they come into contact with the bony lesser sciatic notch. It contains:

  • the tendon of the obturator internus
  • the nerve of the obturator internus
  • the pudendal nerve
  • the internal pudendal artery

Pudendal nerve - dorsal view

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Show references

References:

  • Frank H. Netter, MD, Atlas of Human Anatomy, Fifth Edition, Saunders - Elsevier, Chapter 5 Pelvis & Perineum, Subchapter 33. Bones & Ligaments, Guide Pelvis & Perineum: Bones and Ligaments, Page 168 to 170.
  • Arthur S. Schneider and Philip A. Szanto, Board Review Series Pathology, 1st Edition, Wolters Kluwer - Lippincott, Williams and Wilkins, Chapter 22, Musculoskeletal System, Page 346.

Author:

  • Dr. Alexandra Sieroslawska

Illustrators:

  • Pelvis - ventral view - Yousun Koh
  • Lumbosacral joint - dorsal view - Liene Znotina
  • Sacrococcygeal symphysis - ventral view - Liene Znotina
  • Sacroiliac joint - dorsal view - Yousun Koh
  • Pubic symphysis - ventral view - Liene Znotina
  • Sacrotuberous ligament - dorsal view - Liene Znotina
  • Sacrospinous ligament - dorsal view - Liene Znotina
  • Sciatic nerve - dorsal view - Liene Znotina
  • Pudendal nerve - dorsal view - Liene Znotina
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