Pelvis and perineum

Describe the superior region of the pelvis

Related to upper parts of the pelvic bone and lower lumbar vertebrae - false pelvis (greater pelvis) and is considered part of the abdominal cavity

Describe the true pelvis

Related to the inferior parts of the pelvic bones, sacrum, coccyx and has an inlet and outlet

What is the shape of the pelvic cavity ?

Bowl shaped

• enclosed by the true pelvis consists of the pelvic inlet, walls and floor

• continuous with abdominal cavity

• contains support elements of the urinary. gastrointestinal tract and reproductive system

Describe the perineum

• inferior to the floor of the pelvic cavity

• boundaries form the pelvic outlet

• contains and supports the external genitalia and external openings of genitourinary and gastrointestinal system

What is the lumbosacral joint ?

Joint beaten the sacrum and the vertebra LV

What is the sacroiliac joint ?

Articulation between the pelvic bones and the sacrum

What is the pubic symphysis ?

The joint between the two pelvic bones

Describe the pelvic bone

• Irregular in shape

• 2 major parts separated by an oblique line on the medial surface of the bone

• The bone above this line represents the lateral wall of the false pelvis (abdominal cavity)

• The bone below this line represents the lateral wall of the true pelvis, which tonics the pelvic cavity

What is the linea terminalis ?

It is the lower 2/3 of the medial oblique line and contributes to the margin of the pelvic inlet

What is the acetabulum?

Large articular socket found on the lateral reface of the pelvic bone

• Forms the hip joint with the head of the femur

What is the obturator foramen?

A large foramen inferior to the acetabulum

• most of which is enclosed by flat connective tissue membrane —> obturator membrane

What is the obturator canal?

A small canal that remains open superiorly between the membrane and adjacent bone, providing a route of communication between the lower limb and pelvic cavity

What other notches do we find?

We find them on the posterior margin of the bone and they are separated by the ischial spine

1. Greater sciatic notch

2. Lesser sciatic notch

What is the large ischial tuberosity ?

It is the terminal part of the posterior margin

What is the superior iliac spine?

An irregular anterior margin of the pelvic bone

—> we also find the anterior inferior iliac spine and the pubic tubercle

What is the name of the part of the ilium under the arcuate line?

Pelvic part of the ilium - it contributes to the wall of the lesser or true pelvis

Describe the upper part of the ilium

Expands to form a flat, fan shaped wing which provides bony support to the lower abdomen or false pelvis —> provides attachment for muscles functionally associated with the lower limb

What is the iliac fossa?

A fossa formed on the anteromedial surface of the wing because of its concaveness

Describe the external surface of the wing

Marked by lines and roughening and are related to the gluteal region of the lower limb

What is the tuberculum of iliac crest ?

It is a prominent tubercle, projects laterally near the anterior end of the crest, the posterior end of the crest thickens to form the iliac tuberosity

What is the anterior inferior iliac spine important for?

For the attachment of the rectus femoris muscle of the anterior compartment of the thigh and the iliofemoral ligament associated with the hip joint

Describe the body of the pubis

• Flattened dorsoventrally

• Articulates with the body of the pubic bone on the other side at the pubic symphysis

• Rounded pubic crest on its superior surface that ends laterally as the prominent pubic tubercle

Describe the superior ramus of the pubis

• Posterolaterally from the body and joints with the ilium and ischium at its base

• Sharp superior margin of this triangular surface is termed the pecten pubis (forms part of the line terminalis of the pelvic bone and pelvic inlet)

• Marked on its inferior surface by the obturator groove, which forms the upper margin of the obturator canal

Describe the inferior ramus of the pubis

• laterally and inferiorly to join the ramus of the ischium

What is the ischial spine ?

A bony prominence on the posterior margin of the bone that separates the lesser sciatic notch below from the greater sciatic notch above

What is the most prominent feature in the ischium?

The ischial tuberosity

• Posteroinferior aspect of the bone

• Important site for the attachment of the lower limb muscles and for supporting the body when sitting

What is the large L-shaped facet of the lateral surface of the sacrum for ?

