Anatomy of the Pelvis

Bony Pelvis

• Composed of 4 bones:

  • Two Hip Bones

  • Sacrum

  • Coccyx

Bony Pelvis (Pelvic Girdle)

• Main Functions:

  • Transmits weight from the vertebral column to the femurs.

  • Contains, supports, and protects the pelvic viscera.

Hip Bone

• Large irregular bone.

• Each hip (coxal) bone is made of 3 bones fused together:

  • Ilium: Forms the superior portion of the hip bone and articulates with the sacrum.

  • Ischium: Forms the postero-inferior part of the hip bone.

  • Pubis: Forms the anterior-medial portion of the hip bone.

• All 3 bones meet at the acetabulum.

• The acetabulum: A concave socket that articulates with the head of the femur to form the hip joint.

Ischium

• Ischial Tuberosity:

  • A roughened area that receives body weight when sitting.

• Ischial Spine:

  • Superior to the tuberosity, important, especially in pregnant women.

• Greater Sciatic Notch:

  • Allows the sciatic nerve & vessels to pass from the pelvis to the thigh.

• Lesser Sciatic Notch:

  • Allows vessels & nerves to pass from the pelvis to the perineum.

Pubis

• Two pubic Rami: Superior & Inferior, bounding the Obturator Foramen (for passage of blood vessels & nerves into the anterior part of the thigh).

  • The obturator foramen is closed partially by the obturator membrane.

Sacrum Bony Landmarks

• Base – articulates superiorly with the fifth lumbar vertebra.

• Apex – abuts the coccyx inferiorly.

• Auricular surfaces – located laterally on the sacrum, articulates with the auricular surface of the ilium.

• Anterior and posterior surfaces – attachment to pelvic ligaments and muscles.

Coccyx Bony Landmarks

• The coccyx consists of an apex, base, anterior surface, posterior surface, and two lateral surfaces.

• The base is located most superiorly and contains a facet for articulation with the sacrum.

• The apex is situated inferiorly, at the terminal part of the vertebral column.

Joints of the Pelvis

Sacroiliac Joints

• Synovial joints formed between the sacrum and the iliac bones.

• Covered by cartilage and supported by anterior, posterior, and interosseous sacroiliac ligaments.

• Transmits the weight of the body to the hip bone

• Movements: A small but limited amount of movement is possible at these joints.

Symphysis Pubis

• The symphysis pubis is a cartilaginous joint between the two pubic bones.

• The joint is surrounded by ligaments that extend from one pubic bone to the other.

• Movements: Almost no movement is possible at this joint.

Sacrococcygeal Joint

• The sacrococcygeal joint is a cartilaginous joint between the bodies of the last sacral vertebra and the first coccygeal vertebra.

• The sacrum and coccyx are joined by ligaments.

• Movements: Flexion and extension are possible at this joint.

Ligaments of the Pelvis

• Sacrotuberous Ligament

• Sacrospinous Ligaments

Divisions of the Pelvis

• The pelvis is divided into a true pelvis and false pelvis by the plane of the pelvic brim.

False (greater) pelvis:

• Lies above the pelvic brim and supports the abdominal organs.

• The false pelvis is bounded posteriorly by the lumbar vertebrae, laterally by the iliac fossae and the iliacus muscles, and anteriorly by the lower part of the anterior abdominal wall.

True (lesser) pelvis (in female, also called obstetric pelvis):

• Lies below the pelvic brim and contains the pelvic organs.

• Bounded by the pelvic surfaces of the hip bones, sacrum, and coccyx.

• Includes the true pelvic cavity and the deep parts of the perineum, specifically the ischio-anal fossae.

• The true pelvis has:

  • an inlet,

  • an outlet,

  • and a cavity.

• The pelvic brim include the prominence of the sacrum, the arcuate and pectineal lines, and the upper margin of the pubic symphysis

Pelvic Inlet

• The pelvic inlet is oval in females; its diameters are particularly important in females, especially in preparation for delivery

• Boundaries:

  • Posteriorly: Sacral promontory

  • Laterally: Ileopectineal lines (a line that runs around the inner surface of the ileum).

  • Anteriorly: Symphysis pubis (joint between bodies of pubic bones) anteriorly.

• Structures crossing the pelvic inlet:

  • Sympathetic chains, lumbosacral trunks, obturator nerves, ureters, ovarian vessels and round ligament of uterus in female or vas deferens in male, internal iliac arteries, median sacral artery, superior rectal artery, iliolumbar branch of internal iliac artery.

Types of Pelvic Inlet

1. Gynecoid pelvis:

   • The pelvic inlet is circular, and its transverse diameter is maximum. This is the normal female pelvis.

   • Rounded—slightly oval inlet

   • Straight pelvic sidewalls with roomy pelvic cavity

   • Good sacral curve

   • Ischial spines are not prominent

   • Pubic arch is wide

2. Android pelvis:

   • The pelvic inlet is heart-shaped. This poses difficulty during childbirth.

   • Narrow pubic arch

   • Prominent spines

3. Anthropoid pelvis:

   • The anteroposterior (AP) diameter of the pelvic inlet is more than the transverse diameter. Causes difficulty for normal delivery. Most common in males.

4. Platypelloid (flat) pelvis:

   • The transverse diameter is very wide compared to the AP diameter.

Pelvic Cavity

• The pelvic cavity lies between the pelvic inlet and pelvic outlet.

• It is continuous above with the abdominal cavity and closed below by the pelvic diaphragm.

• It contains the terminal parts of the ureters, urinary bladder, rectum, pelvic genital organs, blood vessels, lymphatics, and nerves, loops of small intestine (mainly ileum) and, large intestine (appendix and transverse and/or sigmoid colon).

• The assessment of the size of the cavity is important before delivery.

Pelvic Outlet

• The Pelvic outlet is wider in females.

• Boundaries:

  • Anteriorly: pubic arch

  • Laterally: ischial tuberosities

  • Posteriorly: tip of coccyx

• The angle beneath the pubic arch is known as the sub-pubic angle and is of a greater size in women.

Differences: Male and Female Bony Pelvis

Female Pelvis: Adaptation for Childbirth

• A wider and broader structure.

• An oval-shaped inlet.

• Less prominent Ischial spines.

• A greater angled sub-pubic arch.

• A sacrum which is shorter, more curved, and with a less pronounced sacral promontory.

• In addition to the bony adaptations, the sacrotuberous and sacrospinous ligaments can stretch under the influence of progesterone and increase the size of the outlet further.

Clinical Relevance: Assessment of the Female Bony Pelvis

Diagonal Conjugate:

• The distance between the sacral promontory and the inferior margin of the pubic symphysis.

• Importance:

  • It is measurable during a vaginal exam and is used to estimate the obstetric conjugate.

• Normal Measurement: Typically $\geq$11.5 cm.

• How It Is Measured:

  • The obstetrician inserts two fingers into the vagina and palpates the sacral promontory.

  • The other hand marks the inferior margin of the pubic symphysis at the point where the examining fingers reach.

  • The distance from the sacral promontory to this point is the diagonal conjugate.

Obstetric Conjugate:

• The shortest anteroposterior distance between the sacral promontory and the midpoint of the pubic symphysis.

• Importance:

  • It is the true available space for the fetal head to pass through the pelvic inlet.

• Normal Measurement: Typically $\geq$10 cm (a measurement below this may indicate cephalopelvic disproportion).

• How It Is Measured:

  • It cannot be measured directly via a vaginal examination.

  • Instead, it is estimated using the diagonal conjugate (see below).

• Formula: Obstetric Conjugate = Diagonal Conjugate − ~1.5 cm.

Blood Supply to Pelvis

Branches of Common Iliac Artery:

• EXTERNAL ILIAC ARTERY:

  • Branches:

    • Superficial circumflex artery

    • Inferior epigastric artery

  • femoral artery

    • Branches:

      • Profunda femoris artery

      • Perforating branches

      • Lateral femoral circumflex artery

      • Medial femoral circumflex artery

Arteries of the Pelvis

• Internal iliac artery (IIA):

  • One of the 2 terminal branches of the Common iliac artery.

  • Arises in front of the sacroiliac joint

  • It descends downward & backwards over the pelvic inlet.

  • It divides at the upper border of the greater sciatic foramen into:

    • Anterior & Posterior divisions.

Posterior division supplies:

• Posterior abdominal wall.

• Posterior pelvic wall.

• Gluteal region.

Anterior division supplies:

1. Gluteal region.

2. Perineum.

3. Pelvic viscera.

4. Medial (adductor) region of thigh

5. The fetus (through the umbilical arteries).

Branches of Posterior Division:

1. Iliolumbar artery:

   • Ascends across pelvic inlet posterior to external iliac vessels, psoas, and iliacus muscles.

2. Lateral sacral arteries:

   • Descends in front of the sacral plexus, giving off branches to neighboring structures.

3. Superior gluteal artery:

   • Leaves the pelvis through the greater sciatic foramen above the piriformis muscle.

From anterior division

• Umbilical artery:

  • The distal part of this artery fibrosed and forms the Medial Umbilical Ligament.

• Superior vesicular artery

• Obturator artery

• Inferior Vesicular artery in the male or vaginal in the female:

  • In the male, it supplies the Prostate and the Seminal Vesicles.

  • It also gives the artery of the Vas Deferens.

1. Middle rectal artery

2. Internal pudendal artery

   • It is the main arterial supply to the perineum.

3. Inferior Gluteal artery.

4. Uterine artery in the female:

   • Crosses the Ureter superiorly and supplies the uterus & uterine tubes.

