26 The Pelvis

Pelvis Overview (Netter 330; Moore 327)

   The pelvis is the massive ring of bone formed by the two Os Coxae and the Sacrum (Netter 332, 333, 334). Within this bony ring, one would find the PELVIC CAVITY, which contains the pelvic viscera. In each sex, from anterior to posterior, one finds the bladder, the internal sex organs and the rectum.

The bony walls of the pelvis do not form a complete bowl, and so there is a muscular “sling” at the bottom of the pelvic cavity to stop everything from falling out. This is the PELVIC DIAPHRAGM, a group of muscles which arise from the pelvic bones and meet in the midline to form the “bowl-like” closure of the inferior outlet.

From the superior view of the pelvis, you should be able to identify the following:

1. iliac crest
2. anterior superior iliac spine
3. anterior inferior iliac spine
4. acetabulum
5. obturator foramen
6. ischiopubic ramus
7. pubic tubercle
8. pectineal line of the pubis
9. pubic crest
10. pubic symphysis
11. pelvic brim (separates the true from the false pelvis)
12. iliac fossa
13. sacral promontory
14. sacrum
    • anterior sacral foramen
    • ala of sacrum
15. coccyx
16. ischial spine

The pelvic brim extends from promontory of the sacrum, arcuate line of the ilium, pectineal line (pectin of pubis) and pubic crest. The pelvis may be divided into a greater (or false) pelvis and lesser (or true) pelvis. They are separated by using the pelvic brim as the limiting line. The greater pelvis is located above the pelvic brim and the lesser pelvis below the brim.

In the image above, the pelvis is shown as it would be in the erect posture. The anterior superior iliac spine and pubic tubercle are in the same vertical plane.

From a lateral view, identify the:

1. sacrum
2. posterior superior iliac spine
3. iliac crest
4. tubercle of the crest
5. anterior superior iliac spine
6. anterior inferior iliac spine
7. pubic tubercle
8. inferior pubic ramus
9. superior pubic ramus
10. ischial tuberosity
11. greater sciatic notch
12. ischial spine
13. lesser sciatic notch
14. obturator foramen

Strong ligaments are necessary to hold the hip bone to the sacrum (Netter 333). These are found anteriorly and posteriorly. Anteriorly, you can identify the anterior sacroiliac ligaments.

Posteriorly, there are even stronger ligaments:

• sacrotuberous
• sacrospinous
• posterior sacroiliac

The pelvic diaphragm (see below) supports the pelvic viscera in both sexes.

The other important anatomical area of this region is the PERINEUM, which is found on the inferior aspect of the pelvic diaphragm. The perineum is basically speaking the area of the external sexual organs and the anus, between the thighs. The perineum too has its own muscular diaphragm, the UROGENITAL DIAPHRAGM, which stretches between the two ischiopubic rami in the anterior half of the perineum. It is inferior (thus superficial) to the pelvic diaphragm; the pelvic diaphragm is superficially covered by much fat — the ISCHIORECTAL FAT PADS, which lie anteriorly between the pelvic and urogenital diaphragms and posteriorly between pelvic diaphragm and skin.

 

PELVIC DIAPHRAGM (Netter 335, 336, 337, 338, 339)

The lateral wall of the pelvis is formed by obturator internus muscle and its fascia. The pudendal canal is split in fascia to allow passage of internal pudendal artery and pudendal nerve.

Netter 335, 336– female pelvis Levator ani (iliococcygeus portion) arises from the fascia of the internal obturator. The Ischiorectal fossa  (Netter 370) is a fat-filled potential space between internal obturator muscle and levator ani. The retropubic space lies just superior to levator ani muscle. The puborectalis is actually a part of the pubococcygeus muscle that wraps around the posterior aspect of the rectum forming a sling that holds the rectum forward in the pelvis.

The pubococcygeus and iliococcygeus muscles make up the levator ani. The muscles of the levator ani are important supportive muscles for the midline organs of the pelvis. Any weakness in these muscles can cause clinical problems of urinary or fecal incontinence

Netter 338, 339 – Male pelvis The male pelvic muscles are the same as the female except that there is no vagina to support in the male.

Netter 370 –  Coronal section of the pelvis illustrating the main part of the funnel-shaped pelvic diaphragm formed mainly by the two levator ani muscles. The pelvic diaphragm forms the floor of the abdominal and pelvic cavities and consists of the paired levatores ani and coccygeus muscles  together with their superior and inferior fasciae. The rectum is anchored to the pelvic diaphragm in the middle. Observe that only the pelvic diaphragm (levator ani portion) intervenes between the ischiorectal fossa and the retropubic space.