For the articulation with the ilium of the pelvic bone

• posterior we also find a large roughened area for the attachment of ligaments that support the sacroiliac joint

Describe the sacral foramina

• 4 pairs of anterior sacral foramina on the anterior surface of the sacrum for anterior rami and

• 4 pairs of posterior sacral foramina on the posterior surface for the posterior rami

What is the sacral canal ?

A continuation of the vertebral canal → terminates as the sacral hiatus

How is the base of the coccyx structured ?

It is superior

• bears a face for articulation with the sacrum and 2 horns one on each side

• these project upward to article or fuse with similar downward projection cornua from the sacrum

Describe the lateral surface of the coccyx

• Each surface has a small rudimentary transverse process extending from the first coccygeal vertebra

What does the lumbosacral joint consist of?

• 2 zygapophyseal joints - occur between adjacent inferior and superior articular processes and

• An intervertebral disc that joins the bodies of vertebra LV and SI

• Reinforced by strong iliolumbar and lumbosacral ligaments that extend from the expanded transverse processes of vertebra LV to the ilium and the sacrum respectively

What does the sacroiliac joint do?

It transmits forces from the lower limbs to the vertebral column

Describe the sacroiliac joint

• Joint surface have an irregular contour and interlock to resist movement

• joint often become floras with age and may become completely ossified

• each one has 3 ligaments:

1. Anterior sacroiliac ligament

2. Interosseous sacroiliac ligament

3. Posterior sacroiliac ligament

Describe the anterior sacroiliac ligament

• thickening of the fibrous membrane of the joint capsule

• runs anteriorly and inferior to the joint

Describe the interosseous sacroiliac ligament

• Largest and strongest one

• positioned immediately posterosuperior to the joints and attaches to adjacent expansive roughened areas on the ilium and sacrum

• fills the gap between the 2 bones

Describe the posterior sacroiliac ligament

• Which covers the interosseous sacroiliac ligament

Describe the pubic symphysis joint

• Each joint is covered by hyaline cartilage and is likened across the midline to adjacent surfaces of fibrocartilage

• Joint is surrounded by interwoven layers of collagen fibers and 2 major ligaments associated with it are:

  1. Superior pubic ligament - above the joint

  2. Inferior pubic ligament - below the joint

Where is the pelvis in the anatomical position ?

• 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

• Pelvic inlet which marks the entrance of pelvic cavity is titled to face anteriorly and the bodies of the pubic bone and the pubic arch are positioned in a nearly horizontal plane facing the ground

What is the pelvic inlet ?

• Circular opening between the abdominal cavity through which structures traverse between the abdomen and pelvic cavity

• surrounded by bones and joints

• promontory of sacrum protrude into the inlet forming its posterior margin of the midline

• on either side of the promontory the margin is formed by the alae of the sacrum

• the margin crosses the sacroiliac joint and continues along the linea terminalis to the pubic symphysis

Describe the pelvic wall

Walls consists of the sacrum, coccyx, pelvic bones inferior to the linea terminalis, 2 ligaments and 2 muscles

Ligaments of the pelvic wall

1. Sacrospinous and sacrotuberous ligaments

• Major components of the lateral pelvic wall that helps define the apertures between the pelvic cavity and adjacent regions through which structures pass

• Smaller of the sacrospinous one is triangular with the apex attached to the ischial spine and its base attached to the margins of the sacrum and coccyx

• Sacrotuberous also triangular and superficial to sacrospinous

• Base has a broad attached that extends form the posterior superior iliac spine of the pelvic bone, laterally apex is attached to the medial margin of the ischial tuberosity

What do the ligaments of the pelvic wall do?

They stabilise the sacrum on the pelvic bone by rising upward tilting of the inferior aspect of the sacrum

• Also convert the greater and lesser sciatic notches of the pelvic bones into foramina

Where is the greater sciatic foramen ?

Superior to the sacrospinous ligament and the ischial spine

Where is the lesser sciatic foramen ?

Lies inferior to the ischial spine and sacrospinous ligament between the sacrospinous and sacrotuberous ligaments

How many apertures does the pelvic wall have?

3

What are the apertures in the pelvic wall ?

