Anatomy of Pelvis in a Nutshell

Anatomy of Pelvis

Skeletal structures:  Pelvic girdle (342)

• Lower portion of vertebral column (sacrum , coccyx)
• Right and left bones = Hip bones. Each forms joint with sacrum = Sacroiliac joint and forms joint anteriorly with itself = Pubic symphysis (fibrocartilage plate at midline). Posteriorly, sacroiliac joint is synovial joint between sacrum and hip bone.
• Pelvis in normal position is tipped anterioly (almost facing inferiorly).  Pubic tubercle is in same frontal plane as ASIS.  In that position, the pubis (anterior part of hip bone) is weight bearing).
• Female: Modified to allow passage of fetus at time of birth down into pelvic cavity and out through pelvic floor.  Dimensions of the cavity are thus greater in female.
• Sub-pubic arch that crosses pubic symphysis from pubis to pubis: in female, this arch approaches a 90 degree angle.  The same region in a male has a sub pubic arch of only 70 degrees.
• 2 Processes projecting into cavity: ischial spines. Project farther into cavity of male than female.
• Large aperture in each hip bone = obturator foramen.  The obturator nerve and vessels leaves the pelvis by passing through here.  Most of the foramen is closed off by obturator membrane.  This space allows for a lighter bone structure.

Pelvic Girdle:

• Hip bone is 3 separate bones until puberty.
  • Ilium (not to be confused with ileum of bowel).
    • Has a large fan shaped crest with anterior superior iliac spine.
    • Has a deep notch posteriorly = greater sciatic notch (slightly on ischium).

1. • Pubis.
   • Ischium.

1. 1. • Has a ischial spine on posterior side.
      • Also has ischial tuberosity (we sit on it).

• • • Also has lesser sciatic notch between ischial tuberosity and greater notch.

• These 3 bones meet in lateral depression that is socket for hip joint = Acetabulum, which receives femur.  This fusion occurs at puberty.  The hip bone is part of the appendicular skeleton of the lower limb. Equivalent in upper limb is scapula.  The sacrum is part of the axial skeleton.

Ligaments:

• Obturator foramen has membrane.
• Greater sciatic notch and lesser notch have been transformed into foramen by placement of ligaments. Their function is to fix the sacrum so that weight of vertebral column doesn’t cause sacrum to fix posteriorly.  They anchor sacrum to hip bone. Both from sacrum.
  • One goes to ischial tuberosity – sacrotuberous ligament.  This forms the large area that once divided forms the greater and lesser sciatic foramen
  • One goes to the ischial spine – sacrospinous ligament. This forms the division between the superior greater sciatic foramen and the inferior lesser sciatic foramen.

Importance of elements: Using planes to find dimensions of pelvic cavity

• Plane from pubic symphysis to S1.  S1 has sharp edge = sacral promontory.
  • The pelvic inlet is the line from the sacral promontory to the pubic symphysis.
  • This is called the conjugate diameter of pelvic inlet and is normally 11 cm (minimum for normal vaginal delivery)
• Transverse diameter:
  • Between the ischial tuberosities
  • Normally about 11 cm
• Plane from inferior aspect of pubic symphysis to tip of coccyx:
  • Called the pelvic outlet
  • Should be 12 cm
  • Called anteroposterior diameter, AKA diagonal conjugate.
• Relaxin hormone: Causes joints of pelvis to relax.
  • Thus connective tissues in joints are more mobile.
  • Causes pubic symphysis to be more gelatinous, thus the pubic bones separate slightly.
  • This increases the transverse dimension by up to 15%.
  • This will not change the anterior-posterior dimension.

Pelvic Diaphragm:

• At level of pelvic outlet: a muscular sheet closes the outlet called the pelvic diaphragm.
• This is not on a flat plane, it has sides that attach lateral pelvic wall and then meet in midline.
• Male diaphragm has anal canal and urethra.
• Female has additional vaginal canal present along with anal canal and urethra.

The pelvic cavity has 2 parts: they are separated by circumferential ridge of bone = pelvic brim. Corresponds with pelvic inlet.

• False Pelvis (greater pelvis)
  • Superior to pelvic brim, still have bone. This is the false pelvis / greater pelvis.
  • Extends up to level of the crest of the ilium.
  • This part of the pelvis doesn’t contain pelvic organs, only abdominal organs (intestines.)
  • Between crests of ilium and pelvic brim.
• True Pelvis
  • Below the pelvic brim, have true pelvis.
  • It contains pelvic organs (bladder, prostate, uterus, seminal vesicles).
  • True pelvis below pelvic brim, is closed off by pelvic diaphram.
  • It is very concave on superior surface.
  • Everything superior to pelvic diaphragm is the pelvis.
  • Everything inferior to the pelvic diaphragm is in the perineum.