Nerve Supply of Pelvis

Somatic Nerve Supply (Voluntary Control)

• These nerves arise from the sacral plexus (L4–S4) and provide motor and sensory innervation to the pelvic muscles, skin, and perineum.

Sacral Plexus (L4–S4)

Pudendal Nerve (S2–S4) (Main nerve of the perineum):

• Motor:

  • Supplies external urethral sphincter, external anal sphincter, and perineal muscles.

• Sensory:

  • Provides sensation to external genitalia, perineum, and anus.

• Travels through the greater sciatic foramen, then re-enters via the lesser sciatic foramen.

Sciatic Nerve (L4–S3):

• Largest nerve of the body, exiting through the greater sciatic foramen.

• Supplies the posterior thigh, leg, and foot muscles.

Inferior Gluteal Nerve (L5–S2):

• Supplies gluteus maximus.

Superior Gluteal Nerve (L4–S1):

• Supplies gluteus medius, gluteus minimus, and tensor fasciae latae.

Posterior Femoral Cutaneous Nerve (S1–S3):

• Sensory:

  • Skin of the posterior thigh and perineum.

Autonomic Nerve Supply (Involuntary Control)

Parasympathetic Supply (S2–S4)

• Arises from the pelvic splanchnic nerves (S2–S4).

• Functions:

  • Relaxation of internal urethral & anal sphincters (allowing urination & defecation).

  • Erection (via vasodilation of penile & clitoral arteries).

  • Stimulates bladder contraction for urination.

Sympathetic Supply (T10–L2)

• Originates from the lumbar and sacral sympathetic chains.

• Forms the superior and inferior hypogastric plexuses.

• Functions:

  • Vasoconstriction of pelvic blood vessels.

  • Contraction of internal urethral and anal sphincters (urinary & fecal continence).

  • Ejaculation in males.

Pelvic Wall

Anterior Wall

• Pubic rami and symphysis

Posterior Wall

• Sacrum, coccyx, and piriformis

Lateral Walls

• Pelvic bone below the pelvic inlet, the obturator membrane, sacrospinous and sacrotuberous ligaments, and the obturator internus muscle

Muscles of the Pelvic Wall

Piriformis:

• Origin: pelvic surface of S2-S4 segments, superior margin of the greater sciatic notch & sacrotuberous ligament

• Insertion: greater trochanter of femur

• Action: laterally rotates thigh, abducts thigh

• Innervation: Anterior rami of S1-S2

Obturator Internus muscle:

• Origin: pelvic surface of ilium & ischium, & obturator membrane

• Insertion: greater trochanter

• Action: laterally rotates thigh

• Nerve supply: nerve to obturator internus (L5, S1, S2)

Pelvic Floor

• The floor of the pelvis (Pelvic diaphragm) is formed by the levator ani and coccygeus muscles and their covering fasciae, and it supports the pelvic viscera.

• The pelvic floor separates the pelvic cavity from the perineum.

• It is incomplete anteriorly to allow passage of the urethra in males and urethra and vagina in females.

• These muscles connect the sacrum and coccyx to the ischium and pubis

• Functions of the pelvic diaphragm:

  1. Seal the pelvic outlet.

  2. Support organs of the pelvic cavity.

  3. Flex sacrum and coccyx.

  4. Control the movement of materials through the urethra and anus.

  5. Resist increased intra-abdominal pressure (which would expel contents of the bladder, rectum, and uterus).

Levator ani muscles

• It is a wide thin sheet-like muscle that has a linear origin from the back of the body of the pubis, a tendinous arch formed by a thickening of the obturator internus fascia, and ischial spine.

• It is divided into 3 fiber types:

  • Anterior fibers

  • Intermediate fibers X 2

  • Posterior fibers

• Anterior fibers:

  • The levator prostatae or sphincter vaginae form a sling around the prostate or vagina & are inserted into a mass of fibrous tissue called the perineal body, in front of the anal canal.

• Intermediate fibers:

  • The puborectalis forms a sling around the junction of the rectum and anal canal.

    • It has a very important role in maintaining fecal continence.

Pubococcygeus

• Intermediate fibers:

  • The pubococcygeus passes posteriorly to be inserted into a small fibrous mass, called the anococcygeal body, between the tip of the coccyx and the anal canal.

    • Originates from the posterior surface of the body of the pubis

    • Inserted into the anococcygeal body, between the tip of the coccyx and the anal canal.

    • Supports the prostate (or constricts the vagina)

    • Stabilizes the perineal body

• Posterior fibers:

  • The iliococcygeus is inserted into the anococcygeal body and the coccyx.

Coccygeus muscle:

• Small triangular muscle.

• Origin: Ischial spine.

• Insertion:

  • Lower end of sacrum and coccyx

• Action:

  • Assist the levator ani in supporting the pelvic viscera

  • Supports the coccyx and pulls it forward after it has been reflected posteriorly by defecation and childbirth.

• Nerve supply:

  • Branches of the 4th and 5th sacral nerves