Osseous Anatomy

The pelvic girdle consists of two coxal bones. As seen on an anterior-posterior radiograph of the pelvis, these two bones unite anteriorly at the symphysis pubis and
posteriorly at the sacrum.  3-D views of the pelvis can be made from a CT scan by electronically "erasing" the soft tissues and then stacking the slices. The
sacrum, coccyx,  and the two coxal bones form the pelvis. The pelvis is divided into the greater, false, pelvis and the lesser, true, pelvis. The greater pelvis
lies superior of the pelvic brim.  It consists laterally of the ilium  and posteriorly of the superior portion of the sacrum. The anterior aspect has no

osseous components, it is formed by the abdominal wall. The lesser pelvis lies inferior and posterior to the pelvic brim. It is formed by the lower aspect of the ilium,
the pubis,  the lower aspect of the sacrum, and the coccyx. The superior opening of the lesser pelvis is known as the pelvic inlet,  while the inferior
opening is the pelvic outlet.

The two coxal bones grow from three separate bones, the ilium, ischium,  and pubis. The area where the three fuse is a deep, lateral fossa called the
acetabulum, which is more clearly shown on an oblique view.  The acetabulum is also the socket for the head of the femur.

The largest of the three divisions of the coxal bone is the ilium. The superior border of the ilium is known as the iliac crest, which is best seen in a lateral view.
It ends anteriorly at the anterior superior iliac spine  and posteriorly at the posterior superior iliac spine.  The anterior inferior iliac
and posterior inferior iliac spines  are located just inferior to their respective superior spines. These structures of the ilium are important because they are the
points of attachment for muscles of the abdominal wall. Inferior to the posterior inferior iliac spine is the greater sciatic notch.  The medial internal surface of
the ilium is concave. It is called the iliac fossa and is the point of attachment for the iliacus muscles.

The inferior, posterior portion of the coxal bone is the ischium.  It's most prominent features are an ischial spine,  a lesser sciatic notch, which lies just
below the spine,  and an ischial tuberosity, which is more fully displayed by rotating the pelvis to an oblique view.  The remainder of the ischium, joins
with the pubis to form the obturator foramen.

The pubis is the anterior and inferior part of the coxal bone. It consists of the superior ramus, the inferior ramus,  and the body. The symphysis pubis is the
joint that connects the two coxal bones at this area.
 

Ligamentous Anatomy

fibers that connects the two bones of the pubis.

The sacrotuberous and sacrospinous ligaments are best observed on a lateral view. The sacrotuberous ligament  is a cartilaginous attachment of the sacrum to
the ischial tuberosity. In the same respect, the sacrospinous ligament  connects the sacrum to the ischial spine.

Changing our view from a lateral to a posterior, the iliolumbar ligament  can be seen and the posterior sacroiliac ligaments  becomes apparent. The
sacroiliac ligament is a strong interosseous connection between the sacrum and ilium on the posterior surface. The sacrospinous  and sacrotuberous ligaments
are seen here, as well. All of these ligaments are supportive and connective in nature and serve to stabilize the muscles and organs within the pelvic girdle.
 

Functions of the Pelvis

The pelvis has three important functions. First, it transmits weight from the axial skeleton to the lower limbs in the standing position or to the ischial tuberosities when

sitting. Second, it provides attachments for numerous muscles which insert onto and control the lower limbs. Third, it houses the terminal parts of the digestive and

urinary tracts and the reproductive systems of males and females.
 

Muscular Anatomy

Sagittal and coronal cross-sectional images afford the best views and will be used throughout the MRI portion of pelvic anatomy. Each image is accompanied by a

reference in the upper right corner. Notice that the red line shows the level of the cross section. As the cross sections change throughout the anatomical descriptions,

make sure to check this reference. This information is necessary to correctly interpret the image and keep everything in perspective.

Pelvic Diaphragm

Laterally, anteriorly and posteriorly the pelvis is bony  and significant support is provided through ligaments. The floor of the pelvis is made of muscles and

fascia. The pelvic diaphragm, which consists of the levator ani and puborectalis muscles, are part of the pelvic floor. The pelvic floor closes the pelvic outlet and
supports the pelvic contents, including the uterus and vagina in females and the bladder and rectum in both males and females.

The pelvic diaphragm is a sheet of muscles that run from the anterior pelvis at the posterior surface of the symphysis pubis to the ischial spine and coccyx.

