Anatomy: Pelvis

Bony Pelvis

• The bony pelvis consists of four bones: two hip bones (innominate bones), sacrum, and coccyx.
• The two hip bones form the lateral and anterior walls, and the sacrum and coccyx form the posterior wall.

False Pelvis

• The false pelvis is of little clinical importance.
• It is bounded by the lumbar vertebrae behind, iliac fossae and iliacus muscles laterally, and the lower part of the anterior abdominal wall in front.
• The false pelvis flares out at its upper end and is considered part of the abdominal cavity.
• It supports the abdominal contents and, after the 3rd month of pregnancy, helps support the gravid uterus.
• During the early stages of labor, it helps guide the fetus into the true pelvis.

True Pelvis

• The true pelvis is the lower part of the pelvis, which is important for obstetrics.
• It forms a strong basin-shaped structure.
• It is divided into an inlet, outlet, and a cavity.
• The inlet is bounded by the sacral promontory posteriorly, the iliopectineal lines laterally, and the symphysis pubis anteriorly.

Pelvic Brim

• The pelvic brim divides the pelvis into two parts: the false pelvis and the true pelvis.
• The pelvic brim is formed by the sacral promontory, iliopectineal lines, and the symphysis pubis.

Pelvic Joints

• The four major joints in the adult pelvis include paired sacroiliac joints, the symphysis pubis, and the sacrococcygeal joint.
• The sacroiliac joints are strong synovial joints between the auricular surfaces of the sacrum and the iliac bones.
• The symphysis pubis is a cartilaginous joint between the two pubic bones.
• The sacrococcygeal joint is a cartilaginous joint between the bodies of the last sacral vertebra and the first coccygeal vertebra.

Muscles and Ligaments

• The levator ani muscles of the two sides form an efficient muscular sling that supports and maintains the pelvic viscera in position.
• The coccygeus muscle forms the smaller, more posterior portion of the pelvic diaphragm.
• The nerve supply to the pelvic floor muscles is from branches of the sacral spinal nerves.
• Ligaments surround the pelvic joints, providing support and stability.

Movements

• A small amount of movement is possible at the sacroiliac joints.
• Almost no movement is possible at the symphysis pubis.
• Extensive flexion and extension are possible at the sacrococcygeal joint.

Clinical Notes

• Joint changes occur during pregnancy, with softening of the symphysis pubis and sacroiliac joints.
• Sacroiliac joint disease can produce low back pain and pain referred along the sciatic nerve (sciatica).

Pelvic Nerves

• Pelvic nerves cross the front of the sacroiliac joint and run forward on the lateral pelvic wall in the angle between the internal and external iliac vessels.
• These branches supply the pelvis and perineum, as well as the lower abdomen and lower limb.

Lumbar Plexus

• The lumbar plexus lies in the posterior abdominal wall (see Chapter 7).
• Two branches, the lumbosacral trunk and the obturator nerve, have notable relations to the pelvis.

Obturator Nerve

• The obturator nerve reaches the obturator canal (i.e., the upper part of the obturator foramen, which is devoid of the obturator membrane), and then splits into anterior and posterior divisions.
• The distribution of the obturator nerve in the thigh is considered in Chapter 11.

Lumbosacral Trunk

• Part of the anterior ramus of the fourth lumbar nerve emerges from the medial border of the psoas muscle and joins the anterior ramus of the fifth lumbar nerve to form the lumbosacral trunk.

Sacral Plexus

• The sacral plexus lies on the posterior pelvic wall in front of the piriformis muscle (see Fig. 8.18).
• It is formed from the lumbosacral trunk (see above) and the anterior rami of the first to third sacral nerves.

Pelvic Branches

• Sensory branches supply the parietal peritoneum on the lateral wall of the pelvis.
• The internal iliac, superior rectal, ovarian, and median sacral arteries run into the pelvic cavity.

Pelvic Splanchnic Nerves

• The pelvic splanchnic nerves form the parasympathetic part of the autonomic nervous system in the pelvis.
• The preganglionic fibers arise from the second, third, and fourth sacral nerves and synapse in ganglia in the inferior hypogastric plexus or in the walls of the viscera.

Arteries

• The common iliac artery ends at the pelvic inlet in front of the sacroiliac joint by dividing into the external and internal iliac arteries (see Figs. 8.10 and 8.18).
• The external iliac artery runs along the medial border of the psoas muscle, following the pelvic brim (see Fig. 6.61).

Pelvis

• The bony pelvis consists of the paired hip bones (os coxae), the sacrum, and the coccyx.
• The pelvic brim divides the pelvis into an upper false pelvis and a lower true pelvis.
• The true pelvis has an inlet (pelvic brim) above and an outlet below.