1. 1. • Between skin and pelvic diaphragm- perineum.
      • Very small but dense area.

(345) The pelvic floor:

Pelvic diaphragm with urogenital diaphragm (contains urogenical hiatus).

• Subdivided into parts
  • Lateral wall has muscle of lower limb = obterator internis.

1. 1. • The fascia of this muscle thickens to form a band = tendinous arch→ extends from pubis (inner aspect of pelvis) along the lateral pelvic wall and passes inferiorly to attach the ischial spine.
      • Some fibers of pelvic diaphragm have origin from this.

1. • Pubococcygeal muscle:

1. 1. • Muscle fibers attach to inner side of pubisà extends deep, some go toward coccyx. This is the pubococcygeal muscle.
      • A tough fibrous tissue is between anorectal hiaus and coccyx = anococcygeal body (tough knot of tissue right off of coccyx)

1. • Iliococcygeal muscle:

1. 1. • See region laterally where fibers take origin from tendinous arch and descend into pelvis.
      • They are associated with ilium, thus are iliococcygeal muscle.

• • • These three muscles (iliococcygeal (2) and pubocyccygeal (1 central) form the levator ani. (note: Netter considers Puborectalis a separate muscle from pubococcygeus, we combine them)

1. • Ischiococcygeal muscle:

1. 1. • 3^rd component is more posteriorly: is mostly fibrous tissue.
      • Takes origin from ischial spine and radiates toward the coccyx and analcoccygeal body = ischiococcygeus muscle.

• • • Most often called coccygeus.

• • • Not a part of levator ani, but is a part of the pelvic diaphragm.

1. • Piriformis:

1. 1. • On posterior pelvic wall, have muscle taking origin from anterior portion of sacrum, passes through greater sciatic foramen (?) = piriformis.
      • Is a muscle of lower limb.

• Pelvic diaphragm innervated by S4.
• More on pubococcygeus (343):

• • Not all of the fibers attach to analcoccygeal body. Some meet its partner fiber on other side and form a sling at anorectal junction.
  • This subdivision forming the sling = puborectalis.
  • These fibers digitate with fibers on other side.  This creates an angle at the junction between rectum and anal canal.
    • This is under tonic contraction except when passing solid (or liquid in some cases) through anal canal.
    • This must relax in order for poo to pass.
    • Incontinence can be caused by dysfunction of this tonus.  Entire pelvic floor relaxes during defecation.

Clinical: The wrath of childbirth

• Pelvic floor musculature is very vulnerable during childbirth.
• As fetus passes into lower birth canal, these muscles are pushed aside.
• If fetus too large / cavity too small, these muscles can be damaged.
• After delivery, the woman should re-tone these muscles (together with UG diaphragm will be needed to support weight of viscera).
•  If these muscles are compromised, organs may sag (leading to problems like cystocele etc.).
• The underlying problem for these conditions is lack of tone in pelvic diaphragm.  Kegels are used to retone these muscles.
• Now have advice against “pushing” during labor to avoid damage to pelvic floor.

Superior to Pelvic Diaphragm = true pelvis: (349) (looking into true pelvis)

• Urinary Bladder:
  • Anterior in midline just posterior to pubic symphysis.
  • Urachus is coming off of apex of urinary bladder, goes to midline abdominal wall and forms median umbilical fold / ligament.
  • Lateral umbilical fold is created by inferior epigastric vessels
  • Medial umbilical fold has remnants from obliterated umbilical vessels.
• When urinary bladder is empty, the uterus rests on posterior surface of bladder.
• The most posterior structure is the rectum.
• Extending from lateral sides of uterus:
  • Round ligament of the uterus:

1. 1. • A fibrous cord that disappears into deep inguinal ring.
      • Is an embryonic remnant, is left over gubernaculum.

• • • There is gubernaculum left over in the male scrotum from lower pole of testical to wall of scrotum.

• • • In female: it goes into labia majora.

1. • Ligament of the ovary:

• • • Another remnant of gubernaculum extending from ovary to side of uterus.
    • Gubernaculum always associated with gonad.
      1. It went from the ovary of deep ring and into labia majora.
      2. The uterus expanded out of pelvic floor.
      3. As it did, it came into contact with gubernaculum and divided it into part that goes from ovary to uterus and the portion that goes out to the labia majora.

1. 1. 1. 1. Another structure from sides of uterus are the uterine tubes = fallopian tubes. They exend over the lateral pelvic wall, as they reach it, they become associated with ovary.

