Regional anatomy
The pelvis and perineum are interrelated regions associated with the pelvic bones and the terminal parts of the vertebral column. The pelvis is divided into two regions:
-The superior region related to upper parts of the pelvic bones and lower lumbar vertebrae is the false pelvis (greater pelvis) and is generally considered part of the abdomen.
- The true pelvis (lesser pelvis) is related to the inferior parts of the pelvic bones, sacrum, and coccyx, and has an inlet and an outlet.
The bowl-shaped pelvic cavity enclosed by the true pelvis consists of the pelvic inlet, walls, and floor.
This cavity is continuous superiorly with the abdominal cavity and contains and supports elements of the urinary, gastrointestinal, and reproductive systems.
The perineum is inferior to the floor of the pelvic cavity; its boundaries form the pelvic outlet. The perineum contains and supports the external genitalia and external openings of the genitourinary and gastrointestinal systems.
PELVIS
Bones
The bones of the pelvis consist of the right and left pelvic (hip) bones, the sacrum, and the coccyx. The sacrum articulates superiorly with vertebra LV at the lumbosacral joint.
The pelvic bones articulate posteriorly with the sacrum at the sacro-iliac joints and with each other anteriorly at the pubic symphysis.
Pelvic bone
The pelvic bone is irregular in shape and has two major parts separated by an oblique line on the medial surface of the bone :
- The pelvic bone above this line represents the lateral wall of the false pelvis, which is part of the abdominal cavity.
- The pelvic bone below this line represents the lateral wall of the true pelvis, which contains the pelvic cavity.
The linea terminalis is the lower two-thirds of this line and contributes to the margin of the pelvic inlet.
The lateral surface of the pelvic bone has a large articular socket, the acetabulum, which together with the head of the femur, forms the hip joint .
Inferior to the acetabulum is the large obturator foramen, most of which is closed by a flat connective
tissue membrane, the obturator membrane. A small obturator canal remains open superiorly between the membrane and adjacent bone, providing a route of communication between the lower limb and the pelvic cavity. The posterior margin of the bone is marked by two notches separated by the ischial spine The posterior margin terminates inferiorly as the large ischial tuberosity.
The irregular anterior margin of the pelvic bone is marked by the anterior superior iliac spine, the anterior inferior iliac spine, and the pubic tubercle.
Components of the pelvic bone Each pelvic bone is formed by three elements: the ilium,
pubis, and ischium. At birth, these bones are connected by cartilage in the area of the acetabulum; later, at between Ilium
Of the three components of the pelvic bone, the ilium is the most superior in position.
The ilium is separated into upper and lower parts by a ridge on the medial surface .
- Posteriorly, the ridge is sharp and lies immediately superior to the surface of the bone that articulates with the sacrum. This sacral surface has a large L-shaped facet for articulating with the sacrum and an expanded, posterior roughened area for the attachment of the strong ligaments that support the sacro-iliac joint .
Anteriorly, the ridge separating the upper and lower parts of the ilium is rounded and termed the arcuate line .
The arcuate line forms part of the linea terminalis and the pelvic brim.
The portion of the ilium lying inferiorly to the arcuate line is the pelvic part of the ilium and contributes to the wall of the lesser or true pelvis.
The upper part of the ilium expands to form a flat, fanshaped “wing,” which provides bony support for the lower abdomen, or false pelvis . This part of the ilium provides attachment for muscles functionally associated with the lower limb. The anteromedial surface of the wing
is concave and forms the iliac fossa. The external (gluteal surface) of the wing is marked by lines and roughenings and is related to the gluteal region of the lower limb (Fig. 5.4B).
The entire superior margin of the ilium is thickened to form a prominent crest (the iliac crest), which is the site of attachment for muscles and fascia of the abdomen,
Pubis
The anterior and inferior part of the pelvic bone is the pubis . It has a body and two arms (rami). The body is flattened dorsoventrally and articulates with the body of the pubic bone on the other side at
the pubic symphysis. The body has a rounded pubic crest on its superior surface that ends laterally as the prominent pubic tubercle.
The superior pubic ramus projects posterolaterally from the body and joins with the ilium and ischium at its base, which is positioned toward the acetabulum.
The sharp superior margin of this triangular surface is termed the pecten pubis (pectineal line), which
forms part of the linea terminalis of the pelvic bone and the pelvic inlet. Anteriorly, this line is continuous with the pubic crest, which also is part of the linea terminalis and pelvic inlet. The superior pubic ramus is marked on its inferior surface by the obturator groove, which forms the upper margin of the obturator canal.
- The inferior ramus projects laterally and inferiorly to join with the ramus of the ischium.
Ischium The ischium is the posterior and inferior part of the pelvic
bone . It has:
⌀ a large body that projects superiorly to join with the
ilium and the superior ramus of the pubis; and
- a ramus that projects anteriorly to join with the inferior ramus of the pubis.
The posterior margin of the bone is marked by a prominent ischial spine that separates the lesser sciatic notch, below, from the greater sciatic notch, above.
Coccyx
The small terminal part of the vertebral column is the coccyx, which consists of four fused coccygeal vertebrae and, like the sacrum, has the shape of an inverted triangle. The base of the coccyx is directed superiorly.
The superior surface bears a facet for articulation with the sacrum and two horns, or cornua, one on each side, that project upward to articulate or fuse with similar downward-projecting cornua from the sacrum. These processes are modified superior and inferior articular processes
that are present on other vertebrae. Each lateral surface of the coccyx has a small rudimentary transverse process, extending from the first coccygeal vertebra. Vertebral arches are absent from coccygeal vertebrae; therefore no bony vertebral canal is present in the coccyx.
Joints
Lumbosacral joints
The sacrum articulates superiorly with the lumbar part of the vertebral column. The lumbosacral joints are formed between vertebra LV and the sacrum and consist of:
- the two zygapophyseal joints, which occur between adjacent inferior and superior articular processes; and an intervertebral disc that joins the bodies of vertebrae LV and SI.
These joints are similar to those between other vertebrae, with the exception that the sacrum is angled posteriorly on vertebra LV. As a result, the anterior part of the intervertebral disc between the two bones is thicker than the posterior part.
The lumbosacral joints are reinforced by strong iliolumbar and lumbosacral ligaments that extend from the expanded transverse processes of vertebra LV to the ilium
and the sacrum, respectively .
Sacro-iliac joints
The sacro-iliac joints transmit forces from the lower limbs to the vertebral column. They are synovial joints between the L-shaped articular facets on the lateral surfaces of the sacrum and similar facets on the iliac parts of the pelvic bones . The joint surfaces have an irregular
contour and interlock to resist movement. The joints often become fibrous with age and may become completely ossified.
Each sacro-iliac joint is stabilized by three ligaments:
-the anterior sacro-iliac ligament, which is a thickening of the fibrous membrane of the joint
capsule and runs anteriorly and inferiorly to the joint
- the interosseus sacro-iliac ligament, which is the largest, strongest ligament of the three, is positioned immediately posterosuperior to the joint and attaches to adjacent expansive roughened areas on the ilium and sacrum, thereby filling the gap between the two bones
The most prominent feature of the ischium is a large tuberosity (the ischial tuberosity) on the posteroinferior aspect of the bone . This tuberosity is an important site for the attachment of lower limb muscles and for supporting the body when sitting.
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