Pelvis

What is Pelvis?

The pelvis is a basin shaped bony structure formed by the combination of two pelvic bones (hip bones or innominate bones) and the sacrum. It is strengthened and supported by several joints and ligaments. It provides attachment to some important muscles in the region, and forms a cavity which accommodates several important internal organs.

Where is the pelvis located?

Picture 1 : Location of Pelvis (with a fracture)

The pelvis is located between the fifth lumbar vertebra and the femoral heads. It forms an irregular bony girdle connecting the lower limbs to the trunk.

Anatomy

The bony pelvis

The bony pelvis is made up of two pelvic bones – the sacrum and the coccyx. Each pelvic bone (hip bone) is made by the combination three bones namely, the ilium, pubis, and ischium. These three bones fuse at a cup-shaped concavity called the acetabulum which articulates with the head of the femur to form the hip joint.

Picture 2: Features of the hip bone (right, posterolateral aspect)

• The ilium is a large, flat bone located in the superolateral aspect of the pelvis on either sides.
• Each ilium consists of a body and an ala. The body is located close to the upper margin of the acetabulum, and a flat plate-like ala extends upwards and laterally from the body.
• The uppermost margin of the ala is known as the iliac crest, which can be felt by palpating the lower parts of the flank on either sides.
• The most superior point of this curved crest is at a position just behind its midpoint, and corresponds to the L4 vertebral level.
• The anterior end of the crest forms a bony prominence called the anterior superior iliac spine, which can be palpated in the lower parts of the anterolateral aspect of either flank. Few centimeters below this is another bony prominence called the anterior inferior iliac spine.
• The posterior end of the iliac crest also forms a bony prominence called the posterior superior iliac spine, which can be felt at the dimple just above the buttock on either sides. This corresponds to the S2 vertebral level. Few centimeters below this point is another bony prominence called the posterior inferior iliac spine.
• The ala of each ilium consists of two surfaces: the inner (medial) surface and the outer (lateral) surface. The inner surface is concave medially, and forms the iliac fossa which demarcates the lateral wall of the greater pelvis (the upper division of the pelvis). At the lower end of this surface is a bony ridge extending from behind forwards, named the arcuate line. The outer surface is convex laterally and provides attachment to the gluteal muscles in the region.
• The posterior aspect of the ala is curved inwards to form a broad, rough region which is divided into two surfaces: the sacropelvic surface and the auricular surface. The auricular surface of each ilium articulates with the ala of the sacrum, to form the sacroiliac joints on either sides.

The pubis is located in the anteroinferior aspect of the pelvis. Each pubis consists of a body and two rami. The bodies are located medially, and are joined with one another by a cartilaginous joint called the symphysis pubis which lies in the midline. The anterior surface of the body has a bony ridge named the pubic crest, in its upper part. The lateral end of this crest forms the pubic tubercle. The posterior surface of the body has a sharp ridge extending laterally, named the pectineal line. The superior pubic ramus runs as a bony extension upwards and laterally from the body of the pubis, and joins the acetabulum. The inferior pubic ramus extends downwards and laterally to join the ramus of the ischium. The pubis and the ischium are separated by an oval opening called the obturator foramen. The angle formed between the two inferior pubic rami is called the subpubic angle.

The ischium is located in the posteroinferior aspect of the pelvis, and is comprised of a body and a ramus. The posteriorly located body has a spinous process in its upper end. The greater sciatic notch is formed between this spinous process and the posterior inferior iliac spine. The lower part of the body forms a laterally directed prominent rough surface named the ischial tuberosity. The lesser sciatic notch is formed between the ischial spine and the ischial tuberosity.

The sacrum is a roughly triangular bone formed by the fusion of five vertebrae. It has a central mass and two lateral masses (forming the ala) on either sides. The upper end of the sacrum has a forwardly projecting prominence named the sacral promontory.

The coccyx is also a roughly triangular bone formed by the fusion of three to five vertebrae, and it articulates with the lower end of the sacrum to form the sacrococcygeal joint.

The bony cavity formed by the combination of these bones is the pelvic cavity, and is divided into two parts by the pelvic brim (pelvic inlet) which is the oval shaped demarcation made by the continuation of the pectineal line of the pubis along with the arcuate line of the ilium, and the sacral promontory. The pelvic brim lies at an angle of 60° to the horizontal. The part of the cavity above the pelvic brim is called the false pelvis (greater pelvis/upper pelvis), and it accommodates lower abdominal contents such as the sigmoid colon and coils of small intestine. The cavity below the pelvic brim is called the true pelvis (lesser pelvis/lower pelvis), and it accommodates pelvic organs such as the urinary bladder, rectum, and genital organs. The lower end of the true pelvis is called the pelvic outlet, and it is demarcated anteriorly by the lower margin of the ischiopubic rami, laterally by the ischial tuberosities and the lower margin of sacrotuberous ligaments, and posteriorly by the tip of the coccyx.