Colon:

• Sigmoid colon becomes rectum at the body of S3 (point of demarcation)
• Not much difference in anatomical features.
• Called rectum till it reaches the pelvic floor.

Peritoneum comes into the true pelvis (347) (female)

• Peritoneum lines many structures:
  • The inner aspect of the anterior abdominal wallà
  • Goes toward pubis where it encounters the urinary bladder→
  • Reflects away and onto the superior surface to coat the superior surface of the urinary bladder→
  • Runs along the urinary bladder posteriorly until reach uterus, at midline it reflects and coats the surface of uterus down posterior wall till reach rectumà
  • Reflects onto rectum.  Not all of rectum is covered by peritoneum.  The lower 1/3 does not have peritoneum associated with it- only the upper 2/3 has it.
• Peritoneal reflections: at their lowest points, they create recesses in the peritoneal cavity.  fluids accumulating in peritoneal cavity will pool in these recesses. Named by organ.
  • Anterior one between bladder and uterus = vesicouterine pouch.  This is a recess in the greater sac of the peritoneal cavity.
  • Posterior one extends more inferiorly, is the lowest point in female peritoneal cavity. lies between rectum and uterus = rectouterine pouch (retrouterine, Douglas, culdesac).
• The upper vaginal wall is very closely associated with this recess. If you need to access this fluid, one approach is to pierce the vagina at upper end to reach peritoneal cavity.
• Bad part of that relationship: non-sterile instrument reaches peritoneal cavity will induce peritonitis.

(347). Urinary bladder

• Is sitting posterior to the pubic symphysis.
• Has a drainage duct (urethra) that comes out of neck→ pierces pelvic floor, empties into vestibule.
• Interior of urinary bladder (353).
  • At the floor of the urinary bladder, have points of entry for ureter.
  • Thus, interior has 3 aptertures: 2 ureters and one urethra.
  • The ureters encountered the bladder wall more superiorly, course through muscular wall for a few cm before enter the bladder itself.

1. 1. • It takes the oblique course to prevent reflux of urine back into ureters.
      • The muscular wall will retain it.

Trigone:

• A triangular shape formed from the 3 apertures inside the bladder.
• The mucosal lining the bladder at this point is tightly bound the underlying muscularis.
• It is smooth all of the time, never changes texture.
• Other areas in the bladder have mucosa that changes appearance and texture depending on fullness of bladder.
• When empty, it has low ridges = rugae that line the inner surface (except in trigone).
• As bladder fills, the rugae disappear.
• Muscle of the bladder wall is collectively called the Detruser muscle (smooth muscle).
• Surrounding the bladder is loose CT (as it is around all pelvic organs).
  • Distributed in this region, see lots of small veins that form a venous plexus around the bladder in the female.

1. 1. • It picks up blood from bladder, other organs, and a structure in the superficial perineal pouch in female = clitoris.
      • The deep dorsal vein on the clitoris / penis passes inferior to pubic symphysis. In female, it dumps into this venous plexus.

1. • In the male, the lose CT venous plexus is on external surface of prostate, and it is into this prostatic venous plexus that the deep dorsal vein drains into (which drain erectile tissue).

Reproductive organs: Female. (356)

Uterus:

• Is coated by peritoneum.
• Peritoneum is also represented lateral to the uterus as a wide extensive double layer of peritoneum called the broad ligament.
  • Is also associated with uterine tubes, and is also suspending the ovaries.
  • Divided into parts.

• Largest part on lateral side of uterus = mesometrium.
• Portion associated with uterine tube (wraps around it) is a double layer called the mesosalpinx.

• Stalk that suspends the ovary is the mesovarium. It is also coating the ovary. The ovary has a coat of peritoneum on it and is also suspended by the double layer. The ovarian ligament fuses with the ovary, was once continuous with round ligament of uterus.

Uterine tube (356)

• Extends to lateral pelvic wall, and is open laterally into infundibulum:
  • Is a trumpet shaped region that expands outward.
  • Has short finger like processes called fimbria at the opening.

1. 1. • They project from the infundibular region.
      • One fimbria is anchored to the surface of the opening.

• Ampulla: Long region of tube moving toward uterus.
  • This is the region in which fertilization should occur.
  • Uterine tube narrows to the isthmus as it approaches the uterus.
  • Fertilization should occur in the ambulla, if this occurs, the egg will move along the fallopian tube and 5-7 days after it will implant.
• A section of the tube passes through the wall of the uterus= called the uterine / intramural portion of the uterine tube.
• Ectopic pregnancies:
  • Some eggs may implant in wall of uterine tube.
  • Having gonorrhea may have destroyed ciliary motion and are thus more prone to this early implantation.
  • It may also implant on external surface of uterus / fallopian tube / on broad ligament. 2 theories.