The common iliac arteries (Netter 376) each divide into an internal and an external iliac artery; the external artery continues into the thigh as the femoral artery. The vessels that supply the pelvis and perineum arise from the INTERNAL ILIAC ARTERIES. The internal iliac artery of each side descends into the pelvic cavity giving off branches to supply the gluteal region (superior and inferior gluteal arteries), the medial thigh (obturator artery), the perineum (internal pudendal artery), and the viscera of the pelvis (vesical, rectal, uterine, vaginal, etc. arteries).

The veins (Netter 377) draining the pelvic viscera form an anastomotic basket lining the walls of the pelvic cavity; various portions of this network are singled out, e.g., the prostatic plexus of veins Batson’s plexus-– but is functionally one network. This is clinically important in the spread of tumor cells. Finally, the draining veins unite to form the internal iliac veins of each side, which unite with the corresponding external iliac veins to form the common iliac veins to the inferior vena cava.

FEMALE PELVIS-  MEDIAN (SAGITTAL) SECTION (Netter 340, 346)

Note the anterior to posterior relationships of: pubic symphysis with anterior abdominal wall muscles, bladder/urethra and uterus/vagina and posteriorly the rectum/anal canal. Note the following peritoneal relatlons: l) anterior abdominal wall muscles 2) pubic symphysis 3) supravesicle (bladder) 4) vesicouterine pouch (tends to disappear   as bladder fills) 5) posterior fornix of vagina 6) rectouterine pouch (Pouch of Douglas) 7) lateral and anterior walls of rectum 8) sigmoid mesocolon

  

Clinical  Note: The rectouterine pouch (Pouch of Douglas) is the most inferior extent of peritoneum in the female. It is a frequent location for ectopic pregnancy. Collection of fluid (blood, pus, etc. ) will tend to pool in this pouch and may be palpated with a finger in the vagina and another in the rectum.

 

A better understanding of the relationships to the broad ligament can be gained if you also look at a section through the broad ligament. In Netter 340 you are looking at the posterior aspect of the broad ligament and the posterior wall of the vagina has been opened up.

Netter 341 -: Drawing of a posterior view of the uterus, ovaries, uterine tubes, and related structures. On the left side the broad ligament of the uterus is removed, thereby setting free the uterine tube, the round ligament of the uterus, and the ligament of the ovary. These three structures are attached to the side of the uterus close together, at the junction of its fundus and body. On the right side is the ‘mesentery” of the uterus and uterine tube, called the broad ligament. Observe that the ovary is attached (1 ) to the broad ligament by a mesentery of its own called the mesovarium; (2) to the uterus by the ligament of the ovary; and (3) near the pelvic brim by the suspensory ligament of the ovary which transmits the ovarian vessels. The part of the broad ligament above the level of the mesovarium is called the mesosalpinx.

 Major blood supply to upper vagina and body of uterus comes through the uterine artery, a branch from the internal iliac artery (Netter 380). The ovary receives arterial blood through the ovarian artery, which branches directly from the aorta just inferior to the renal arteries.

There is a substantial collateralization between the two vessels in the area where the uterine tube enters the wall of the uterus.

Clinical Notes:

Uterine prolapse (Netter 348a)  is the protrusion of the cervix into the lower part of the vagina close to the vestibule and causes a bearing down sensation in the uterus and an increased frequency of and burning sensation on urination. The prolapse occurs as a result of advancing age and menopause and results from weakness of the muscles, ligaments and fascia of the pelvic floor such as the pelvic diaphragm, urogenital diaphragm, ovarian and cardinal (transverse cervical) ligaments and broad and round ligaments of the uterus that constitute the support of the uterus and other pelvic viscera. Note also the changes in the uterus with age (Netter 353a)

Fibromyoma or leiomyoma (Netter 342c) is the most common benign neoplasm of the female genital tract derived from smooth muscle. A fibroid is a benign uterine tumor made of smooth muscle cells and fibrous connective tissue in the wall of the uterus. A large fibroid can cause bleeding, pressure and pain in the pelvis, heavy menstrual periods and infertility.

Endometriosis (Netter 342b) is a benign disorder in which a mass of endometrial tissue occurs aberrantly in various locations, including the uterine wall, ovaries, or other extraendometrial sites. It frequently forms cysts containing altered blood. A video showing ann endometrial biopsy may be viewed by clicking here – from the NEJM’s series of  Videos in Clinical Medicine

Endometrial cancer is the most common type (about 90%) of uterine cancer and develops from the endometrium of the uterus usually in the uterine glands. Its main symptom is vaginal bleeding which allows for early detection; other symptoms are clear vaginal discharge, lower abdominal pain and pelvic cramping.

