Pelvis Anatomy: Inlet and Outlet

Pelvis

• The pelvis consists of the sacrum, coccyx, and two hip bones (coxae).
• The Pelvis Major refers to the greater, or false, pelvis superior to the pelvic brim.
• The Pelvis Minor refers to the lesser, or true, pelvis inferior to the pelvic brim.

Pelvic Inlet and Outlet

• Pelvic inlet = pelvic brim = superior pelvic aperture.
• Pelvic outlet = inferior pelvic aperture, which is closed by the pelvic diaphragm.

Superior Pelvic Aperture (Pelvic Inlet)

• The male pelvis with ligaments shows the inclination of the pelvis corresponding to upright posture, ventral superior view.
• The anterior superior iliac spine and upper border of the symphysis pubis are located in an approximate frontal plane.
• The aperture of the lesser pelvis is heart-shaped.

Female Pelvis

• The female pelvis with ligaments shows ventral superior view; the iliac alae are broader than in the male pelvis.
• The aperture of the lesser pelvis is transverse oval.
• The transverse diameter of the aperture is longer than the sagittal diameter.

Boundaries of the Pelvic Inlet

• Superior margin of pubic symphysis.
• Linea terminalis (pubic crest, pecten pubis, arcuate line of ileum).
• Anterior border of ala sacrum.
• Sacral promontory.

Measurements of Pelvic Inlet in Females

• The four diameters of pelvic inlets are the anteroposterior (true conjugate) (AP Diameter), diagonal conjugate, obstetric conjugate and transverse diameter.
• The diagonal conjugate can be measured clinically.
• The Anteroposterior (true conjugate) (AP Diameter) has a measurement of 11.5cm.
• The diagonal conjugate has a measurement of 12cm.
• The obstetric conjugate has a measurement of 10.5cm.
• The transverse diameter measures a maximum of 13.5cm (12-13cm).
• The obstetric conjugate is clinically important, and can be estimated by subtracting 1.5-2cm from the diagonal conjugate.
• The fetus head is about 9.5-10cm in diameter.

Inferior Pelvic Aperture (Pelvic Outlet) Boundaries

• Coccyx and sacrum (posteriorly).
• Symphysis pubis (anteriorly).
• Ischial tuberosities (laterally).
• Sacrotuberous ligaments.

Pelvic Axis

• The pelvic axis is an imaginary line joining the center point of each of the planes of the pelvis, indicating the center of the cavity throughout the cavity.

Pelvic Diameters

• Oblique Diameter (AB) measures 12.5cm.
• Transverse line between the anterior, superior iliac spines measures 27.5cm.
• Transverse Diameter (CD) measures 12-13cm.
• Conjugate Diameter (EF) measures 11.5cm.
• AP Diameter measures 9-11cm
• Transverse Diameter measures 10-12cm.

Differences between Male and Female Pelvises

• The female pelvis is wider and shallower.
• The female pelvis has a broader sacrum (wider hips) and ischial tuberosities that are farther apart.
• The female pelvis has a greater subpubic angle and a less curved sacrum.

Boundaries of the Pelvis Minor

• The interior wall of the pelvis minor includes the body and superior and inferior rami of the pubis, and the obturator membrane.
• The posterior wall of the pelvis minor includes the sacrum and coccyx.
• The lateral wall of the pelvis minor has the medial view of the arcuate line of ilium, obturator internus muscle and fascia (cut), tendinous arch of levator ani muscle, obturator canal, the iliococcygeus part of levator ani muscle, rectum, ubococcygeus part of levator ani muscle, vagina, and the urethra.
• The inferior wall (or floor) of the Pelvic Cavity, consists of the Levator ani, Coccygeus and Fasciae known as the Pelvic Diaphragm

Anterior Wall of the Pelvis Minor

• Consists of the body of pubis, superior ramus of pubis, symphysis pubis, obturator membrane, and inferior ramus of pubis.

Lateral Wall of the Pelvis Minor

• Includes the piriformis muscle, ischial spine, coccygeus muscle, arcuate line of ilium, obturator internus muscle and fascia (cut), and tendinous arch of levator ani muscle.

Vessels and Nerves in Pelvis Minor

• Common iliac artery and vein, ureter, lumbosacral trunk, sciatic nerve, and pudendal nerve.
• Inferior gluteal artery & obturator nerve/vessels, internal iliac artery and vein, lateral sacral artery, and middle rectal artery are also present in the area.
• External iliac artery and vein, deep circumflex iliac artery, inferior epigastric artery, and obliterated umbilical artery also populate this location.

Inferior Wall of Pelvis Minor

• The pelvic diaphragm includes the dorsal, superficial sacrococcygeal ligament and the sacro-tuberous ligament.
• The anterior half of the inferior pelvic aperture is closed by the urogenital diaphragm (between the two sides of the pubic arch).
• Anterior half of the inferior pelvic aperture is inferior to the pelvic diaphragm.

Pelvic Diaphragm

• Includes the Puborectalis part of levator ani muscle, Pubococcygeus part of levator ani muscle, Iliococcygeus part of levator ani muscle, and the Tendinous arch of levator ani muscle.

Lateral View of the Pelvic Floor

• Key features in the lateral view of the pelvic floor include the piriformis muscle, greater sciatic foramen, and ischial spine.
• The iliococcygeus and pubococcygeus parts of the levator ani muscle are also visible.
• The Anococcygeal ligament attaches external anal sphincter muscle

Ischio-rectal Fossa

• The pelvic diaphragm forms the roof of the ischio-rectal fossa

Urogenital Diaphragm

• Anterior half of pelvic outlet and inferior to the pelvic diaphragm.

Internal Iliac Artery

• The internal iliac artery supplies the pelvis.
• It gives rise to the superior gluteal, obturator, umbilical, piriformis muscle, internal pudendal, inferior gluteal, coccygeus muscle, middle rectal, uterine, and vaginal arteries.
• The posterior division: internal iliac and the anterior division.
• The pelvis is innervated mainly by the sacral and coccygeal nerves via the parasympathetic division, and by the pelvic part of the autonomic nervous system; pudendal n innervates most of the perineum

Deep Perineal Space

• Referred to as the Urogenital Diaphragm.
• Contains the sphincter urethrae muscle.

Ischiorectal Fossa Boundaries

• Lateral boundary: ischium and obturator internus muscle.
• Medial boundary: rectum and anal canal.
• Posterior boundary: sacrotuberous ligament and overlying gluteus maximus muscle.
• Anterior boundary: base of the urogenital diaphragm.
• Contents: Fat, internal pudendal vessels and pudendal n (they give off the inf rectal vessels and nerves).

Pudendal Canal (Alcock's Canal)

• Begins in the posterior border of the ischiorectal fossa.
• Extends from the lesser sciatic notch to the posterior edge of the urogenital diaphragm.

Perineum

• Anteriorly is the pubic symphysis.
• Separated into a urogenital triangle and anal triangle.