Sacrum and Coccyx

• The sacrum and coccyx have apical processes of the sacrum (see Fig. 8.22) that are fused with each other in the midline to form the median sacral crest.
• The sacral hiatus is situated on the posterior aspect of the lower end of the sacrum, where the extradural space ends.

Surface Anatomy

• The pubic tubercle can be felt on the upper border of the pubis (see Figs. 8.20 to 8.22).
• The iliac crest can be felt through the skin along its entire length (Figs. 8.20 to 8.22).
• The symphysis pubis lies in the midline between the bodies of the pubic bones and can be palpated as a solid structure through the fat in this region.

Pelvis, Part II: Pelvic Cavity

• The pelvic cavity (cavity of the true pelvis) is the area between the pelvic inlet and the pelvic outlet.
• The pelvic diaphragm subdivides the pelvic cavity into the main pelvic cavity above and the perineum below.

Components of the Pelvic Cavity

• The pelvic cavity contains the lower ends of the intestinal and urinary tracts and the internal organs of reproduction.
• It also contains the nerve supply, blood supply, and lymphatic drainage of these organs.

Gastrointestinal Tract

• The sigmoid colon is part of the pelvic cavity.

Importance of the Pelvic Cavity in Medicine

• The physician often encounters problems involving infections, injuries, and prolapses of the rectum, uterus, and vagina.
• Emergency situations involving the bladder, the pregnant uterus, ectopic pregnancy, spontaneous abortion, and acute pelvic inflammatory disease are examples of problems found in the female.
• The urinary bladder and the prostate in the male are frequent sites of disease.

Urinary Tract Nervous Control

• Nerves that control the urinary tract originate from the hypogastric plexuses and enter the spinal cord at the first and second lumbar segments
• Efferent parasympathetic impulses leave the cord from the second, third, and fourth sacral segments and pass through the pelvic splanchnic nerve and the inferior hypogastric plexus to the bladder wall
• These impulses cause the detrusor muscle to contract and the sphincter vesicae to relax
• Somatic efferent impulses also pass to the external urethral sphincter via the pudendal nerve, causing relaxation
• Afferent impulses pass up the pelvic splanchnic nerve and enter the second, third, and fourth sacral segments of the spinal cord

Micturition

• In young children, micturition is a simple reflex action that occurs whenever the bladder becomes distended
• In adults, this reflex is inhibited by the cerebral cortex until the time and place for micturition are favorable
• Voluntary control of micturition is accomplished by contracting the sphincter urethrae, which closes the urethra
• This is assisted by the sphincter vesicae, which compresses the bladder neck
• Voluntary control of micturition normally develops during the 2nd or 3rd year of life

Ureteric Calculi

• Ureteric calculi are more likely to be arrested at anatomically narrowed areas, such as the junction of the renal pelvis and ureter, and where the ureter bends down into the pelvis
• When a calculus enters the lower pelvic part of the ureter, pain is often referred to the testis and the tip of the penis in males and the labium majus in females

Bladder Distention

• The normal adult bladder has a capacity of about 500 mL
• In the presence of urinary obstruction in males, the bladder may become greatly distended without permanent damage to the bladder wall
• In such cases, it is routinely possible to drain 1000 to 1200 mL of urine through a catheter

Urinary Retention

• In adult males, urinary retention is commonly caused by obstruction to the urethra by a benign or malignant enlargement of the prostate
• Acute urethritis or prostatitis can also be responsible
• In females, acute inflammation around the urethra (e.g., from herpes) is the only anatomic cause of urinary retention

Uterus

• The uterus is a hollow, pear-shaped organ with thick muscular walls, measuring approximately 3 inches (8 cm) long, 2 inches (5 cm) wide, and 1 inch (2.5 cm) thick.
• Divided into three parts: fundus, body, and cervix.
• The fundus is the part of the uterus above the entrance of the uterine tubes.
• The body is the part of the uterus below the entrance of the uterine tubes.
• The cervix is the lower, narrow part of the uterus that pierces the anterior wall of the vagina.

Uterine Positions

• In most women, the long axis of the uterus is bent forward at a 90-degree angle to the long axis of the vagina (anteversion).
• The body of the uterus is bent forward at the level of the internal os with the long axis of the cervix (anteflexion).
• In some women, the fundus and body of the uterus are bent backward on the vagina, lying in the rectouterine pouch (retroversion).
• If the body of the uterus is bent backward on the cervix, it is said to be retroflexed.