Cervical_cancer is a slow-growing cancer that develops from the epithelium covering the cervix. The major risk factor for development of cervical cancer is human papillomavirus (HPV) infection. Cancer cells grow upward toward the endometrial cavity, downward toward he vagina and laterally toward the pelvic wall, invadinig the bladder and rectum directly. A Papanicolau (Pap) smear or cervical smear test is effective in detecting cervical cancer early. The cancer metastasizes to extrapelvic lymph nodes, liver, lung and bone and can be treated by surgical removal of the cervix or by a hysterectomy.

Hysterectomy is surgical removal of the uterus, performed either through the abdominal wall or through the vagina. It may result in injury to the ureter which lies in the transverse cardinal ligament beneath the uterine artery.

Ovarian cancer develops from germ cells that produce ova, stromal cells that produce estrogen and progesterone and epithelial cells that cover the outer surface of the ovary. Its symptoms include a feeling of pressure in the pelvis or changes in bowel or bladder habits. Metastasis occurs via lymph and blood vessels or by direct spread to nearby structures. Diagnosis involves feeling a mass during a pelvic examination, visualizing it by usiing an ultrasound probe placed in the vagina or using a blood test for a protein associated with ovarian cancer (CA-125). Some germ cell cancers release certain protein markers such as human chorionic gonadotropin (HCG) and alpha-fetoprotein (AFP) into the blood. Ovarian cancev causes several signs and symptoms such as ususual vaginal bleeding, postmenopausal bleeding, bleeding after intercourse and pain during intercourse, pelvic pressure, abdominal and pelvic pain, back pain, indigestion and loss of appetite. It can be treated by surgical removal of the ovary, uterine tubes and uterus.

 

Lymphatics of Uterus and Vagina (Netter 384)

External portions of rectum and vagina drain via superficial inguinal lymph nodes to superior, anterior thigh, then follow external iliac nodes.

Upper vagina, uterus, and ovary pass through broad ligament to internal iliac nodes and lateral sacral nodes, then follow up common iliac to point of bifurcation on the promontory.

MALE PELVIS – MEDIAN (SAGITTAL) SECTION (Netter 344)

This is the male pelvis as seen on sagittal section. Along with this image is a small image of the pelvic skeleton seen from the midline. You should always find something easy to identify so that you can tell where the front and back of the diagram are. I usually start by looking for the pubic symphysis for the front and sacrum for the back. Starting from the pubic symphysis, work your way back and identify the following structures: pubic symphysis retropubic space pubovesical and puboprostatic ligaments urinary bladder prostate urethra rectovesical space (pouch) rectum presacral space

Look at Netter 348, 345. A coronal section of the male pelvis to show the genitourinary organs from behind.

Note the following relationships:

1) Prostate gland resting on superior surface of levator ani (pubococcygeus) muscle with urethra passing through urogenital hiatus of muscle.

2) Vas deferens entering posterior aspect of prostate in mid-section of the gland.

3) Ureter passes inferior and then medial to vas deferens to enter posterolateral aspect of bladder.

4) Bulbo-urethral (Cowper’s) glands lying within substance of U.G. diaphragm.

5) Bulb of penis located in superficial pouch (inferior to perineal membrane).

6) Obturator internus muscle forming lateral wall of ischiorectal fossa (Netter 372) (anterior recess, levator ani muscle forming medial wall of ischiorectal fossa and U.G. diaphragm forming “floor” of anterior recess of ischiorectal fossa).

 

Bladder and Prostate (Netter 362)

The urinary bladder is a musculomembranous sac whose shape is affected by factors including the person’s age and sex – as well as the volume of urine it contains at the time.It collects urine excreted by the kidneys prior to disposal by urination. A hollow muscular, and distensible (or elastic) organ, the bladder sits on the pelvic floor. Urine enters the bladder via the ureters and exits via the urethra.

Embryologically, the bladder is derived from the urogenital sinus and, it is initially continuous with the allantois. In males, the base of the bladder lies between the rectum and the pubic symphysis. It is superior to the prostate, and separated from the rectum by the rectovesical pouch. In females, the bladder sits inferior to the uterus and anterior to the vagina. It is separated from the uterus by the vesicouterine pouch.

The superior” or “abdominal” surfaces, and the “lateral” surfaces of the bladder are covered by peritoneum, so most of the bladder is referred to as a “sub-peritoneal” structure.