1. Obturator canal - at the top of the obturator foramen, bordered by the obturator membrane, associated obturator muscles and the superior pubic ramus —> the obturator nerve and vessels pass through here

2. The greater sciatic foramen - major route of communication between the pelvic cavity and the lower limb, formed by greater sciatic notch in pelvic bone, sacrotuberous and sacrospinous ligaments and the spine of the ischium —> piriformis muscle passes through dividing into 2 parts

3. The lesser sciatic foramen - formed by lesser sciatic notch of pelvic bone, the ischial spine, sacrospinous ligament and sacrotuberous ligaments, tendon of obturator interns muscle passes through this foramen to enter gluteal region —> acts as a route of communication between the perineum and the gluteal region

• Pudendal nerve and internal pudendal vessels pass between the pelvic cavity and the perineum by first passing out of the pelvic cavity through the greater sciatic foramen and then looping around the ischial spine and sacrospinous ligament to pass through the lesser sciatic foramen to enter perineum, the nerve to obturator internus follows a similar course

Greater sciatic foramen is divided into 2 parts

1. The superior gluteal nerves and vessels pass through the foramen above the piriformis

2. Passing through the foramen below the piriformis are the inferior gluteal nerves and vessels, the sciatic nerves, pudental nerves, internal pudental vessels, the posterior femoral cutaneous nerve and the nerves to the obturator internus and quadratus femoris muscles

Describe the pelvic outlet

• Diamond shaped - anterior part defined by bone : posterior part defined by ligaments

• Midline boundary is pubic symphysis

By what is the pubic arch formed ?

1. Inferior border of the body of the pubis

2. Inferior ramus of the pubis

3. Ramus of the ischium

4. Ischial tuberosity

—> these boundaries continue from the ischial tuberosity posteriorly and medially along the sacrotuberous ligament on both sides of the coccyx

What is the perineum ?

The area enclosed by the boundaries of the pelvic outlet and below the pelvic floor

Describe the pelvic floor

• Formed by pelvic diaphragm

• And in the anterior midline the perineal membrane and the muscles in the deep perineal pouch

Where does the circular line of attachment of the cylindrical pelvic wall pass?

It passes on each side between the greater sciatic foramen and the lesser sciatic foramen thus:

1. Greater sciatic foramen is above the level of the pelvic floor and is a route of communication between the pelvic cavity and the gluteal region of the lower limb

2. Lesser sciatic foramen is below the pelvic floor providing a route of communication between gluteal region of the lower limb and perineum

How are the levator ani muscles attached to the pelvic wall?

It follows the circular control of the wall ad includes:

1. Posterior aspect of body of pubic bone

2. Linear thickening called tendinous arch in the fascia covering the obturator internus muscle

3. The spine of the ischium

By what are the levator ani muscles separated?

By a U-shaped defect or gap termed the urogenital hiatus

• the margins of the hiatus merge with the walls of the associated viscera and with muscles in the deep perineal pouch below

• hiatus allows urethra and the vagina to pass through the pelvic diaphragm

What are the 3 collections of muscle fibers of the lavatory ani muscles?

1. Pubococcygeus muscle - originates from the pubis and passes posteriorly to attach along the midline as far back as the coccyx, this part of muscle is further subdivided on the basis of associated structures in the midline into:

   • Puboprostaticus (levator prostatae)/ Pubovaginalis

   • Puboanalis muscles

2. Puborectalis muscle - originates from the the pubis and passes inferiorly on each side to form a sling sound the terminal part of the gastrointestinal tract, this muscular sling maintains an angle or flexure called perineal flexure at the anorectal junction, this angle functions as part of the mechanism that keeps the end of the GI system closed

3. Iliococcygeus muscle - originates from the fascia that covers the obturator internus muscle, joins the same muscles on the other side in the midline to form a ligament or raphe that extends form the anal aperture to the coccyx

What does the levator ani muscle do?

Helps support pelvic viscera and maintain closure of the rectum and vagina

What is the perineal membrane ?

Its a thick fascial, triangular structure attached to the bony framework of the pubic arch

• oriented in the horizontal plane and has a free posterior margin

• anteriorly there is a small gap between the membrane and the inferior pubic ligament

To what is the perineal membrane related to ?

Above to a thin space called the deep perineal pouch (deep perineal space) which contains a layer of skeletal muscle and various neurovascular elements

Where is the perineal pouch ?

Is open above and is separated from more superior structures by a distinct layer of fascia

• the parts of the perineal membrane and structures in the deep perineal pouch, encloses by the urogenital hiatus above therefore contributes to the pelvic floor and support elements of the urogenital system in the pelvic cavity —> even though the perineal membrane and deep perineal pouch are considered parts of the perineum

What do the perineal membrane and adjacent pubic arch provide attachment for ?

The roots of the external genitalia and the muscle associated with them

What can we find within the deep perineal pouch ?

A sheet of skeletal muscle and it functions as a sphincter mainly for the urethra and as a stabilised for the posterior edge of the perineal membrane

What are the three skeletal muscles that function as a sphincter for the urethra?

1. Anteriorly a group of muscle fibers surround the urethra and collectively from the external urethral sphincter

2. 2 additional groups of muscle fibers are associated with the urethra and vagina in women, one group forms the sphincter urethrovaginalis - which surrounds the urethra and vagina as a unit - the second group forms the compress urethrae on each side, which originate from the ischiopubic rami and meet anterior to the urethra —> together with the external urethral sphincter the sphincter urethrovaginalis and compressor urethra facilitate closing of the urethra

3. In men and women a deep transverse perineal muscle on each side parallel the free margin of the perineal membrane and joins with its partner at the midline —> these muscles are thought to stabilise the position of the perineal body, which is a midline structure along the posterior edge of the perineal membrane

What is the perineal body ?

• an ill-defines but important connective tissue structure into which muscles of the pelvic floor and the perineum attach

• positioned in the midline along the posterior border of the perineal membrane to which it attaches

• posterior end of the urogenital hiatus in the levator ani muscles is also connected to it

• deep transverse perineal muscles intersect at the perineal body: inwoven the sphincter urethrovaginalis also attached to the perineal body, other muscles that connect to the perineal body include the external anal sphincter, superficial transverse perineal muscles and the bulbospongiosus muscles of the perineum

What is the peritoneum ?

Of the pelvis is continuous at the pelvic inlet with the peritoneum of the abdomen

• It drapes over the pelvic viscera in the midline forming

1. Pouches between adjacent viscera

2. Folds and ligaments between viscera and pelvic wall

Where are the sacral and coccygeal plexuses?

They are situated on the posterolateral wall of the pelvic cavity and generally occur in the plane between the muscles and blood vessels

What do the sacral and coccygeal plexus supply ?

Nerves from these, mainly from somatic plexus, contribute to the innervation of the lower limb and muscles of the pelvis and perineum

• Cutaneous branches supply skin over the lateral side of the foot, the posterior aspect of the lower limb and most of the perineum

Where do sacral contributions to the sacral plexus pass?

Out of the anterior sacral foramina and courses laterally and inferiorly to the pelvic wall

What does the lumbosacral part consist of?

They consist part of the anterior ramus of L4 and all of the anterior ramus of L5

• it courses vertically into the pelvic cavity from the abdomen by passing immediately anterior to the sacroiliac joint

Describe the gray rami communicates

They are from ganglia of the sympathetic trunk

• connect with each of anterior rami and carry postganglionic sympathetic fibers destined for the periphery to the somatic nerves

What can we also find in the sacral plexus ?

We can find special visceral nerves (pelvic splanchnic nerves) originating from S2 to S4 deliver preganglionic parasympathetic fibers to the pelvic part of the prevertebral plexus

What branches originate from the sacral plexus ?

1. Sciatic nerve

2. Gluteal nerves

—> major nerves of the lower limb and the pudendal nerve which is the nerve of the perineum

• numerous smaller branches supply the pelvic wall, floor and lower limb

Where do nerves from the sacral plexus leave the pelvic cavity ?

They leave the pelvic cavity by passing through the greater sciatic foramen inferior to the piriformis muscle and enter the gluteal region of the lower limb

• other nerves leave the pelvic cavity using different routes, a few never do not leave the pelvic cavity and course directly into muscle in the pelvic cavity

• 2 nerves that leave the pelvic cavity through the greater sciatic foramen loop around the ischial spine and sacrospinous ligament and pass medially through the lesser sciatic foramen to supply structures in the perineum and lateral pelvic wall

Describe the sciatic nerve

• largest nerve of the body and carries contributions from L4 to S3

• forms on the anterior surface of the piriformis muscle and leaves the pelvic cavity through the greater sciatic foramen inferior to piriformis

Where does the sciatic nerve pass ?

