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Pelvic Girdle: Hip Bones, Sacrum & Coccyx
Pelvic Girdle: Hip Bones, Sacrum & Coccyx
Includes labeled diagrams, downloadable notes, anatomy quizzes, and interactive learning tools
The pelvic girdle is formed by a single bone on each side called the hip bone. Each hip bone is firmly connected to the axial skeleton through its articulation with the sacrum of the vertebral column. The right and left hip bones also join to each other anteriorly. Together, the two hip bones, the sacrum, and the coccyx form the bony pelvis.
Unlike the pectoral girdle, which is highly mobile, the bones of the pelvis are strongly united to form a largely immobile structure. This immobility is important because it allows the pelvis to function as a stable, weight-bearing framework. The weight of the upper body is transferred from the vertebral column through the pelvis and into the lower limbs, especially when one limb is lifted. In this way, the pelvis provides a strong and stable foundation for the body.
Hip Bone
The hip bone is a large, curved bone that forms the lateral and anterior parts of the pelvis. Each hip bone is formed by the fusion of three separate bones:
the ilium
the ischium
the pubis
These bones are separate during early life but fuse together in the late teenage years. In the adult, their names are still used to describe the three regions of the hip bone.
These three parts meet at a deep, cup-shaped cavity called the acetabulum, which is located on the lateral side of the hip bone and forms part of the hip joint.
A large opening called the obturator foramen is located between the ischium and pubis and serves as a site for muscle attachment.
Ilium
The ilium is the superior and largest portion of the hip bone and has a broad, fan-like shape. It extends upward from the acetabulum and forms the lateral boundary of the greater pelvis. The upper border of the ilium is the iliac crest, which is curved and can be felt along the waist. The crest ends anteriorly at the anterior superior iliac spine and posteriorly at the posterior superior iliac spine.
Below the anterior superior iliac spine is the anterior inferior iliac spine. Similarly, below the posterior superior iliac spine is the posterior inferior iliac spine. These projections serve as important attachment points for muscles and ligaments.
The auricular surface of the ilium is located on its posterior aspect and articulates with the sacrum to form the sacroiliac joint. Behind this is a roughened area that provides attachment for strong ligaments that support this joint.
The internal surface of the ilium contains a large, smooth depression called the iliac fossa. Inferior to this is the arcuate line, which marks a change in curvature and contributes to the pelvic brim.
On the posterior border of the ilium is the greater sciatic notch, a large indentation that helps form an opening for the passage of structures from the pelvis to the lower limb.
Ischium
The ischium forms the posteroinferior part of the hip bone. It plays an important role in supporting the body when sitting. The large roughened area at its lower part is the ischial tuberosity, which bears weight in the seated position and serves as an attachment point for muscles.
Extending from the ischial tuberosity is the ischial ramus, which projects forward to join with the pubis. Above the tuberosity is the lesser sciatic notch, and between this and the greater sciatic notch is a projection called the ischial spine.
Pubis
The pubis forms the anterior part of the hip bone. Its medial portion, called the pubic body, joins with the pubic body of the opposite side at the pubic symphysis. A small projection called the pubic tubercle is located on the pubic body.
The pubis has two extensions called the superior and inferior pubic rami. The superior pubic ramus connects the pubis to the ilium, and along its upper edge runs the pectineal line. The inferior pubic ramus extends downward and joins the ischial ramus to form the ischiopubic ramus.
Together, the inferior pubic rami from both sides form an arch called the pubic arch, and the angle formed between them is known as the subpubic angle.
Pelvis
The pelvis is composed of four bones: the two hip bones, the sacrum, and the coccyx. Its main functions are to support the weight of the upper body, transfer this weight to the lower limbs, provide attachment for muscles, and protect the pelvic organs.
When standing, the pelvis is tilted slightly forward. In this position, the anterior superior iliac spines and the pubic tubercles lie in the same vertical plane, and the sacrum faces forward and downward.
The pelvis is divided into two regions. The greater pelvis is the broad, upper part and is associated with the abdominal cavity. The lesser pelvis is the lower, narrower part that contains the pelvic organs. The boundary between them is called the pelvic brim.