The detrusor muscle is a layer of the urinary bladder wall made of smooth muscle fibers arranged in spiral, longitudinal, and circular bundles. When the bladder is stretched, this signals the parasympathetic nervous system to contract the detrusor muscle. This encourages the bladder to expel urine through the urethra.

For the urine to exit the bladder, both the autonomically controlled internal sphincter and the voluntarily controlled external sphincter must be opened. Problems with these muscles can lead to incontinence. If the amount of urine reaches 100% of the urinary bladder’s capacity, the voluntary sphincter becomes involuntary and the urine will be ejected instantly.

The urinary bladder usually holds 300-350 mL of urine; a full adult bladder holds about 500mL of urine, 15 times its empty volume. As urine accumulates, the rugae flatten and the wall of the bladder thins as it stretches, allowing the bladder to store larger amounts of urine without a significant rise in internal pressure

The fundus of the urinary bladder is the base of the bladder, formed by the posterior wall. It is lymphatically drained by the external iliac lymph nodes. The peritoneum lies superior to the fundus.

The trigone is a smooth triangular region of the internal urinary bladder formed by the two ureteral orifices and the internal urethral orifice. The area is very sensitive to expansion and once stretched to a certain degree, the urinary bladder signals the brain of its need to empty. The signals become stronger as the bladder continues to fill. Embryologically, the trigone of the bladder is derived from the caudal end of mesonephric ducts, which is of mesodermal origin (the rest of the bladder is endodermal). In the female the mesonephric ducts regress, causing the trigone to be less prominent, but still present.

Netter 362 represents a split in the anterior wall of the prostatic urethra and bladder, thus exposing the interior and posterior walls.

Note the urethral crest leading to the seminal colliculus (“verumontanum” Netter 363) which is an elevation of the posterior wall of the prostatic urethra. On the summit of the elevation is the blind pouch (utriculus masculinus or utricle) which represents the terminal portion of the embryologic paramesonephric ducts. The ejaculatory ducts (vas deferens and seminal vesicles) open onto the summit on either side of the utricle. Within the depressions on either side of the veru-montanum, one notes numerous openings for the prostatic ducts. It is into this region that the components of the semen (sperm, fluid from prostate and seminal vesicles) are deposited at the time of emission.

 

Distention of the urethra due to the presence of the semen causes the sensation of “ejaculatory inevitability” and a reflex arc through pudendal nerve brings about clonic contraction of the muscles in the perineal region to expel the semen – ejaculation. Thus, a “sexual episode” is: 1) erection: blood filling cavernous bodies of penis (parasympathetic), 2) emission: contraction of smooth muscle in walls of vas deferens, seminal vesicles, prostate to deposit semen in urethra (sympathetic), 3) ejaculation proper: clonic contraction of muscles in perineum to expel semen through urethra (parasympathetic – pudendal nerve), and 4) detumescence: return of penis to flaccid state through loss of blood from cavernous bodies (sympathetic). Note within posterior wall of bladder the “trigone” area: supero-lateral orifices of ureters and inferior-median urethral opening into prostatic urethra.

Clinical Correlation: Urination (aka micturation) is initiated by stimulating stretch receptors in the detrusor muscle in the bladder wall by the increasing volume (about 300 ml for adults) of urine. Afferent (sensory) impulses arise from these receptors in the bladder wall and enter the spinal cord (S2 – S4) via the pelvic splanchnic nerves. Urination can be assisted by contraction of the abdominal muscles, which increases the intra-abdominal and pelvic pressures. It also involves the following processes:   Sympathetic fibers induce the relaxation of the bladder wall and constrict the internal sphincter, inhibiting emptying. They may also activate the detrusor to prevent the reflux of semen into the bladder during ejaculation). (However, certain drugs that help to alleviate the symptoms of benign prostatic hypertrophy (BPH), such as Flomax, over time, may counteract this effect and allow semen to pass back into the bladder during ejaculation, giving rise to retrograde ejaculation) Parasympathetic preganglionic fibers in the pelvic splanchnic nerves synapse in the inferior hypogastric plexus; postganglionic fibers to the bladder musculature induce a reflex contraction of the detrusor muscle and relaxation of the internal sphincter, enhancing the urge to void. (This is a common problem for men who have BPH and overactive bladders) Somatic motor fibers in the pudendal nerve cause voluntary relaxation of the external urethral sphincter, and the bladder begins to void (often causing embarrassment for men with BPH). At the end of micturation, the external urethral sphincter contracts and bulbospongiosus muscles in the male expel the last few drops of urine from the urethra. (However, “No matter how you shake and dance, you always leave some in your pants” – from Dana Stearns)