Picture 3 & 4: The true pelvis and the false pelvis

Position of the pelvis

The pelvis is positioned in an anteriorly tilted manner, so that in the anatomical (erect) position, the anterior superior iliac spine and the upper margin of the pubis lie in the same vertical plane. This can be demonstrated by holding the aforementioned points against a wall.

Joints and ligaments of the pelvis

The two sacroiliac joints are synovial joints, and are further strengthened by the very strong posterior sacroiliac ligaments which run along the posterior aspect of the joint.

Picture 5: Joints and ligaments of the pelvis (anterior view)

The sacrospinous ligament extends from the ischial spine to the sides of the sacrum and coccyx, and it converts the greater sciatic notch into an opening called the greater sciatic foramen.

The sacrotuberous ligament extends from the ischial tuberosity to the sides of the sacrum and coccyx, and it converts the lesser sciatic notch into an opening called the lesser sciatic foramen.

The symphysis pubis Is further strengthened by fibrous ligaments along its superior and inferior surfaces.

Muscles of the pelvis

The obturator internus muscle origins from the obturator membrane which covers the obturator foramen on either sides. This muscle and its fascia, along with the hip bones, form the lateral walls of the pelvis.

The piriformis muscle origins from the anterior aspect of the middle three segments of the sacrum on either sides. The piriformis muscles and the sacrum together form the posterior wall of the pelvis.

The floor of the pelvis is formed by the two muscles named levator ani and coccygeus. These muscles origin in continuity from the body of the pubis, along a tendinous arch over the obturator internus fascia, and the ischial spine. Each of these muscles, together with their counterparts in the opposite side, form a gutter extending from above downwards, forming a sling around the pelvic visceral organs. This sheet of muscle is also called the pelvic diaphragm, and it separates the true pelvis from the perineum underneath.

Picture 6: The pelvic inlet

Picture 7: The pelvic outlet

Differences between the Male pelvis and the Female pelvis

The pelvis in males and females differ from one another in various features, due to the functional distinction between the two sexes. The male pelvis is more adapted to bear the heavier build of the upper body and for stronger muscle attachment, while the female pelvis is adapted to provide enough space for the birth canal (or birth passage) in the region. Some of these differences are compared in the table below.

	MALE	FEMALE
General Features	Heavy, Thick	Light, Thin
Muscle attachments	Well marked, prominent	Less well marked, less prominent
Greater pelvis	Narrow and deep	Wide and shallow
Shape of the pelvic inlet	Heart shaped	Transversely oval
Subpubic angle	Less than 60 degrees	More than 60 degrees
Sacrum	Evenly curved, Long and narrow	Less curved, Short and wide
Ischial tuberosities	Inturned	Everted
Acetabulum	Large concavity	Small concavity
Obturator foramen	Round in shape	Oval in shape

Functions of the pelvis

• Protects and supports the pelvic visceral organs
• Supports the weight of the body (transmitted along lumbar vertebrae) and transmits this force via the sacroiliac joints and the pelvic bones, to the acetabulum and then to the head of the femur on both sides
• Provides muscle attachment
• Aids in walking, by allowing swinging/rotatory movements at the lumbosacral joint and hence allowing the lower limbs to swing
• In females, forms a bony passage for the birth canal

Clinical significance

Pelvis fracture

Fractures of the pelvis may occur due to direct trauma or due to compression injuries. Direct trauma on the ilium, pubis, or sacrum with falls or road traffic accidents may cause fracture in the relevant bones.

Lateral compression injuries cause fractures in the pubic rami on both sides, or dislocation at the symphysis with fractures on one side. Anteroposterior compression also causes fracture of the pubic rami or dislocation of the symphysis pubis, or fracture of the rami associated with dislocation of the sacroiliac joint. Falls in the leg may cause central dislocation of the hip, pushing the head of the femur through the acetabulum.

Along with such fractures, internal organs in the region (urethra, urinary bladder, rectum etc.) may also get injured due to penetration by bone fragments. Along with pelvic displacements, the iliolumbar artery may also get ruptured as it traverses over the sacroiliac joint, and may lead to fatal internal hemorrhage.