The pelvic brim forms the upper opening of the lesser pelvis, known as the pelvic inlet. The lower opening is called the pelvic outlet.
These openings define the shape and orientation of the pelvic cavity, which is curved and directed from anterosuperior (pelvic inlet) to posteroinferior (pelvic outlet).
The sacrum is a large, triangular bone formed by the fusion of five sacral vertebrae. It is located at the base of the vertebral column and forms the posterior part of the pelvis.
The upper end, known as the base, articulates with the fifth lumbar vertebra, while the lower end, called the apex, articulates with the coccyx. The sacrum is curved and positioned obliquely, with its base projecting forward.
The anterior (pelvic) surface of the sacrum is concave and marked by four transverse ridges that indicate the lines of fusion between the original vertebrae.
On either side are four anterior sacral foramina, which allow the passage of the anterior divisions of the sacral spinal nerves. The segments decrease in size from above downward, with the first segment being the largest.
The posterior surface is convex and irregular. It features a median sacral crest formed by the fused spinous processes, as well as posterior sacral foramina for the passage of the posterior divisions of the sacral nerves.
Lateral to these are roughened areas and crests formed by fused transverse and articular processes, which provide attachment for ligaments and muscles.
The lateral surfaces of the sacrum are broad at the top and narrow below. The upper part contains the auricular surface, which articulates with the hip bones at the sacroiliac joints, while the rough area behind it provides attachment for strong ligaments.
The sacral canal runs through the bone as a continuation of the vertebral canal and transmits sacral nerves.
The coccyx is formed from four fused vertebrae, although the number may vary. It is small and triangular and articulates with the sacrum.
The anterior surface is slightly concave, while the posterior surface is convex and marked by rudimentary processes. The first segment is the largest and resembles a vertebra, while the remaining segments decrease in size.
The coccyx provides attachment for ligaments and muscles and may bear weight when sitting.
Ligaments and Openings
The bones of the pelvis are stabilized by several strong ligaments. The sacroiliac joint is supported by the anterior and posterior sacroiliac ligaments, which connect the sacrum and ilium.
Additional support is provided by the sacrospinous ligament, which connects the sacrum to the ischial spine, and the sacrotuberous ligament, which connects the sacrum to the ischial tuberosity. These ligaments also help form openings in the pelvis.
The greater sciatic foramen is formed by the greater sciatic notch, sacrum, and sacrospinous ligament, while the lesser sciatic foramen is formed by the lesser sciatic notch along with the sacrospinous and sacrotuberous ligaments. These openings allow the passage of muscles, nerves, and blood vessels from the pelvis to the lower limb.
Need a recap? Take a look at the pelvic girdle anatomy notes here.
Need a recap? Take a look at the pelvic girdle anatomy notes here.
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References
References
Gray, H. (1918). Anatomy of the human body (W. H. Lewis, Ed.; 20th ed.). Lea & Febiger.
J. Gordon Betts, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix. (2013, April 25). Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/8-3-the-pelvic-girdle-and-pelvis.
J. Gordon Betts, Kelly A. Young, James A. Wise, Eddie Johnson, Brandon Poe, Dean H. Kruse, Oksana Korol, Jody E. Johnson, Mark Womble, Peter DeSaix. (2013, April 25). Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/7-3-the-vertebral-column.
Based on OpenStax, Anatomy and Physiology (2013), licensed under CC BY 4.0.
Access for free at https://openstax.org/books/anatomy-and-physiology/pages/1-introduction.
Content paraphrased; adaptations were made.
Sobotta, J. (1914). Atlas and text-book of human anatomy (J. P. McMurrich, Ed.; W. H. Thomas, Trans.). W.B. Saunders Company.
Image Credis
Image Credis
Images used in this guide and games are from the following source:
Sobotta, J. (1914). Atlas and text-book of human anatomy (J. P. McMurrich, Ed.; W. H. Thomas, Trans.). W.B. Saunders Company.
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