The perineum (Fig. 5.1) is inferior to the floor of the pelvic cavity; its boundaries form the pelvic outlet. The perineum contains the external genitalia and external openings of the genitourinary and gastrointestinal systems.

Functions

Contains and supports the bladder, rectum, anal canal, and reproductive tracts

Within the pelvic cavity, the bladder is positioned anteriorly and the rectum posteriorly in the midline.

As it fills, the bladder expands superiorly into the abdomen. It is supported by adjacent elements of the pelvic bone and by the pelvic floor. The urethra passes through the pelvic floor to the perineum, where, in women, it opens externally (Fig. 5.2A) and in men it enters the base of the penis (Fig. 5.2B).

Fig. 5.2 The pelvis and perineum contain and support terminal parts of the gastrointestinal, urinary, and reproductive systems. A. In women. B. In men.

Continuous with the sigmoid colon at the level of vertebra SIII, the rectum terminates at the anal canal, which penetrates the pelvic floor to open into the perineum. The anal canal is angled posteriorly on the rectum. This flexure is maintained by muscles of the pelvic floor and is relaxed during defecation. A skeletal muscle sphincter is associated with the anal canal and the urethra as each passes through the pelvic floor.

The pelvic cavity contains most of the reproductive tract in women and part of the reproductive tract in men.

In women, the vagina penetrates the pelvic floor and connects with the uterus in the pelvic cavity. The uterus is positioned between the rectum and the bladder. A uterine (fallopian) tube extends laterally on each side toward the pelvic wall to open near the ovary.

In men, the pelvic cavity contains the site of connection between the urinary and reproductive tracts. It also contains major glands associated with the reproductive system—the prostate and two seminal vesicles.

Anchors the roots of the external genitalia

In both genders, the roots of the external genitalia, the clitoris and the penis, are firmly anchored to:

the bony margin of the anterior half of the pelvic outlet, and

a thick, fibrous, perineal membrane, which fills the area (Fig. 5.3).

Fig. 5.3 The perineum contains and anchors the roots of the external genitalia. A. In women. B. In men.

The roots of the external genitalia consist of erectile (vascular) tissues and associated skeletal muscles.

Component parts

Pelvic inlet

The pelvic inlet is somewhat heart shaped and completely ringed by bone (Fig. 5.4). Posteriorly, the inlet is bordered by the body of vertebra SI, which projects into the inlet as the sacral promontory. On each side of this vertebra, wing-like transverse processes called the alae (wings) contribute to the margin of the pelvic inlet. Laterally, a prominent rim on the pelvic bone continues the boundary of the inlet forward to the pubic symphysis, where the two pelvic bones are joined in the midline.

Structures pass between the pelvic cavity and the abdomen through the pelvic inlet.

During childbirth, the fetus passes through the pelvic inlet from the abdomen, into which the uterus has expanded during pregnancy, and then passes through the pelvic outlet.

Pelvic walls

The walls of the true pelvis consist predominantly of bone, muscle, and ligaments, with the sacrum, coccyx, and inferior half of the pelvic bones forming much of them.

Two ligaments—the sacrospinous and the sacrotuberous ligaments—are important architectural elements of the walls because they link each pelvic bone to the sacrum and coccyx (Fig. 5.5A). These ligaments also convert two notches on the pelvic bones—the greater and lesser sciatic notches—into foramina on the lateral pelvic walls.

Fig. 5.5 Pelvic walls. A. Bones and ligaments of the pelvic walls. B. Muscles of the pelvic walls.

Completing the walls are the obturator internus and piriformis muscles (Fig. 5.5B), which arise in the pelvis and exit through the sciatic foramina to act on the hip joint.

Perineum

The perineum lies inferior to the pelvic floor between the lower limbs (Fig. 5.9). Its margin is formed by the pelvic outlet. An imaginary line between the ischial tuberosities divides the perineum into two triangular regions.

Anteriorly, the urogenital triangle contains the roots of the external genitalia and, in women, the openings of the urethra and the vagina (Fig. 5.9A). In men, the distal part of the urethra is enclosed by erectile tissues and opens at the end of the penis (Fig. 5.9B).

Posteriorly, the anal triangle contains the anal aperture.

Relationship to other regions

Abdomen

The cavity of the true pelvis is continuous with the abdominal cavity at the pelvic inlet (Fig. 5.10A). All structures passing between the pelvic cavity and abdomen, including major vessels, nerves, and lymphatics, as well as the sigmoid colon and ureters, pass via the inlet. In men, the ductus deferens on each side passes through the anterior abdominal wall and over the inlet to enter the pelvic cavity. In women, ovarian vessels, nerves, and lymphatics pass through the inlet to reach the ovaries, which lie on each side just inferior to the pelvic inlet.

Lower limb

Three apertures in the pelvic wall communicate with the lower limb (Fig. 5.10A):

the obturator canal,

the greater sciatic foramen, and

the lesser sciatic foramen.

The obturator canal forms a passageway between the pelvic cavity and the adductor region of the thigh, and is formed in the superior aspect of the obturator foramen, between bone, a connective tissue membrane, and muscles that fill the foramen.

The lesser sciatic foramen, which lies inferior to the pelvic floor, provides communication between the gluteal region and the perineum (Fig. 5.10B).

The pelvic cavity also communicates directly with the perineum through a small gap between the pubic symphysis and the perineal membrane (Fig. 5.10B).

Key features

The pelvic cavity projects posteriorly

In the anatomical position, the anterior superior iliac spines and the superior edge of the pubic symphysis lie in the same vertical plane (Fig. 5.11). Consequently, the pelvic inlet is angled 50°–60° forward relative to the horizontal plane, and the pelvic cavity projects posteriorly from the abdominal cavity.

Fig. 5.11 Orientation of the pelvis and pelvic cavity in the anatomical position.

Meanwhile, the urogenital part of the pelvic outlet (the pubic arch) is oriented in a nearly horizontal plane, whereas the posterior part of the outlet is positioned more vertically. The urogenital triangle of the perineum therefore faces inferiorly, while the anal triangle faces more posteriorly.

Important structures cross the ureters in the pelvic cavity

The ureters drain the kidneys, course down the posterior abdominal wall, and cross the pelvic inlet to enter the pelvic cavity. They continue inferiorly along the lateral pelvic wall and ultimately connect with the base of the bladder.

An important structure crosses the ureters in the pelvic cavity in both men and women—in women, the uterine artery crosses the ureter lateral to the cervix of the uterus (Fig. 5.12A), and in men, the ductus deferens crosses over the ureter just posterior to the bladder (Fig. 5.12B).

Fig. 5.12 Structures that cross the ureters in the pelvic cavity. A. In women. B. In men.

The prostate in men and the uterus in women are anterior to the rectum

In men, the prostate gland is situated immediately anterior to the rectum, just above the pelvic floor (Fig. 5.13). It can be felt by digital palpation during a rectal examination.

Fig. 5.13 Position of the prostate gland.

In both sexes, the anal canal and the lower rectum also can be evaluated during a rectal examination by a clinician. In women, the cervix and lower part of the body of the uterus also are palpable. However, these structures can more easily be palpated with a bimanual examination where the index and middle fingers of a clinician’s hand are placed in the vagina and the other hand is placed on the lower anterior abdominal wall. The organs are felt between the two hands. This bimanual technique can also be used to examine the ovaries and uterine tubes.

The perineum is innervated by sacral spinal cord segments

Dermatomes of the perineum in both men and women are from spinal cord levels S3 to S5, except for the anterior regions, which tend to be innervated by spinal cord level L1 by nerves associated with the abdominal wall (Fig. 5.14). Dermatomes of L2 to S2 are predominantly in the lower limb.

Fig. 5.14 Dermatomes of the perineum. A. In women. B. In men.

Most of the skeletal muscles contained in the perineum and the pelvic floor, including the external anal sphincter and external urethral sphincter, are innervated by spinal cord levels S2 to S4.

Much of the somatic motor and sensory innervation of the perineum is provided by the pudendal nerve from spinal cord levels S2 to S4.

Nerves are related to bone

The pudendal nerve is the major nerve of the perineum and is directly associated with the ischial spine of the pelvis (Fig. 5.15). On each side of the body, these spines and the attached sacrospinous ligaments separate the greater sciatic foramina from the lesser sciatic foramina on the lateral pelvic wall.

The pudendal nerve leaves the pelvic cavity through the greater sciatic foramen and then immediately enters the perineum inferiorly to the pelvic floor by passing around the ischial spine and through the lesser sciatic foramen (Fig. 5.15). The ischial spine can be palpated transvaginally in women and is the landmark for administering a pudendal nerve block.

Parasympathetic innervation from spinal cord levels S2 to S4 controls erection

The parasympathetic innervation from spinal cord levels S2 to S4 controls genital erection in both women and men (Fig. 5.16). On each side, preganglionic parasympathetic nerves leave the anterior rami of the sacral spinal nerves and enter the inferior hypogastric plexus (pelvic plexus) on the lateral pelvic wall.

Fig. 5.16 Pelvic splanchnic nerves from spinal levels S2 to S4 control erection.

The two inferior hypogastric plexuses are inferior extensions of the abdominal prevertebral plexus that forms on the posterior abdominal wall in association with the abdominal aorta. Nerves derived from these plexuses penetrate the pelvic floor to innervate the erectile tissues of the clitoris in women and the penis in men.

Muscles and fascia of the pelvic floor and perineum intersect at the perineal body

Structures of the pelvic floor intersect with structures in the perineum at the perineal body (Fig. 5.17). This poorly defined fibromuscular node lies at the center of the perineum, approximately midway between the two ischial tuberosities. Converging at the perineal body are:

the levator ani muscles of the pelvic diaphragm, and

muscles in the urogenital and anal triangles of the perineum, including the skeletal muscle sphincters associated with the urethra, vagina, and anus.

The course of the urethra is different in men and women

In women, the urethra is short and passes inferiorly from the bladder through the pelvic floor and opens directly into the perineum (Fig. 5.18A).

Fig. 5.18 Course of the urethra. A. In women. B. In men.

In men the urethra passes through the prostate before coursing through the deep perineal pouch and perineal membrane and then becomes enclosed within the erectile tissues of the penis before opening at the end of the penis (Fig. 5.18B). The penile part of the male urethra has two angles:

The more important of these is a fixed angle where the urethra bends anteriorly in the root of the penis after passing through the perineal membrane.

Another angle occurs distally where the unattached part of the penis curves inferiorly—when the penis is erect, this second angle disappears.

It is important to consider the different courses of the urethra in men and women when catheterizing patients and when evaluating perineal injuries and pelvic pathology.

Regional anatomy

The pelvis is the region of the body surrounded by the pelvic bones and the inferior elements of the vertebral column. It is divided into two major regions: the superior region is the false (greater) pelvis and is part of the abdominal cavity; the inferior region is the true (lesser) pelvis, which encloses the pelvic cavity.

The bowl-shaped pelvic cavity is continuous above with the abdominal cavity. The rim of the pelvic cavity (the pelvic inlet) is completely encircled by bone. The pelvic floor is a fibromuscular structure separating the pelvic cavity above from the perineum below.

The perineum is inferior to the pelvic floor and its margin is formed by the pelvic outlet. The perineum contains:

the terminal openings of the gastrointestinal and urinary systems,

the external opening of the reproductive tract, and

the roots of the external genitalia.

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 (Fig. 5.19A):

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 (Fig. 5.19B).

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 greater sciatic notch, and

the lesser sciatic notch.

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 16 and 18 years of age, they fuse into a single bone (Fig. 5.20).

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 (Fig. 5.21A).

Fig. 5.21 Components of the pelvic bone. A. Medial surface. B. Lateral 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 (Fig. 5.21).

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, fan-shaped “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. 5.21B).

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, back, and lower limb and terminates anteriorly as the anterior superior iliac spine and posteriorly as the posterior superior iliac spine.

A prominent tubercle, the tuberculum of the iliac crest, projects laterally near the anterior end of the crest; the posterior end of the crest thickens to form the iliac tuberosity.

Inferior to the anterior superior iliac spine of the crest, on the anterior margin of the ilium, is a rounded protuberance called the anterior inferior iliac spine. This structure serves as the point of attachment for the rectus femoris muscle of the anterior compartment of the thigh and the iliofemoral ligament associated with the hip joint. A less prominent posterior inferior iliac spine occurs along the posterior border of the sacral surface of the ilium, where the bone angles forward to form the superior margin of the greater sciatic notch.

In the clinic

Bone marrow biopsy

In certain diseases (e.g., leukemia), a sample of bone marrow must be obtained to assess the stage and severity of the problem. The iliac crest is often used for such bone marrow biopsies. The iliac crest lies close to the surface and is easily palpated.

A bone marrow biopsy is performed by injecting anesthetic in the skin and passing a cutting needle through the cortical bone of the iliac crest. The bone marrow is aspirated and viewed under a microscope. Samples of cortical bone can also be obtained in this way to provide information about bone metabolism.

Pubis

The anterior and inferior part of the pelvic bone is the pubis (Fig. 5.21). 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 (Fig. 5.21). 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.

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.

Sacrum

The sacrum, which has the appearance of an inverted triangle, is formed by the fusion of the five sacral vertebrae (Fig. 5.22). The base of the sacrum articulates with vertebra LV, and its apex articulates with the coccyx. Each of the lateral surfaces of the bone bears a large L-shaped facet for articulation with the ilium of the pelvic bone. Posterior to the facet is a large roughened area for the attachment of ligaments that support the sacro-iliac joint. The superior surface of the sacrum is characterized by the superior aspect of the body of vertebra SI and is flanked on each side by an expanded wing-like transverse process termed the ala. The anterior edge of the vertebral body projects forward as the promontory. The anterior surface of the sacrum is concave; the posterior surface is convex. Because the transverse processes of adjacent sacral vertebrae fuse lateral to the position of the intervertebral foramina and lateral to the bifurcation of spinal nerves into posterior and anterior rami, the posterior and anterior rami of spinal nerves S1 to S4 emerge from the sacrum through separate foramina. There are four pairs of anterior sacral foramina on the anterior surface of the sacrum for anterior rami, and four pairs of posterior sacral foramina on the posterior surface for the posterior rami. The sacral canal is a continuation of the vertebral canal that terminates as the sacral hiatus.

Coccyx

The small terminal part of the vertebral column is the coccyx, which consists of four fused coccygeal vertebrae (Fig. 5.22) 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.

In the clinic

Pelvic fracture

The pelvis can be viewed as a series of anatomical rings. There are three bony rings and four fibro-osseous rings. The major bony pelvic ring consists of parts of the sacrum, ilium, and pubis, which forms the pelvic inlet. Two smaller subsidiary rings are the obturator foramina. The greater and lesser sciatic foramina formed by the greater and lesser sciatic notches and the sacrospinous and sacrotuberous ligaments form the four fibro-osseous rings. The rings, which are predominantly bony (i.e., the pelvic inlet and the obturator foramina), are brittle rings. It is not possible to break one side of the ring without breaking the other side of the ring, which in clinical terms means that if a fracture is demonstrated on one side, a second fracture should always be suspected.

Fractures of the pelvis may occur in isolation; however, they usually occur in trauma patients and warrant special mention.

Owing to the large bony surfaces of the pelvis, a fracture produces an area of bone that can bleed significantly. A large hematoma may be produced, which can compress organs such as the bladder and the ureters. This blood loss may occur rapidly, reducing the circulating blood volume and, unless this is replaced, the patient will become hypovolemic and shock will develop.

Pelvic fractures may also disrupt the contents of the pelvis, leading to urethral disruption, potential bowel rupture, and nerve damage.

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 zygapophysial joints, which occur between adjacent inferior and superior articular processes, and

an intervertebral disc that joins the bodies of vertebrae LV and SI (Fig. 5.23A).

Fig. 5.23 Lumbosacral joints and associated ligaments. A. Lateral view. B. Anterior view.

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 (Fig. 5.23B).

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 (Fig. 5.24A). 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 (Fig. 5.24B);

the interosseous sacro-iliac ligament, which is the largest, strongest ligament of the three, and 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 (Fig. 5.24A,C); and

the posterior sacro-iliac ligament, which covers the interosseous sacro-iliac ligament (Fig. 5.24C).

Pubic symphysis joint

The pubic symphysis lies anteriorly between the adjacent surfaces of the pubic bones (Fig. 5.25). Each of the joint’s surfaces is covered by hyaline cartilage and is linked across the midline to adjacent surfaces by fibrocartilage. The joint is surrounded by interwoven layers of collagen fibers and the two major ligaments associated with it are:

Fig. 5.25 Pubic symphysis and associated ligaments.

the superior pubic ligament, located above the joint, and

the inferior pubic ligament, located below it.

In the clinic

Common problems with the sacro-iliac joints

The sacro-iliac joints have both fibrous and synovial components, and as with many weight-bearing joints, degenerative changes may occur and cause pain and discomfort in the sacro-iliac region. In addition, disorders associated with the major histocompatibility complex antigen HLA-B27, such as rheumatoid arthritis, psoriasis, and inflammatory bowel disease, can produce specific inflammatory changes within these joints.

Orientation

In the anatomical position, the pelvis is oriented so that the front edge of the top of the pubic symphysis and the anterior superior iliac spines lie in the same vertical plane (Fig. 5.26). As a consequence, the pelvic inlet, which marks the entrance to the pelvic cavity, is tilted to face anteriorly, and the bodies of the pubic bones and the pubic arch are positioned in a nearly horizontal plane facing the ground.

Differences between men and women

The pelvises of women and men differ in a number of ways, many of which have to do with the passing of a baby through a woman’s pelvic cavity during childbirth.

The pelvic inlet in women is circular (Fig. 5.27A) compared with the heart-shaped pelvic inlet (Fig. 5.27B) in men. The more circular shape is partly caused by the less distinct promontory and broader alae in women.

Fig. 5.27 Structure of the bony pelvis. A. In women. B. In men. The angle formed by the pubic arch can be approximated by the angle between the thumb and index finger for women and the angle between the index finger and middle finger for men as shown in the insets.

The angle formed by the two arms of the pubic arch is larger in women (80°–85°) than it is in men (50°–60°).

The ischial spines generally do not project as far medially into the pelvic cavity in women as they do in men.

True pelvis

The true pelvis is cylindrical and has an inlet, a wall, and an outlet. The inlet is open, whereas the pelvic floor closes the outlet and separates the pelvic cavity, above, from the perineum, below.

Pelvic inlet

The pelvic inlet is the circular opening between the abdominal cavity and the pelvic cavity through which structures traverse between the abdomen and pelvic cavity. It is completely surrounded by bones and joints (Fig. 5.28). The promontory of the sacrum protrudes into the inlet, forming its posterior margin in the midline. On either side of the promontory, the margin is formed by the alae of the sacrum. The margin of the pelvic inlet then crosses the sacro-iliac joint and continues along the linea terminalis (i.e., the arcuate line, the pecten pubis or pectineal line, and the pubic crest) to the pubic symphysis.

Pelvic wall

The walls of the pelvic cavity consist of the sacrum, the coccyx, the pelvic bones inferior to the linea terminalis, two ligaments, and two muscles.

Ligaments of the pelvic wall

The sacrospinous and sacrotuberous ligaments (Fig. 5.29A) are major components of the lateral pelvic walls that help define the apertures between the pelvic cavity and adjacent regions through which structures pass.

Fig. 5.29 Sacrospinous and sacrotuberous ligaments. A. Medial view of right side of pelvis. B. Function of the ligaments.

The smaller of the two, the sacrospinous ligament, is triangular, with its apex attached to the ischial spine and its base attached to the related margins of the sacrum and the coccyx.

The sacrotuberous ligament is also triangular and is superficial to the sacrospinous ligament. Its base has a broad attachment that extends from the posterior superior iliac spine of the pelvic bone, along the dorsal aspect and the lateral margin of the sacrum, and onto the dorsolateral surface of the coccyx. Laterally, the apex of the ligament is attached to the medial margin of the ischial tuberosity.

These ligaments stabilize the sacrum on the pelvic bones by resisting the upward tilting of the inferior aspect of the sacrum (Fig. 5.29B). They also convert the greater and lesser sciatic notches of the pelvic bone into foramina (Fig. 5.29A,B).

The greater sciatic foramen lies superior to the sacrospinous ligament and the ischial spine.

The lesser sciatic foramen lies inferior to the ischial spine and sacrospinous ligament between the sacrospinous and sacrotuberous ligaments.

Muscles of the pelvic wall

Two muscles, the obturator internus and the piriformis, contribute to the lateral walls of the pelvic cavity. These muscles originate in the pelvic cavity but attach peripherally to the femur.

Obturator internus

The obturator internus is a flat, fan-shaped muscle that originates from the deep surface of the obturator membrane and from associated regions of the pelvic bone that surround the obturator foramen (Fig. 5.30 and Table 5.1).

Table 5.1

Muscles of the pelvic walls

Muscle	Origin	Insertion	Innervation	Function
Obturator internus	Anterolateral wall of true pelvis (deep surface of obturator membrane and surrounding bone)	Medial surface of greater trochanter of femur	Nerve to obturator internus L5, SI	Lateral rotation of the extended hip joint; abduction of flexed hip
Piriformis	Anterior surface of sacrum between anterior sacral foramina	Medial side of superior border of greater trochanter of femur	Branches from SI, and S2	Lateral rotation of the extended hip joint; abduction of flexed hip