Human Anatomy: Pelvis and Perineum (Lecture Notes)

HUMAN ANATOMY

PELVIS AND PERINEUM
(Lecture Notes)

Necdet Ersöz
Gazi University Medical School

Bony Pelvis

Composition: formed by paired hip bones, sacrum, coccyx, and their articulations.

Two portions: Greater and Lesser pelvis

Lesser pelvis:

ü  Terminal line (pelvic inlet): formed by promontory of sacrum, arcuate line, pectin of pubis, pubic tubercle, upper border of pubic symphysis.

ü  Pelvic outlet: formed by tip of coccyx, sacrotuberous ligament, ischial tuberosity, ramus of ischium, inferior ramus of pubic symphysis.

Muscles of Pelvic Wall: Piriform muscle and Obturator internus muscle

Anteroinferior pelvic wall is formed primarily by the bodies and rami of the pubic bones and the pubic symphysis. It participates in bearing the weight of the urinary bladder.

Lateral pelvic wall is formed by the right and left hip bones -includes an obturator foramen caused by an obturator membrane >> obturator internus muscles cover and thus pad most of the lateral pelvic walls. The fibers of each obturator internus converge posteriorly, become tendinous, and turn sharply laterally to pass from the lesser pelvis through the lesser sciatic foramen to attach to the greater trochanter of the femur. The medial surface of these muscles is covered by obturator fascia, thickened centrally as a tendinous arch that provides attachment for the pelvic diaphragm.

Posterior pelvic wall is, in the anatomical position, consists of a bony wall and roof in the midline (formed by the sacrum and coccyx) and musculoligamentous posterolateral walls, formed by the ligaments associated with the sacroiliac joints and piriformis muscles.

Pelvic Floor

ü  Formed by the bowl – or – funnel-shaped pelvic diaphragm, which consists of the coccygeus and levator ani muscles and the fascias covering the superior and inferior aspects of these muscles.

ü  The pelvic diaphragm lies within the lesser pelvis, separating the pelvic cavity from the perineum, for which it forms the roof     .

ü  The attachment of the diaphragm to the obturator fascia divides the obturator internus into a superior pelvic portion and an inferior perineal portion.

Pelvis: superior fascia of pelvic diaphragm, levator ani muscle, coccygeus, and inferior fascia of pelvic diaphragm.

Muscles of floor of pelvis and pelvic diaphragm

ü  Lavator ani muscle: puborectalis, pubococcygeus, and iliococcygeus

ü  Coccygeus

Coccygeus

ü  Arise from the lateral aspects of the interior sacrum and coccyx, their fleshy fibers lying on and attaching to deep surface of the sacrospinous ligament.

Levator ani muscle (puborectalis, pubococcygeus, and iliococcygeus)

ü  Proximal attachment: body of pubis, tendinous arch of obturator fascia, ischial spine

ü  Distal attachment: perineal body, coccyx, anococcygeal ligament, walls of prostate or vagina, rectum, and anal canal.

ü  Forms most of pelvic diaphragm that helps support pelvic viscera and resists increases in intra-abdominal pressure.

ü  The levator ani muscles help support the pelvic viscera and maintain closure of the rectum and vagina. They are innervated directly by branches from the anterior ramus of S4 and by branches of the pudendal nerve (S2 to S4)

ü  An anterior gap between the medial borders of the levator ani muscles of each side – the urogenital hiatus – gives passage to the urethra and, in females, vagina.

Puborectalis:

ü  The thicker, narrower, medial part of the levator ani muscle.

ü  It forms U-shaped muscular sling (puborectal sling) that passes posterior to the anorectal junction, bounding the urogenital hiatus.

ü  This part plays a major role in maintaining fecal continence.

Pubococcygeus:

ü  The wider but thinner intermediate part of the levator ani, which arises from lateral to the puborectalis.

ü  Its lateral fibers attach to the coccyx and its medial fibers merge with those of the contralateral muscle to form a fibrous raphe or tendinous plate, part of the anococcygeal body or ligament between the anus and the coccyx (often referred to clinically as the “levator plate”)

ü  Shorter muscular slips of the pubococcygeus extending medially and blending with the fascia around midline structures are named for the structure near their termination: pubovaginalis (females), puboprostaticus (levator prostate, males), puboperinealis, puboanalis.

Iliococcygeus:

ü  The posterolateral part of the levator ani, which arises from the posterior tendinous arch and ischial spine.