1. 1. • The fimbria in infundibular region of fallopian tube were not successfully in getting egg into tube. Sperm get out through infundibulum and fertilize egg in greater sac.  Not likely.
      • More likely, egg was fertilized normally in ampulla and then reverses direction and is ejected into peritoneal cavity.

Innervation

Autonomic plexus (390) for pelvic viscera:

• The sympathetic input to these plexuses are coming from grey rami / splanchnic nerves (sacral, lumbar etc) coming from sympathetic trunk.
• They feed into these plexuses of autonomic nerves.
• Also have parasympathetic component.
• These plexuses are seen on bladder, prostate etc.
• Parasympathetic are coming from sacral outflow, from S2-4.
  • The parasympathetic nervous system doesn’t leave the head / trunk.
  • There are no extensions into the extremities.
• One exception: external genitalia
• Parasympethetics do go to the erectile tissue to affect engorgement.

Contributions:

• S2-4 contribute to autonomic portion of plexus.
• S2-4 spinal nerves also feed into plexus (somatic nerve plexus) called sacral plexus.
• The pudendal nerve has contribution from S2-4.
• Confusion is that parasympathetics also come from S2-4, but they are very different.
• Thus, S2-4 spawns autonomics as well as the ventral rami that contribute to sacral plexus (gluteal nerve, sciatic nerve, pudendal nerve etc.).

(382). Sacral plexus:

• Gets contribution from lumbar plexus (L4-5) as well as sacral nerves.
• It comes into blend into S1-5.  All these ventral primary rami form dense plexus deep in pelvis where greater sciatic foramen is.
• S2-4: from sacral plexusAmpullainnervates striated muscle (skeletal muscle) in both triangles of perineum.  Also gets sensory data from skin.
• S2-4 parasympathetic innervates bladder, smooth muscle, muscle in uterine wall, arteries, glands.

Blood vessels of pelvis: has a great deal of variation.  (382)

• Aorta bifurcates at 4^th lumbar vertebrae into common iliac arteries.
• Common iliac arteries are shortà split at sacroiliac joint into internal and external iliac arteries.
• External continues deep to inguinal ligament and becomes the femoral artery in lower limb.
• Internal artery is in the pelvis, most branches distribute to pelvis.  It should divide into posterior division and anterior division.
  • Posterior division has 3 branches:
• One branch ascends out of pelvis, into iliac fossa, into psoas major muscle as iliolumbar artery.
• One or more branches go to sacrum- lateral sacral. Posterior wall, may supply meningnes in spinal canal.

• • Largest branch leaves through greater sciatic foramen as the superior gluteal artery. It leaves between S1 ventral ramus and lumbarsacral trunk.

1. 1. • Anterior division: shows most variation.

• Branch along lateral pelvic wall, goes into obturator foramen – obturator artery.
  • It should become parallel with obturator nerve before leaving through foramen. Goes into the thigh.
  • If not find obturator artery, a variation is for the artery to come off of inferior epigastric artery and go out through obturator foramen = aberrant obturator artery (20%).
• Umbilical artery, is patent for variable length and gives off branches to the superior surface of urinary bladder = superior vesicle artery. The umbilical artery then becomes a fibrous cord = obliterated portion of umbilical artery, creates medial umbilical fold.

• • Gives off a branch to the uterus = uterine artery→ Is parent stem for branch onto vagina = vaginal artery. May come off independently.

• Next trunk has branch to rectum (middle rectal) and branch toward inferior aspect of bladder = inferior vesicle artery. May be common trunk, may not.

1. • Two terminal branches of internal iliac:

1. 1. • Inferior gluteal- main supply to gluteus maximus.
      • Internal pudendal.
      • Common for this common trunk to continue far down.

Female vasculature notes (382).

• Ureter joins urinary bladder.
• An important relationship is that uterine artery as it branches from internal iliac is closely associated with ureter.
  • They cross- Ureter passes inferior to artery (water passes under the bridge).
  • This is important because historectomies require familiarity with this relationship.
  • Don’t want to ligate ureter when ligate the artery.
• In female, the inferior vesicle artery is distinct from vaginal artery.
  • Some say that there is no inferior vesicle artery in the female, it is just the vaginal artery.
  • The vaginal artery in the female gives branches to bladder.
  • Thus, inferior vesicle (present in male) is the same vessel as the vaginal artery in the female.

Also read:

Also Watch: