Bony Pelvis & Pelvic Diaphragm

Bony Pelvis & Pelvic Diaphragm

Objectives

By the end of this lecture, students should be able to:

  • Identify the position of the bladder on diagrams, models, prosections, and cross-sectional imaging.

  • Identify the nerve supply of the bladder and its sphincters.

  • Identify the pelvic diaphragm (or floor), including its attachments, function, and nerve supply. Understand its role in supporting pelvic organs and its contribution to core stability.

  • Describe the blood supply and venous drainage of the pelvic viscera (e.g., rectum and bladder). Detail the major arteries and veins involved, and their clinical significance.

  • Describe the relationship of the peritoneum to the bladder and uterus. Include reflections and pouches formed by the peritoneum.

Male and Female Pelvis

  • False and True Pelvis: Note the distinction between the false and true pelvis. The false pelvis is superior and supports the abdominal contents, while the true pelvis contains the pelvic organs.

  • Pelvic Cavity: Identify the location of the pelvic cavity. The boundaries and contents of this cavity are crucial for understanding pelvic anatomy.

  • Bones: The bony pelvis is composed of the sacrum, coccyx, and the hip bones.

  • Hip Bone: Point out the three parts of the hip bone: ilium, ischium, and pubis.

  • Iliac Fossa: Located on the posterior abdominal wall. Site of iliacus muscle origin.

  • Pelvic Brim (Pelvic Inlet): Defines the boundary between the false and true pelvis. Important obstetrically as the entry point for fetal descent.

Perineum

The perineum is defined as the surface region in both males and females between the pubic symphysis and the coccyx. It is located below the pelvic diaphragm and between the legs. This diamond-shaped area includes the anus and, in females, the vagina. Subdivided into urogenital triangle and anal triangle.

Muscles of the Pelvis

The muscles of the pelvis are classified into:

  • Muscles of the Pelvic Wall:

    • Piriformis (Posterior wall): Originates from the anterior surface of the sacrum and inserts onto the greater trochanter of the femur. Important for external rotation of the hip.

    • Obturator internus (Lateral and front walls): Arises from the inner surface of the obturator membrane and surrounding bony margins, exiting via the lesser sciatic foramen to insert on the greater trochanter. Also an external rotator of the hip.

  • Muscles of the Pelvic Floor:

    • Levator ani: Composed of pubococcygeus, puborectalis, and iliococcygeus.

    • Coccygeus (together known as the pelvic diaphragm): Extends from the ischial spine to the sacrum and coccyx, contributing to pelvic floor support.

Obturator Internus and Piriformis

  • Piriformis:

    • Insertion: It leaves the pelvis through the greater sciatic foramen and runs laterally to become inserted into the top of the greater trochanter. Innervated by a branch of S1S1 and S2S2.

  • Obturator Internus:

    • Insertion: Its tendon leaves the pelvis through the lesser sciatic foramen and bends laterally 90° to become inserted into the greater trochanter just below piriformis. The lesser sciatic foramen also transmits the pudendal nerve and internal pudendal vessels. Innervated by a branch of L5L5, S1S1 and S2S2.

Muscles of the Pelvic Floor (Pelvic Diaphragm)

The pelvic floor is formed by the levator ani and coccygeus muscles.

  • Levator Ani: Forms the major anterior part of the pelvic diaphragm. Critical for urinary and fecal continence.

  • Coccygeus: Forms the small posterior part of the pelvic diaphragm. Helps support pelvic organs and stabilize the sacroiliac joint.

  • Pelvic Diaphragm: The two muscles together form the pelvic diaphragm. Supports pelvic viscera and resists increases in intra-abdominal pressure.

Characteristics of the Pelvic Diaphragm

  • The pelvic diaphragm is convex downwards, unlike the abdominal diaphragm. This curvature supports the pelvic organs.

  • It is traversed by the rectum and urethra in both sexes, as well as the vagina in females. These openings are potential sites of weakness, leading to prolapse.

Structures Traversing the Pelvic Diaphragm

  • Urethra

  • Vagina (in females)

  • Rectum

Origin of Levator Ani Muscle

The levator ani muscle originates from:

  1. Inner surface of the body of the pubis, near the lower margin of the symphysis pubis.

  2. Arcus tendineus (thickened white line of obturator fascia). Extends from the ischial spine to the pubic symphysis.

  3. Inner surface of the ischial spine.

Insertion of Levator Ani Muscle

The fibers of the levator ani muscles from both sides swing downwards and medially to fuse in a median raphe. There are two gaps within the raphe:

  1. Anterior Gap (Urogenital Hiatus): Located just behind the pubic bones and symphysis pubis. It allows the passage of the urethra in both sexes, as well as the vagina in females. This gap is a critical area for pelvic floor support and can be affected by childbirth.

  2. Posterior Gap: Allows the passage of the anal canal.

  • Perineal Body: The part of the raphe between the two gaps is called the perineal body. A crucial fibromuscular node for pelvic floor integrity.

  • Ano-coccygeal Raphe: The part of the raphe between the anal canal and coccyx is called the ano-coccygeal raphe. Provides posterior support to the pelvic floor.

Parts of Levator Ani Muscle

The levator ani muscle consists of:

  1. Pubococcygeus: Fibers arising from the body of the pubis and anterior part of the arcus tendineus. Further subdivided into:

    • Anterior Fibers: Surround the urogenital hiatus, forming the "sphincter vaginae" in females and "levator prostatae" in males, then insert into the perineal body. These fibers help control urinary continence and vaginal closure.

    • Middle Fibers: Surround the anorectal junction, forming the "puborectalis." This part maintains the anorectal angle, essential for fecal continence.

    • Posterior Fibers: Insert into the ano-coccygeal raphe, forming the "pubococcygeus proper." Supports the pelvic organs and helps control intra-abdominal pressure.

  1. Iliococcygeus: Fibers arising from the ischial spine and posterior part of the arcus tendineus, inserted into the ano-coccygeal raphe and tip of the coccyx. Provides lateral support to the pelvic floor.

Perineal Body

The perineal body (central tendon of the perineum) features:

  • A gap for the prostate in males or the vagina in females. This area is prone to injury during childbirth.

  • An opening for the anorectal junction. Provides support and structure to the anal canal.

Puborectalis

The puborectalis part of the levator ani wraps around the anorectal junction. Its contraction pulls the anorectal junction forwards and upwards, which is very important for fecal continence. It can contract for long periods, in contrast to the external anal sphincter, which can only contract for about a minute.

Coronal Section of the Pelvis

  • The pelvic visceral cavity is above the pelvic diaphragm, and the perineum, including the ischiorectal fossa, is below.

  • The pelvic diaphragm is an important sheet of muscle that supports the pelvic viscera while allowing objects to pass through. Maintains pelvic organ position and function.

  • The ischiorectal (ischioanal) fossa is located below the pelvic diaphragm. A fat-filled space that allows for expansion of the anal canal during defecation.

  • The obturator internus muscle and fascia are part of the lateral pelvic wall. Provides a surface for the attachment of the levator ani muscle.

  • The pudendal canal contains the internal pudendal vessels, pudendal nerve, and nerve to obturator internus. Inferior rectal nerves and vessels pass to the anal canal.

Nerve Supply of Levator Ani

  • Perineal branch of S4S4 from the sacral plexus (supplies the superior surface). Primary motor innervation.

  • Pudendal nerve. Supplies sensory and motor innervation to the perineum and external genitalia.

  • Inferior rectal nerve (supplies the inferior surface). Provides sensory and motor innervation to the anal canal and external anal sphincter.

Relations of Pelvic Surface of Levator Ani

  • Prostate

  • Vagina

  • Uterus

  • Rectum

  • Urinary bladder

Arteries to the Pelvis

  • Common Iliac Artery: Divides into internal and external iliac arteries.

  • Internal Iliac Artery: Supplies the pelvic viscera and walls. Major source of blood to the pelvic organs.

  • External Iliac Artery: Supplies the lower limb. Becomes the femoral artery after passing under the inguinal ligament.

  • Lateral Sacral Artery: Small, but supplies sacral nerves and roots. Important for the neural structures in the sacral region.

  • Median Sacral Artery: Small. Supplies the sacrum and coccyx.

  • Superior Rectal Artery: A branch of the inferior mesenteric artery. Supplies the upper rectum.

Internal Iliac Artery

  • Beginning: In front of the sacroiliac joint, opposite the lumbosacral disc, as the smaller of the two terminal branches of the common iliac artery.

  • Course: Runs downwards and backwards in front of the sacroiliac joint.

  • Termination: Opposite the upper margin of the greater sciatic foramen, dividing into anterior and posterior divisions.

Parietal Branches of Internal Iliac Artery

  • From Posterior Division:

    1. Iliolumbar artery: Supplies the iliacus muscle, psoas major, and quadratus lumborum.

    2. Lateral sacral arteries (2 branches): Supply the sacral spinal cord and meninges.

    3. Superior gluteal artery: Exits pelvis superior to piriformis, supplying gluteal muscles.

  • From Anterior Division:

    1. Obturator artery: Passes through obturator canal, supplying adductor muscles of the thigh.

    2. Inferior gluteal artery: Exits pelvis inferior to piriformis, supplying gluteal muscles and hip joint.

Visceral Branches of Internal Iliac Artery (All from Anterior Division)

  • Umbilical artery

    • Gives the superior vesical artery. Supplies the superior part of the bladder.

    • The distal part of the umbilical artery fibroses and forms the medial umbilical ligament.

  • Inferior vesical artery (in males) or vaginal artery (in females):

    • In males, it supplies the prostate and the seminal vesicles.

    • It also gives the artery of the vas deferens. Supplies the vas deferens and epididymis.

  • Middle rectal artery: Supplies the middle and lower rectum.

  • Internal pudendal artery

    • It is the main arterial supply to the perineum. Supplies the external genitalia and perineal muscles.

Visceral Branches of Internal Iliac Artery (in Females)

  • Vaginal Artery: Replaces the inferior vesical artery.

  • Uterine Artery: Crosses the ureter superiorly and supplies the uterus and uterine tubes. Essential for uterine blood supply during the menstrual cycle and pregnancy.

  • Ovarian Vessels

    • Ovarian artery: Arises from the abdominal aorta. Supplies the ovaries and fallopian tubes.

Prostate

The prostate is a conical fibromuscular gland that lies just below the neck of the urinary bladder, surrounding the most proximal part of the urethra.

  • It lies just in front of the ampulla of the rectum, so it can be easily examined by a finger inserted through the anal canal into the rectal ampulla. This is the basis for the digital rectal exam (DRE).

Lobes of Prostate

  • Anterior lobe (isthmus)

  • Median lobe: Contains the ejaculatory ducts.

  • Posterior lobe: Common site for prostate cancer.

Senile Enlargement of Prostate

Senile enlargement of the prostate is a disease that occurs with increasing age in most men. It generally involves the more central regions of the prostate, which gradually enlarge. The prostate feels 'bulky' on digital rectal examination. Owing to the more central hypertrophic change of the prostate, the urethra is compressed, and urinary outflow obstruction may develop.

Blood Supply of Prostate

  • Inferior vesical artery

  • Middle rectal artery

  • Internal pudendal artery

  • Superior vesical artery

  • Obturator artery

Sacral Plexus

  • Formation: Ventral rami of L4L4, 5 (lumbosacral trunk), S1S1, 2, 3, 4.

  • Site: In front of the piriformis muscle. Provides innervation to the lower limb, pelvis, and perineum.

Branches of Sacral Plexus

  • Superior gluteal nerve: Supplies the gluteus minimus, gluteus medius, and tensor fasciae latae muscles.

  • Inferior gluteal nerve: Supplies the gluteus maximus muscle.

  • Nerve to quadratus femoris and inferior gemellus muscles

  • Nerve to obturator internus and superior gemellus muscles

  • Posterior femoral cutaneous nerve: Provides sensory innervation to the posterior thigh and leg.

  • Pudendal nerve: Supplies the perineum and external genitalia.

Sympathetic Nerves in the Abdomen and Pelvis

  • Origin: Preganglionic sympathetic fibers to all abdominal and pelvic organs arise from segments T5 to L2 of the spinal cord.

  • These fibers run within the sympathetic trunk (without synapsing) and leave it via 4 nerves:

    1. Greater splanchnic nerve (T5-9): Supplies the foregut.

    2. Lesser splanchnic nerve (T10 & 11): Supplies the midgut.

    3. Least splanchnic nerve (T12): Supplies the kidneys and upper ureter.

    4. Lumbar splanchnic nerve (T12-L2): Supplies the hindgut and pelvic organs.

Sympathetic Fibers and Prevertebral Ganglia

Sympathetic fibers within the splanchnic nerves synapse within ganglia along the anterior surface of the abdominal aorta, close to the origin of the major arteries. These are collectively referred to as “prevertebral ganglia”:

  • Coeliac ganglia; around coeliac trunk (largest ganglia in ANS): Supplies the stomach, liver, spleen, and pancreas.

  • Superior mesenteric ganglion; at the origin of the superior mesenteric artery: Supplies the small intestine and proximal colon.

  • Inferior mesenteric ganglion; at the origin of the inferior mesenteric artery: Supplies the distal colon and rectum.

  • Aorticorenal ganglion; at the origin of the renal artery: Supplies the kidneys.

Lumbar Splanchnic Nerves

Arise in the abdomen from ganglia T12-L2 and synapse in small ganglia within the inferior mesenteric and superior hypogastric plexuses.

Hypogastric Nerves and Plexuses

  • Right and left hypogastric nerves: Carry sympathetic fibers to the pelvic organs.

  • Right inferior hypogastric (pelvic) plexus: Provides sympathetic and parasympathetic innervation to the pelvic organs.

  • Superior hypogastric plexus: Located at the bifurcation of the abdominal aorta, carrying sympathetic fibers to the pelvis.

  • The postganglionic sympathetic fibers leave their ganglia and travel to their destined organs along the arteries distributed to these organs.

  • Sympathetic fibers arising from the ganglia of the superior and inferior hypogastric plexuses travel directly to the pelvic organs.

Parasympathetic Nerves in the Abdomen and Pelvis

  • Preganglionic parasympathetic fibers to the foregut and midgut: derived from the vagus nerve.

  • Preganglionic parasympathetic fibers to the hindgut and pelvic organs: derived from the pelvic splanchnic nerve (S2, 3, and 4 of the sacral plexus).

Parasympathetic Pathways

  • Vagus nerve provides parasympathetic innervation to the foregut and midgut. Increases digestive activity and relaxes sphincters.

  • Pelvic splanchnic nerves (S2, S3, S4) innervate the hindgut and pelvic organs. Promotes defecation, urination, and sexual function.

Summary

  • Identify the position of the bladder on diagrams, models, prosections, and cross-sectional imaging.

  • Identify the nerve supply of the bladder and its sphincters.

  • Identify the pelvic diaphragm (or floor), including its attachments, function, and nerve supply.

  • Describe the blood supply and venous drainage of the pelvic viscera (e.g., rectum and bladder).

  • Describe the relationship of the peritoneum to the bladder and uterus.