• Passes through the glottal region into the thigh where it divides into its major 2 branches:

1. Common fibular nerve

2. Tibial nerve

—> dorsal division of L4,L5,S1 and S2 are carried in the common fibular part of the nerve, and the ventral divisions of L4,L5,S1,S2 and S3 are carried in the tibial part

• Innervates muscles in the posterior compartment of the thigh and muscles in the leg and foot and

• carries sensory fibers from the skin of the foot and lateral leg

Describe the pudendal nerve

• Forms anteriorly to the lower part of piriformis muscle from ventral division of S2-S4

• Leaves the pelvic cavity through the greater sciatic foramen, inferior to the piriformis muscle and enter the gluteal region

• courses into the perineum by immediately passing around the sacrospinous ligament where the ligament joins the ischial spine and through the lesser sciatic foramen

• Accompanies throughout its course by the internal pudendal vessels and

• Innervates skin and skeletal muscles of the perineum, including the external anal and external urethra sphincters

What are other branches of the sacral plexus ?

1. Motor branches to muscles of the gluteal region, pelvic wall and pelvic floor

2. Sensory nerves to skin over the inferior gluteal region and posterior aspect of the thigh and upper leg

Describe the superior gluteal nerves

• Leaves the pelvic cavity through the greater sciatic foramen superior to the piriformis muscle and supplied muscles in the gluteal region

Describe the inferior gluteal nerves

• leave the pelvic cavity through the greater sciatic foramen inferior to the piriformis muscle and supplies the gluteus maximus

Describe the nerve to the obturator internus and the associated superior gemellus muscle

• leaves the pelvic cavity through the greater sciatic foramen inferior to the piriformis muscle, like the pudendal nerve it passes around the ischial spine and through the lesser sciatic foramen to enter the perineum and supply the obturator internus muscle from the medial side of the muscle inferior to the attachment of the levator ani muscle

Describe the nerve to the quadratus femoris muscle and the inferior gemellus muscle and the posterior cutaneous nerve of the thigh

• Leaves the pelvic cavity through the greater sciatic formed inferior to the piriformis muscle and course to muscle and skin

  —> respectively in the lower limb

What does the perforating cutaneous nerve do?

Leaves the pelvic cavity by penetrating directly through the sacrotuberous ligament and then courses to skin over the inferior aspect of the buttocks

—> the nerve to the piriformis and a number of small nerves to the levator ani and coccygeus muscles originate from the sacral plexus and pass directly into their target muscles without leaving the pelvic cavity

Describe the paravertebral sympathetic chain

• Paravertebral parts of the visceral nervous system

• represented in the pelvis by the inferior ends of the sympathetic trunks

• each true enters the pelvic cavity from the abdomen by passing over the ala of the sacrum medially to the lumbosacral trunks and posteriorly to the iliac vessels

• trunks course inferiorly along the anterior surface of the sacrum where they are positioned medially to the anterior sacral foramen

• 4 ganglia occur along each trunk

• Anteriorly to the coccyx the 2 trunks join to from a single small terminal ganglion (ganglion impar)

What is the function of the sympathetic trunks in the pelvis ?

It is to deliver postganglionic sympathetic fibers to the anterior rami of sacral nerves for distribution to the periphery, mainly to parts of the lower limb and perineum, this is accomplished by gray rami communicants —> these connect the trunks to the sacral anterior rami, other branches (sacral splanchnic nerve) join and contribute to the pelvic part of the prevertebral plexus associated with innervating pelvic viscera

What does the pelvic part of the prevertebral plexus carry ?

Sympathetic, parasympathetics and visceral afferent fibers

• pelvic parts of the plexus are associated with innervating pelvic viscera and erectile tissues of the perineum

How does the prevertebral plexus enter the pelvis?

As 2 hypogastric nerves, one on each side, that cross the pelvic inlet medially to the internal iliac vessels, the hypogastric nerves are formed by the separation of the fibers in the superior hypogastric plexus into right and left bundles

Where is the superior hypogastric plexus ?