Prostate Gland (Netter 362)

The prostate gland is a cone-shaped gland about the size of a chestnut and is made up of connective tissue and smooth muscle. Parts of relations of the gland are:

• the base is directed superiorly against the neck of the bladder
• the apex is directed inferiorly against the urogenital diaphragm
• the posterior surface is separated from the rectum by the rectovesical septum
• the anterior surface is separated from the pubic symphysis by the the retropubic space, that is filled with a venous plexus
• the lateral surfaces face the levator ani and a venous plexus
• it is made up of 5 lobes:
• • two lateral lobes
  • anterior lobe – in front of the prostatic urethra
  • middle lobe – behind the prostatic urethra and between the two ejaculatory ducts
  • posterior lobe

The function of the prostate is to store and secrete a slightly alkaline (pH ~7.29) fluid, milky or white in appearance, that usually constitutes 25-30% of the volume of the semen along with sperm and seminal vesicle fluid. The alkalinity of semen helps neutralize the acidity of the vaginal tract, prolonging the lifespan of sperm. The alkalinization of semen is primarily accomplished through secretion from the seminal vesicles. The prostatic fluid is expelled in the first ejaculate fractions together with most of the spermatozoa. In comparison with the few sperm expelled together with mainly seminal vesicular fluid those expelled in prostatic fluid have better motility, longer survival and better protection of the sperm.

The prostate also contains some smooth muscle that helps to expel semen during ejaculation.

Another important prostate function is controlling the flow of urine during ejaculation. A complex system of valves in the prostate sends the semen into the urethra during ejaculatory process and a prostate muscle called the sphincter seals the bladder, thereby preventing urine entry into the urethra

In Netter 362 note the relationship of the Rectovesical septum  (formed by Denonvillier’s fascia) to the prostate and rectum. This separates the rectum (posterior) from the genito-urinary tract (anterior). It is the adult structure formed from urorectal septum in the embryo. It runs from peritoneum forming the rectovesical pouch superiorly to the central tendinous point of the perineum (posterior margin of U.G. diaphragm).

Hypertrophy of the prostate (benign prostatic hyperplasia – BPH)  is a benign enlargement of the prostate that affects a high proportion of older men and most often occurs in the middle lobe, obstructing the urethral orifice and thus leading to nocturia (excessive urination at night), dysuria (difficulty in urination) and urgency (sudden desire to urinate).

Prostate cancer (Netter 364a) is a slow-growing cancer that occurs particularly in the posterior lobe. It is usually symptomless in the early stages but it can impinge on the urethra in the late stage. Prostate cancer spreads to the bony pelvis, pelvic lymph nodes, vertebral column and skull via the vertebral venous plexus, producing pain in the pelvis, lower back and the bones. This cancer also metastasizes to the heart and lungs through the prostatic venous plexus, internal iliac veins and into the inferior vena cava. It can be detected by digital rectal examination, ultrasound imaging with a device inserted into th rectum or PSA (prostate specific antigen)  test.

Seminal Vesicles (Netter 362)

The seminal vesicles  are a pair of  glands posteroinferior to the bladder of males. They  secrete a significant proportion of the fluid that ultimately becomes semen. The excretory duct of each seminal gland opens into the corresponding vas deferens as it enters the prostate gland and together they become the ejaculatory ducts.

 

 NERVES OF PELVIC VISCERA

The NERVES (Netter 388) of the pelvis are somatic (sacral and coccygeal plexuses) and autonomic. The autonomic innervation is composed of both the sympathetic and parasympathetic portions.

Sympathetic: The sympathetic nerves of the pelvis form two main groups: 1, the bilateral sympathetic trunks continuing from the abdomen into the pelvis, and 2, the continuation of the various preaortic plexuses into the abdomen .

1) The sympathetic trunks descend on each side just anteriorly to the sacral and coccygeal nerve roots, and give off gray rami communicantes to these nerves to be distributed to blood vessels, sweat glands, the erector pili muscles, and bones and joints of the pelvis and lower extremities.

2) The superior hypogastric plexus, found anterior to the bifurcation of the aorta, receives fibers from its superior neighbor, the inferior mesenteric plexus, and from the lower few lumbar splanchnic nerves (= branches of the sympathetic trunk). More importantly, as it descends into the pelvis the superior hypogastric plexus divides into two discrete nerves, the hypogastric nerves, which are the principal sympathetic supply to the bilateral inferior hypogastric plexuses. The inferior hypogastric plexuses, right and left, lie on the lateral surfaces of all the pelvic viscera, inside the ‘basket’ of pelvic veins .