ü  It is thin and often poorly developed (appearing more aponeurotic than muscular) and also blends with the anococcygeal body posteriorly.

Pelvic Fascia (Parietal pelvic fascia + Visceral pelvic fascia)

Parietal pelvic fascia:

ü  A continuation of the transverse fascia into the pelvis. It covers the piriformis and obturator internus muscle.

ü  Attaches to the arcuate line of the pubis and ilium, thickens over the obturator internus muscle to form the arcus tendinous, the origin of portions of the levator ani muscle.

ü  At the tendinous arch of levator ani it splits to cove both superior and inferior surfaces of the levator ani as superior and inferior fascia of pelvic diaphragm.

Visceral pelvic fascia:

ü  Lies between the peritoneum and the pelvic viscera.

ü  It is a continuation of the extraperitoneal connective tissue.

ü  Ensheathes retroperitoneal viscera and forms septa between retroperitoneal organs: rectovesical septum, rectovaginal septum

Retropubic space:

ü  Lies between pubic symphysis and urinary bladder.

Pararectal space:

ü  Lies around the rectus

ü  Retrorectal space

Perineum

General Features

ü  Region of below pelvic diaphragm

ü  A diamond shape space whose boundaries are those of the pelvic outlet: lower border of symphysis pubis, rami of pubis and ischium, ischial tuberosities, sacrotuberous ligament, the coccyx

Superficial Perineal Space

ü  A potential space between the perineal fascia and the perineal membrane, bounded laterally by the ischiopubic rami.

Boundaries of superficial perineal space:

ü  Lies between inferior fascia of urogenital diaphragm and superficial fascia of perineum.

ü  Space open anteriorly (in rupture of cavernous part of urethra, urine can extravasate from scrotum upward in front of symphysis pubis into anterior abdominal wall deep to membranous fascia of Scarpa)

Contents

ü  Posterior part: superficial transverse perineal muscle

ü  Lateral part: crus penis (male), crus of clitoris (female) and ischiocavernosus covering them.

ü  Central part: bulb of urethra (male), bulb of vestibule (female) and bulbocavernosus covering them.

ü  Branches of pudendal nerves and internal pudendal vessels.

In males, the superficial perineal pouch contains the:

ü  Root (bulb and crura) of the penis and associated muscles (ischiocavernosus and bulbospongiosus)

ü  Proximal (bulbous) part of the spongy uretra

ü  Superficial transverse perineal muscles

ü  Deep perineal branches of the internal pudendal nerves

In females, the superficial perineal pouch contains the:

ü  Clitoris and associated muscles (ischiocavernosus)

ü  Bulbs of the vestibule and surrounding muscle (bulbospongiosus)

ü  Greater vestibular glands

ü  Superficial transverse perineal muscles

ü  Related vessels and nerves (deep perineal branches of the internal pudendal vessels pudendal vessels and nerves)

Deep Perineal Space

ü  Lies between superior and inferior fascia of urogenital diaphragm.

Contents

ü  Deep transverse perineal muscle

ü  Bulbourethral gland (male)

ü  Sphincter of urethra (male), urethrovaginal sphincter

ü  Arteries, veins and nerves

The perineal membrane

ü  The perineal membrane is a thick fascial, triangular structure attached to the bony framework of the pubic arch

ü  It is oriented in the horizontal plane and has a free posterior margin.

ü  Anteriorly there is a small gap between the membrane and the inferior pubic ligament (a ligament associated with the pubic symphysis)

ü  The perineal membrane is related above to a thin space called the deep perineal pouch (deep perineal space), which contains a layer of skeletal muscle and various neurovascular elements.

ü  The deep perineal pouch is open above and is not separated from more superior structures by a distinct layer of fascia.

ü  The parts of perineal membrane and structures in the deep perineal pouch, enclosed by the urogenital hiatus above, therefore contribute to the pelvic floor and support elements of the urogenital system in the pelvic cavity.

ü  The perineal membrane and adjacent pubic arch provide attachment for the roots of the external genitalia and the muscles associated with them.

ü  The urethra penetrates vertically through a circular hiatus in the perineal membrane as it passes from the pelvic cavity, above, to the perineum, below.

ü  In women, the vagina also passes through a hiatus in the perineal membrane just posterior to the urethral hiatus.

ü  In both women and men, a deep transverse perineal muscle on each side parallels the free margin of the perineal membrane and joins with its partner at the midline.

ü  These muscles are thought to stabilize the position of the perineal body, which is a midline structure along the posterior edge of the perineal membrane.

Perineal Central Tendon

ü  Wedge-shaped fibromuscular mass

ü  In female, between anal canal and lower end of vagina

ü  In male, between anal canal and root of penis

ü  It is larger in female than in male and five support to the posterior wall of the vagina

Perineal Body

ü  An ill-defined but important connective tissue structure into which muscles of the pelvic floor and the perineum attach.

ü  It is positioned in the midline along the posterior border of the perineal membrane, to which it attaches.

ü  Origin or insertion of several small muscles and insertion of part of pelvic diaphragm

ü  These muscles are: sphincter ani externus, levator ani, superficial transverse muscle perineum, deep transverse muscles perineum, bulbocavernousus, sphincter of urethra (male) or urethrovaginal sphincter (female).

In both sexes, the deep perineal pouch contains:

ü  Part of the urethra, centrally.

ü  The inferior part of the external urethral sphincter muscle, above the center of the perineal membrane, surrounding the urethra.

ü  Anterior extensions of the ischioanal fat pads.

In males, the deep perineal pouch contains:

ü  Intermediate part of the urethra, the narrowest part of the male urethra.

ü  Deep transverse perineal muscles, immediately superior to the perineal membrane (on its superior surface), running transversely along its posterior aspect.

ü  Bulbouretral glands, embedded within the deep perineal musculature.

ü  Dorsal neurovascular structures of the penis.

In females, the deep perineal pouch contains:

ü  Proximal part of the urethra.

ü  A mass of smooth muscle in the place of deep transverse perineal muscles on the posterior edge of the perineal membrane, associated with the perineal body.

ü  Dorsal neurovasculature of the clitoris

Two Triangles

ü  An imaginary line drawn between the two ischial tuberosities divides perineum into anterior and posterior triangles.

ü  Urogenital region (anterior) differs in male and female.

ü  Anal region (posterior) similar in both sexes.

Anal Region

ü  Internal Anal Sphincter: smooth muscle (thickened circular muscle coat), surrounds upper two-thirds of anal canal, autonomic nerve supply.

ü  External Anal Sphincter: striated muscle, surrounds lower two-thirds of anal canal, three parts (subcutaneous, superficial, and deep), innervation by anal nerves of pudendal nerves and branches of S4.

Ischioanal Fossa

ü  Paired, wedge-shaped, fat-filled spaces on either side of anal canal.

Boundaries:

ü  Apex – conjunctive area of inferior fascia of pelvic diaphragm and fascia covering the obturator internus.

ü  Base – skin of anal region.

ü  Medial – sphincter ani externus, levator ani, coccygeus and inferior fascia of pelvic diaphragm

ü  Lateral – ischial tuberosity, obturator internus and fascia

ü  Anterior – posterior border of urogenital diaphragm. Forward projection of anterior recess of fossa between pelvic diaphragm above and urogenital diaphragm below.

ü  Posterior – backward projection of posterior recess of fossa between gluteus maximus, sacrotuberous ligament and coccyx.

Contents

ü  Fat

ü  Internal pudendal artery and vein and their rectal branches.

ü  Pudendal nerve and its inferior rectal branch.

Vessels and nerves enter from gluteal region, through lesser sciatic foramen, travel on a fascial canal – the pudendal canal (Alcocks) – on the lateral wall of fossa, and extend forward into urogenital region.

Urogenital Region

Superficial fascia has two layers:

ü  The superficial or fatty layer

ü  The deep or membranous layer (superficial fascia of perineum or Colles fascia)

ü  Anteriorly it is continuous with Dartos of the scrotum, fascia of the penis, membranous layer of superficial fascia of the abdominal wall known as the fascia of Scarpa.

Deep fascia has two layers:

ü  Superior fascia of urogenital diaphragm.

ü  Inferior fascia of urogenital diaphragm.

Urogenital Diaphragm

ü  Triangular in shape

ü  Attached laterally to ischiopubic rami and ischial tuberosities.

ü  Formed by sphincter of urethra, deep transverse perineal muscle, superior and inferior fascia of urogenital diaphragm.

Note: These notes are taken from Gazi University Faculty of Medicine Prof. Dr. Meltem BAHCELIOGLU's Anatomy lectures.