Anterior structures include levator ani, perineal body, perineal membrane, and several other small muscles. Membranes, as well as small structures and muscles are

hard to differentiate in MRI images.

The anterior fibers of the levator ani,  which can be seen as a thin strip of muscle on the lateral projection, pass caudal until they reach the perineal body,

which can not be seen on radiographs, so now we will take a look at some sketches. The perineal body is located between the vagina and anus in women and is a
central structure for the pelvic floor.  The perineal membrane consists of two layers of fibrous tissue forming a protective and supportive covering of the
anterior portion of the pelvic floor.  It has an opening for the urethra in both sexes and an opening for the vagina in females. The external genitalia are located
superficial to the perineal membrane.  The posterior portion of the perineal membrane attaches to the perineal body. There is no such membrane over the anal
portion of the lower pelvis posteriorly.  The perineal body is an important structure because it is the point of insertion for eight muscles, the deep transverse

perineal muscle, which runs from the perineal body to the ischial tuberosity, the superficial muscles of the perineal membrane, the external sphincter urethrae, the
external anal sphincter, which courses posterior and inserts onto the coccyx, and the anterior levator ani, as mentioned previously.

The posterior fibers of the levator ani also pass downward, around the anorectal junction, to the anococcygeal body and ligament. Returning to MRI images, the
portion of the levator ani that forms the sling around the anorectal junction is also known as the puborectalis muscle.  The anococcygeal body also separates

the external anal sphincter from the coccyx. The coccygeus is a small muscle of the pelvic diaphragm, which connects the ischial spine to the sacrum and the coccyx.
As the levator ani passes downward both anteriorly and posteriorly, it forms an arch of muscular tissue, also called a sling.  This can be best visualized in the

coronal projection, as seen on this MRI slice.

In males, the anterior portion of this arch is where the bladder and prostate gland are located. In females, the bladder, the uterus and the upper portion of the vagina
are supported by this sling.  The vagina and urethra pass between the anterior fibers of the levator ani to reach and pierce the perineal membrane, with the

urethra anterior to the vagina. Posteriorly, the muscles of the levator ani hold the anorectal junction in a sling. The space between the levator ani and the skin of the
posterior region of the pelvic outlet is filled with fat and is known as the ischiorectal fossa.

Functions of the Pelvic Diaphragm

While the muscles of the anterior pelvic floor support the urinary and reproductive organs, the posterior pelvic floor supports the rectum and anus with several other

muscles. Very few of these muscle are distinguishable on radiographic images, but they have important functions and need to be reviewed. Besides providing

support, these muscles control the action of the rectum and anus.

Rectum and Anus

The anal sphincter complex consists of an external anal sphincter, an internal anal sphincter and the puborectalis component of the levator ani. The external anal
sphincter is held laterally by the ischial spines, the sacrospinous ligaments and the sacrotuberous ligaments.  The external anal sphincter surrounds the full length
of the anal canal. The internal anal sphincter is located around the upper and middle portion of the anal canal. It is normally contracted to close the canal.  The

puborectalis, as mentioned previously, is the sling around the junction of the rectum and the anal canal. The rectum starts at the rectosigmoid junction and follows the
curve of the sacrum and coccyx until it ends just anterior and below the tip of the coccyx, where it meets the anus.  The lower part of the rectum is the rectal

ampulla. The anorectal junction is where the anal canal turns to run inferior and posterior from the rectum, forming an angle. This angle is called the anorectal angle.

Notice here, that at rest the angle is about 90 degrees. The anal canal ends at the anus, which is the external orifice, or opening, of the digestive tract.

Gluteal Muscles

Another important function of the pelvis is to serve as an attachment for muscles that control the lower extremities. There are several groups of muscles that are

involved in this function. One such group of muscles are called the gluteal muscles. They consist of the gluteus maximus, tensor fasciae latae, gluteus medius, gluteus

minimus, piriformis, obturator internus, superior gemellus, inferior gemellus, and the quadratus femoris.

The gluteus maximus arises from the posterior aspect of the iliac crest, the sacrum, the coccyx and the sacrotuberous ligament. It inserts onto the posterior aspect of

the femur and the lateral aspect of the thigh at the posterior portion of fascia lata. It is a superficial muscle that gives the buttock its shape and covers the gluteus
medius.

The gluteus medius arises from the lateral aspect of the ilium and inserts onto the lateral aspect of the greater trochanter. It is also covered by the fasciae lata. The
tensor fasciae latae arises posterior to the anterior superior iliac spine and inserts into the same region as the gluteus on the lateral aspect of the thigh.  It

continues as a strong fascia supporting the lateral thigh and eventually supports the lateral aspect of the knee. The gluteus minimus is deep to all of these muscles. It
arises from the lateral part of the ilium and inserts into the anterior aspect of the greater trochanter.

The piriformis originates from the anterior surface of the sacrum and inserts into the upper part of the greater trochanter, passing out of the pelvis through the greater
sciatic notch.  Lying inferior to the piriformis, the obturator internus muscle also arises within the pelvic girdle. It originates at the medial surface of the pubis,
covers the obturator foramen, and passes by the lesser sciatic notch to insert onto the greater trochanter laterally.

As the obturator internus passes through the lesser sciatic notch, leaving the pelvic cavity, it is joined by the superior gemellus, which originates at the ischium just

above the foramen, and the inferior gemellus, which also originates at the ischium, but below the foramen. The two gemelli muscles can not be appreciated on either
the sagittal or coronal MRI. The quadratus femoris lies inferior to the gemelli muscles.  It arises from the lateral aspect of the ischial tuberosity and inserts onto

the posterior aspect of the greater trochanter.

Actions of the Gluteal Muscles

The gluteal muscle group has a number of actions at the hip. The gluteus maximus extends and laterally rotates the hip. The gluteus medius and minimus abduct and

rotate the hip internally. The rest of the muscles externally rotate the hip.

Flexor Muscles

Another group of muscles of the pelvic girdle act to flex the hip, and include the psoas, the iliacus, a combination of the two called the iliopsoas, the pectineus, and

the sartorius muscles.

The psoas muscle originates from the sides of the 12th thoracic through the 5th lumbar intervertebral discs and the lateral and anterior sides of all five lumbar
vertebrae. It then runs caudal and into the pelvis where it has a triangular shape.  In the pelvis it combines with the iliacus muscle to form the iliopsoas. The

iliacus arises from the anterior and medial surface of the ilium. The iliopsoas then traverses the pelvis, exits anteriorly over the superior pubic ramus, and inserts into
the lesser trochanter of the femur.

The pectineus muscle originates at the iliopubic ramus. It inserts just distal to the lesser trochanter of the femur.  The sartorius arises from the anterior superior

iliac spine. It then runs inferiorly and medially all the way down and across the thigh until it inserts into the tibia, which is not shown in this view.

Actions of the Flexor Muscles

All of these muscles flex the hip. The iliopsoas also rotates the hip medially, the pectineus also adducts the hip, and the sartorius also laterally rotates the hip.

Abdominal Muscles

These muscles work in relation with those of the abdominal wall, which includes the quadratus lumborum, rectus abdominis, external oblique, internal oblique, and

transversus abdominis.

The quadratus lumborum is the posterior muscle of the abdominal wall.  It originates at the medial part of the iliac crest and iliolumbar ligament and inserts onto
the lowest rib through the 4th lumbar vertebrae. The rectus abdominis is the anterior muscle of the abdominal wall.  It arises from the middle ribs, passes down

over the abdomen and becomes more narrow distally as it inserts onto the pubis. The rectus abdominis is covered by a fibrous sheath, called the rectus sheath.

The rectal sheath also partially invests the external oblique, which is the outermost layer of muscle of the anterolateral wall of the abdomen. The external oblique
originates from the lowest eight ribs and inserts onto the iliac crest and the anterior superior iliac spine.  The internal oblique lies deep to the external oblique. It

arises from the iliac crest and lower lumbar spine, runs upwards and inserts into the lower four ribs. The deepest layer is the transversus abdominis and runs from the
lower six ribs to the iliac crest.

Actions of the Abdominal Muscles

The quadratus lumborum, if bilaterally activated, depresses the whole rib cage, or will cause lateral flexion of the vertebral column by contracting one side. The rectus

abdominis flexes the thoracic and lumbar spine. The external obliques, internal obliques and transversus abdominis assist in movement of the trunk with lateral flexion

and rotation. Their actions also control the intra-abdominal pressure. In coordination with the quadratus lumborum, rectus abdominis, diaphragm and pelvic

diaphragm, these muscles raise the intra-abdominal pressure in defecation.