For Most People, The Bony Pelvis Is Very Difficult To

Pelvis For most people, the bony pelvis is very difficult to visualize threedimensionally. Even artists have a difficult time getting it right. For this reason, it becomes important for anatomy students to spend some time identifying and memorizing the various parts of the pelvis before learning about the contents. In knowing the bony pelvis well, you will be more confident in visualizing how structures enter and leave the pelvis and how muscles of the pelvic floor are attached. You should look at the pelvis from all possible aspects. The hip bone is originally made up of three bones that have fused: 1)ilium, 2)ischium and 3)pubis. These come together at the acetabulum.

Bony Pelvis 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. Some people divide the pelvis 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. No muscle crosses the pelvic brim. If they did, they would be in the way during childbirth. Turn the pelvis over and identify the structures on the back: 1. sacrum ○

posterior sacral foramen

2. coccyx 3. posterior superior

iliac spine 4. iliac crest 5. anterior superior

iliac spine 6. tubercle of the crest 7. ischial tuberosity 8. acetabulum 9. ischiopubic ramus 10. pubic symphysis 11. obturator foramen 12. ischial spine 13. greater sciatic notch 14. lesser sciatic notch

From the 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

(not labeled)

In this image, 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. Looking at the pelvis from the inside, you should be able to identify the following items: 1. anterior superior

iliac spine 2. anterior inferior iliac

spine

3. pectineal line of

pubis 4. pubic tubercle 5. pubic symphysis

6. obturator foramen 7. ischial tuberosity 8. lesser sciatic notch 9. ischial spine 10. greater sciatic notch 11. articulation of

sacrum

12. posterior superior

iliac spine 13. iliac fossa 14. pelvic brim - not

labeled

Ligaments of the Pelvis Strong ligaments are necessary to hold the hip bone to the sacrum. 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 fifth lumbar vertebra also has a strong tie-in with the ilium through the iliolumbar ligament.

The sacrotuberous and sacrospinous ligaments complete the greater and lesser sciatic foraminae.

View of Pelvic Structures on Sagittal Section The best way to get a good idea of how the structures of the male and female pelvis are arranged is to view them on a sagittal section. That way you can see the way the different midline structures relate to one another.

Male Pelvis

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

•

rectum

•

presacral space

Note that, in the small diagram, two lines have been drawn. One from the sacral promontory to the upper pubic symphysis represents the pelvic inlet. Above this line is the false (or greater) pelvis and the abdominal cavity. The second line (2) extends from the coccyx to the lower border of the pubic

symphysis and represents the pelvic outlet. Below this line is the region called the perineum. Between the two lines is the true (or lesser) pelvis. This is the area we are interested in for now.

Female Pelvis In the sagittal section of the female pelvis, identify the following items, staring again from the front: •

pubic symphysis

•

retropubic space

•

urinary bladder ○

•

urethra

uterus ○

vagina

•

rectouterine pouch of Douglas

•

rectum

•

presacral space

Again the pelvic inlet and outlet is represented as two lines. You can see exactly what structures are within the lesser pelvis. Again, they are midline structures. Since, in both male and female, the organs are centrally located, that means that their blood and/or nerve supply must come in from laterally or posteriorly and we will find this to

be true when we examine the vasculature of the pelvis. We will also note that most of the muscles found in the pelvis lie laterally. These midline structures are supported by a musculature pelvic diaphragm which we will discuss in a moment.

Muscles of the Pelvis

Muscles of the female pelvis are the: •

pelvic diaphragm ○

pubococcygeus 

puborectalis

○

iliococcygeus

○

coccygeus

•

piriformis

•

iliacus

•

psoas major

The male pelvic muscles are the same as the female except that there is no vagina to support in the male. Identify the following: •

pelvic diaphragm ○

pubococcygeus 

puborectalis

○

iliococcygeus

○

coccygeus

•

piriformis

•

iliacus

•

psoas major

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.

Arteries of the Pelvis

With one exception, the arteries of the pelvis are branches of the internal iliac artery. The exception is the superior rectal artery which is a branch of the inferior mesenteric artery. Starting posteriorly, the branches of the internal iliac artery are as follows: •

iliolumbar

•

superior gluteal ○

lateral sacral

•

inferior gluteal

•

internal pudendal

•

middle rectal

•

inferior vescical (the uterine in the female)

•

obturator

•

superior vesical

•

terminal part of the internal iliac is occluded and becomes the lateral umbilical ligament of the lower anterior abdominal wall.

Nerves of the Pelvis

The nerves of the pelvis are derived from the: 1. lumbosacral

plexus 2. inferior

mesenteric plexus

3. sympathetic chain

The lumbosacral plexus is made up of: •

L4

•

L5

•

S1

•

S2

•

S3

•

○

L4 and L5 merge to form the lumbosacral trunk

○

L4, L5, S1, S2, S3 forms the sciatic nerve and other combination s form the superior and inferior gluteal

○

S2, S3, S4 join to form the pudendal nerve that supplies structures in the perineum.

S4

The inferior mesenteric plexus starts out in the

abdomen at the point of origin of the inferior mesenteric artery and passes along the aorta to the presacral region. As the plexus drops into the pelvis, it usually splits up into a right and left hypogastric plexus that lies behind the rectum. The sacral sympathetic chain is the continuation of the lumbar chain. The sacral part of the parasympathetic nervous system arises from S2, S3, S4 and supplies the pelvic structures as well as the left colic flexure, descending colon and sigmoid colon.

Urinary bladder Compare the male and female bladders.