Anterior to vertebra LV between the promontory of the sacrum and the bifurcation of the aorta

→ when the hypogastric nerves are joined b pelvic splanchnic nerves carrying preganglionic parasympathetic fibers from S2 to S4 the pelvic plexuses (inferior hypogastric plexuses are formed)

Where are the inferior hypogastric plexuses?

One on each side, course in an inferior direction around the pelvic walls, medially to major vessels and somatic nerves

To what does the inferior hypogastric plexus give rise to?

they give origin to the subsidiary plexuses, which innervate the pelvic viscera:

1. rectal plexuses

2. uterovaginal plexus

3. Prosthetics plexus

4. vesicle plexus

Describe the sympathetic fibers

• enter the inferior hypogastric plexuses from the hypogastric nerves and from branches of the upper sacral parts of the sympathetic trunks

• these nerves are derived form preganglionic fibers that leave the spinal cord in the anterior roots - mainly of T10 to L2

• these fibers:

1. innervate blood vessels

2. Cause contraction of smooth muscle in the internal urethral sphincter in men and the internal anal sphincters in men and women

3. Cause smooth muscle contraction associated with the reproductive tract and with the accessory glands of reproductive system

4. important in moving secretions from the epididymis and associated glands into the urethra to form semen during ejaculation

Describe parasympathetic fibers

• enter the pelvic plexus in pelvic splanchnic nerves that originate from spinal cord levels S2 to S4

• they:

1. Genrally vasodilatory

2. Stimulate bladder contraction

3. Stimulate erection

4. Modulate activity of the enteric nervous system of the colon distal to the left colic flexure

Describe visceral afferent fibers

• follow the course of the sympathetic and parasympathetic fibers to the spinal cord

• afferent fibers that enter the cord in low thoracic levels and lumbar levels with sympathetic fibers carry pain,

What is the perineum ?

It is a diamond shaped region positioned inferiorly to the pelvic floor between the things

• peripheral boundary is the pelvic outlet

• ceiling is the pelvic diaphragm

What is the perineum divided into ?

1. anterior urogenital triangle - opening of urinary system and reproductive system and functions to anchor the external genitalia

2. posterior anal triangle - contains the anus and the external anal sphincter

Describe the anal triangle

• posteroinferiorly

• ceiling is the pelvic diaphragm which is formed by the levator ani and coccygeus muscle

• major muscle is the external anal sphincter - innervated by inferior rectal branches of the pudendal nerve and by branches directly from the anterior ramus of S4

Describe the urogenital triangle

• anterior half

• contains the perineal membrane - a deep perineal pouch which is attached to the pubic arch

Superficial perineal pouch - 3 muscles

1. Ischiocavernous

2. Bulbospongiosus

3. Superficial transverse perineal muscles

Describe the pudendal nerve (somatic nerves)

• Major somatic nerve of perineum

• travels along the wall of the ischia-anal fossa in the pudendal canal which is a tubular compartment formed in the fascia that covers the obturator interns muscle

• 3 major branches which is accompanied by branches of the internal pudendal artery:

1. Inferior rectal nerve - external anal sphincter and related regions of the levator ani muscles, general sensory for the skin of the anal triangle

2. Perineal nerve - motor branches supply skeletal muscles in the superficial and deep perineal pouches

   • largest of sensory branches is the posterior scrotal nerve/posterior labial nerve

3. Dorsal nerve of penis and clitoris - body of the clitoris and penis

What are other somatic nerves ?

Mainly sensory and include branches of the olio-inguinal, genitofemoral, posterior femoral cutaneous and anococcygeal nerves

Through where do visceral nerves enter the perineum ?

2 routes:

1. Those to the skin (postganglionic sympathetic) delivered to the region along the pudendal nerve, these fibers join the pudendal nerve from gray rami communicantes that connect pelvic parts of the sympathetic trunks to the anterior rami of the sacral spinal nerves

2. Those to erectile tissue enter the region mainly by passing through the deep perineal pouch from the inferior hypogastric plexus in the pelvis cavity, the fibers that stimulate erection are parasympathetic fibers, which enter the inferior hypogastric plexus via pelvic splanchnic nerves from spinal cord levels of S2 to S4