Uterine Wall and Cavity

• The cavity of the uterine body is triangular in coronal section, but merely a cleft in the sagittal plane.
• The cavity of the cervix, the cervical canal, communicates with the cavity of the body through the internal os and with the vagina through the external os.

Blood Supply and Lymph Drainage

• The uterine artery crosses the ureter on each side of the cervix.
• The uterine vein follows the artery and drains into the internal iliac vein.
• Lymph vessels from the fundus of the uterus accompany the ovarian artery and drain into the paraaortic nodes at the level of the first lumbar vertebra.
• Vessels from the body and cervix drain into the internal and external iliac lymph nodes.

Nerve Supply

• Sympathetic and parasympathetic nerves from branches of the inferior hypogastric plexuses innervate the uterus.

Relations

• Anteriorly: The body of the uterus is related to the uterovesical pouch and the superior surface of the bladder.
• Posteriorly: The body of the uterus is related to the rectouterine pouch (pouch of Douglas) with coils of ileum or sigmoid colon within it.

Broad and Round Ligaments

• The broad ligaments and the round ligaments of the uterus are lax structures and can be pulled up or pushed down for a considerable distance before they become taut.
• The round ligament of the uterus extends between the superolateral angle of the uterus and the subcutaneous tissue of the labium majus.

Development of the Uterus

• The fundus and body of the uterus remain small until puberty, when they enlarge greatly in response to estrogens secreted by the ovaries.

Clinical Notes

• In most women, the uterus is anteverted and anteflexed.
• A retroverted, retroflexed uterus can be palpated through the posterior vaginal fornix.
• Varicose veins and hemorrhoids are common conditions in pregnancy, caused by pressure of the gravid uterus on the inferior vena cava and inferior mesenteric vein, and increased progesterone levels in the blood.

Radiographic Anatomy of the Pelvis

• The sacrum, sacroiliac joints, hip bones, and upper ends of the femurs are visible in a radiograph of the pelvis.
• Gas and fecal material may be seen in the large bowel, and soft tissue shadows of the skin and subcutaneous tissues may also be visualized.

Cross-Sectional Anatomy of the Pelvis

• Study of labeled cross-sections of the pelvis (Figs. 9.34 and 9.35) assists in interpreting computed tomography (CT) scans of the pelvis (Fig. 9.36).
• Sections are photographed on their inferior surfaces.

Gastrointestinal Tract

• The pelvic colon and rectum can be demonstrated effectively following a barium enema.
• A distended sigmoid colon usually shows no haustral-like sacculations.
• The rectum has a wider caliber than the colon.
• A contrast enema is sometimes useful for examining the mucous membrane of the sigmoid colon.

Female Genital Tract

• Instillation of viscous iodine preparations through the external os of the uterus allows visualization of the lumen of the cervical canal, the uterine cavity, and the uterine tubes (Fig. 9.37).
• Hysterosalpingography demonstrates the patency of these structures by some of the opaque medium entering the peritoneal cavity.
• Sonography can be used to visualize the pelvic organs and the developing fetus (Figs. 9.38 to 9.40).

Urinary System

• The urinary bladder lies in the pelvic cavity and is surrounded by the peritoneum.
• Micturition involves the relaxation of the internal urethral sphincter and the contraction of the detrusor muscle.
• Spinal cord injuries may disrupt the process of micturition and produce an atonic bladder, an automatic reflex bladder, or an autonomous bladder.

Male Internal Genital Organs

• The male internal genital organs are the vas deferens, seminal vesicles, ejaculatory ducts, prostate, prostatic urethra, and membranous urethra.
• The vas deferens arises from the tail of the epididymis, passes through the inguinal canal, and joins the duct of the seminal vesicle to form the ejaculatory duct.

Female Internal Genital Organs

• The uterus is a hollow, pear-shaped organ divided into the fundus, body, and cervix.
• The cervix pierces and enters the upper end of the vagina.
• The long axis of the uterus is typically bent forward at about a 90-degree angle to the long axis of the vagina, in anteversion of the uterus.
• The uterus is supported by pelvic floor muscles (elevators) and pelvic fascia ligaments (transverse cervical, pubocervical, and sacrocervical ligaments).

Pelvic Cavity

• The pelvic cavity is a region of interest in a digital vaginal examination.
• The parietal peritoneum continues down from the anterior abdominal wall onto the upper surface of the urinary bladder and then turns back onto the anterior surface of the uterus, creating the shallow uterovesical pouch.
• The peritoneum also runs over the uterus onto the upper part of the posterior surface of the vagina, and then onto the front of the rectum, creating the rectouterine pouch.