Male bladder This image displays the male urinary bladder opened from the top and front and defining the: •

trigone of the bladder

•

interureteric fold

•

opening of the ureter

•

uvula of the vesical (beginning of the urethra)

•

urethral crest ○

seminal colliculus

○

opening of prostatic utricle

•

prostatic sinus (opening of ejaculatory ducts enter here)

•

membranous urethra

•

ureter

•

vas deferens

Female bladder In the female bladder, identify: •

ureter

•

interureteric fold

•

opening of the ureters

•

trigone

•

internal opening of urethra

•

vesical sphincter

Prostate Gland 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 cephalad against the neck of the bladder

•

the apex is directed caudad 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

If the prostate is opened up from the front, you can identify the following: •

urethral crest

•

seminal colliculus - a slightly enlarged part of the urehtral crest which open the ejaculatory ducts and an embryonic remnant, the prostatic utricle.

•

prostatic sinus - small valleys along side the crest into which the prostatic ducts open

Clinical Considerations 1. middle lobe: important clinically because enlargement of the

mucous glands in this lobe leads to obstruction. Adenomas are frequent in this lobe and they encroach into the urethra, blocking the internal urethral orifice.

2. posterior lobe: adenomas are rare; this lobe can be felt on rectal examination 3. anterior lobe: adenomas are rare; there is not encroachment on the urethra when this lobe enlarges 4. lateral lobe: enlargement of lateral lobes can cause obstruction to the urethra

Uterus and Broad Ligament

The uterus is a midline organ and is held to the lateral walls of the true pelvis by a double layer of peritoneum, called the broad ligament. The broad ligament also encloses the uterine tube in its upper free border, the ovarian artery, the round ligament of the uterus, uterine artery, ovary, and the ovarian ligament. 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 the first image, you are looking at the posterior aspect of the broad ligament and the posterior wall of the vagina has been opened up. These items should be found in relation to the broad ligament. •

uterus

•

uterine tube (oviduct, Fallopian tube) ○

fimbriated end

•

ovarian artery

•

ovary

•

ovarian ligament

•

mesovarium

•

mesosalpinx

•

opening of cervix

•

cervix

•

vagina

•

opening of urethra

•

bladder

In the section through the broad ligament pay attention to the: •

broad ligament

•

uterine tube - in the upper free margin of the broad ligament and connected to the root of the mesovarium by the mesosalpinx

•

ovary - attached to the posterior part of the broad ligament by the mesovarium

•

ovarian ligament - in free margin of the mesovarium

•

anterior layer of the broad ligament

•

posterior layer of the broad ligament

•

round ligament of the uterus - beneath the anterior layer of the broad ligament

•

uterine artery - near the root of the broad ligament

The ovary is also described as having a suspensory ligament but this is nothing more the a fold of peritoneum near where the ovarian artery and veins cross the pelvic brim to enter the true pelvis.

Rectum and Anal Canal

The rectum and anal canal are clinically important parts of the intestinal tract because, by either palpation or rectoscope or sigmoidoscope, they can be easily examined in a routine physical. Tumors, hemorrhoids or abscesses are frequent in this part of the GI tract. The rectum is the continuation of the sigmoid colon and at the point of their junction, the rectum becomes covered by peritoneum only on its anterior surface, and therefore becomes retroperitoneal. The rectum terminates approximately at the attachment of the levator ani to its borders. Also at this point, is the pectinate line which, anatomically, is the anorectal junction. The inside of the

rectum is thrown into folds called rectal valves. These maintain the fecal material until water is removed and a bowel movement occurs. At that point the rectum elongates and the valves become less prominent. At the lower end of the rectum, a series of rectal columns encircle the rectum. Between the column are rectal sinuses. Outside of the columns is found the internal rectal plexus of veins. It is here that internal hemohhroids are found. At the junction of the rectum and anal canal, the columns and sinuses form a dentate or pectinate appearance. This is called the pectinate line and is the starting point of the anal canal which is about 2.5-4.0 cm long. The lining of the

anal canal is continuous with the skin at the white line of Hilton (or intersphincteric line). This line can be felt with the finger as a small indentation between the internal anal sphincter (circular muscle of the rectal wall) and the subcutaneous external anal sphincter. The external anal sphincter is much stronger to the touch than the internal. Note that the external anal sphincter consists of three parts, the deep, superficial and subcutaneous.

Arteries to the rectum There are three sources of arterial supply to the rectum and anus: 1. superior rectal artery - from the inferior mesenteric artery 2. middle rectal arteries - either directly from the internal iliac artery or

from a common branch with the inferior vesical artery

3. inferior rectal arteries - from the internal pudendal artery.

Veins of the Rectum and Anus Surrounding the rectum and anus is a very dense rectal plexus of veins. The upper part of the plexus will send tributaries to form the superior rectal vein which then goes into the inferior mesenteric vein. From the middle part of the plexus, along with tributaries from the bladder, prostate and seminal vesicle pass to the internal iliac vein

From the inferior part of the plexus, drainage is into the internal pudendal vein.

Lymphatic Drainage From the rectum, lymphatics pass eventually into the inferior mesenteric group of preaortic lymph nodes. From the anal canal, lymphatics pass along the middle rectal artery to end in the internal iliac nodes and from these to the common iliac nodes and then to the lateral aortic group of nodes. From the anus, below the white line of Hilton, the lymphatics join those of the perineum and scrotum and pass into the superficial inguinal nodes Clinical Considerations •

Internal hemorrhoids are found above the pectinate line and outside the rectal columns.

•

external hemorrhoids are below the pectinate line and are the more common clinically and can be seen when enlarged.

•

Both types of hemohhroids can be sources of bleeding when abraded. This type of bleeding is bright red compared to bleeding higher up in the GI tract where the blood is occult and must be identified by chemical tests.