Pelvis and Perineum Study Notes

Introduction and Course Details

  • Instructor: Joy Boro, MD

  • Course: SCI 5100

  • Subject: Gross Anatomy 2025

Outline of Subjects Covered

  • Pelvic Osteology

  • Pelvic Ligaments

  • Pelvic Apertures and Divisions

    • Female (Assigned female at birth)

    • Male (Assigned male at birth)

  • Pelvic Organs

    • Urogenital Triangle

    • Anal Triangle

  • Perineum

Osteology and Ligaments

Osteology of the Pelvis

  • Composition:

    • The pelvis is a basin-shaped structure formed by the bony pelvis, which provides support to the vertebral column and articulation with the lower limbs. Each hemipelvis (os coxae) is a single, large irregular bone formed from the fusion of three primary bones: the ilium, ischium, and pubis. These three bones meet and fuse at the acetabulum during puberty.

    • Ilium: The largest and most superior part, forming the superior portion of the os coxae.

    • Ischium: The posteroinferior part, providing attachment for several muscles and supporting the body's weight when seated.

    • Pubis: The anteroinferior part, contributing to the anterior pelvic ring through the pubic symphysis.

    • Sacrum: A large, triangular bone at the base of the spine, formed by the fusion of five sacral vertebrae, providing a strong posterior wall for the pelvic cavity.

    • Coccyx: A small, triangular bone formed by three to five fused coccygeal vertebrae, articulating with the sacrum.

  • Significance of the Acetabulum:

    • The acetabulum is a large cup-shaped articular fossa that accommodates the head of the femur to form the hip joint. It is critically formed by contributions from all three bones of the os coxae (ilium, ischium, and pubis), making it a strong and stable structural component for weight-bearing and locomotion.

Sex Differences in Pelvis Anatomy

  • These differences are primarily adaptations for childbirth in females.

  • Female:

    • Subpubic Angle: Generally greater than 8080 degrees, forming a wider arch, which facilitates passage during childbirth.

    • Iliac Crest: Shorter and less pronounced.

    • Wider space between ischium bones: Specifically, the ischial tuberosities are farther apart, contributing to a broader pelvic outlet.

    • Pelvic Inlet (Superior Pelvic Aperture): Typically more oval or round in shape.

    • Pelvic Outlet (Inferior Pelvic Aperture): Wider and more accommodating.

  • Male:

    • Subpubic Angle: Typically less than 7070 degrees, forming a narrower, more V-shaped arch.

    • Iliac Crest: Taller and more robust.

    • Narrower space between ischium bones: The ischial tuberosities are closer together.

    • Pelvic Inlet (Superior Pelvic Aperture): Generally heart-shaped.

    • Pelvic Outlet (Inferior Pelvic Aperture): Narrower.

Hip Bone Landmarks

  • Acetabulum: As mentioned, a deep socket on the lateral aspect of the hip bone that articulates with the head of the femur to form the hip joint. It features an acetabular fossa and lunate surface.

  • Obturator Foramen: A large, oval or irregularly shaped opening formed by the ischium and pubis. It is almost completely closed by the fibrous obturator membrane, leaving a small gap superiorly called the obturator canal, which transmits the obturator nerve, artery, and veins to the medial compartment of the thigh.

  • Ilium Landmarks:

    • Iliac crest: The prominent superior ridge of the ilium, extending from the Anterior Superior Iliac Spine (ASIS) to the Posterior Superior Iliac Spine (PSIS). It serves as an attachment site for abdominal muscles and the thoracolumbar fascia.

    • Anterior Superior Iliac Spine (ASIS) and Posterior Superior Iliac Spine (PSIS): Important palpable bony prominences that serve as attachment points for ligaments and muscles (e.g., sartorius muscle origin at ASIS).

    • Iliac fossa: A large, smooth, concave area on the medial surface of the ilium, providing origin for the iliacus muscle.

    • Greater Sciatic Notch: A large indentation inferior to the PSIS, converted into the greater sciatic foramen by ligaments.

Pubis and Ischium Landmarks

  • Pubis Features:

    • Pubic body: The main central part.

    • Pubic tubercle: A small, prominent projection on the superior ramus of the pubis, serving as an attachment point for the inguinal ligament.

    • Pubic crest: The superior border of the pubic body, extending laterally from the pubic symphysis to the pubic tubercle.

    • Superior and Inferior pubic rami: Extensions of the pubic body that articulate with the ilium and ischium respectively. The superior ramus connects to the ilium, forming part of the acetabulum, while the inferior ramus connects to the ischial ramus.

  • Ischium Features:

    • Ischial spine: A pointed projection that separates the greater and lesser sciatic notches. It is an important landmark for pudendal nerve block and marks the level of the vaginal introitus.

    • Lesser sciatic notch: A smaller indentation inferior to the ischial spine, converted into the lesser sciatic foramen by ligaments, through which various structures pass into and out of the perineum.

    • Ischial tuberosity: A large, roughened projection on the inferior part of the ischium that provides strong attachment for hamstring and adductor magnus muscles. It supports the body's weight when sitting.

    • Ischial ramus: A thin, flattened part extending anteromedially from the ischial tuberosity.

    • Ischiopubic ramus: The fused structure formed by the inferior ramus of the pubis and the ischial ramus. This forms the inferior boundary of the obturator foramen.

Sacrum and Coccyx

  • Sacrum Landmarks:

    • Base: The broad, superior-most region that articulates with the fifth lumbar vertebra, forming the lumbosacral angle.

    • Sacral promontory: The prominent anterior projecting edge of the body of the first sacral vertebra (S1S_1), forming part of the pelvic inlet.

    • Apex: The inferior-most region that articulates with the coccyx.

    • Sacral foramina (anterior/posterior): Eights (four pairs) on each side of the sacrum, allowing the passage of anterior (ventral) and posterior (dorsal) rami of the sacral spinal nerves.

    • Auricular surface: The lateral articular surface that connects with the ilium to form the sacroiliac joint.

  • Coccyx:

    • Made up of three to five (commonly four) fused rudimentary vertebrae, forming the tailbone. It articulates with the sacrum at the sacrococcygeal joint. While small, it provides attachment for muscles of the pelvic floor (e.g., coccygeus muscle) and can be a source of pain (coccydynia).

Pelvic Ligaments

  • These ligaments provide significant stability to the sacroiliac joints and the overall pelvic girdle.

  • Sacroiliac Ligaments:

    • Anterior Sacroiliac Ligament: A relatively thin ligament connecting the anterior surface of the lateral part of the sacrum to the ilium.

    • Posterior Sacroiliac Ligament: Stronger and more extensive, connecting the posterior superior iliac spine and part of the iliac crest to the sacrum. It is crucial for reinforcing the SI joint posteriorly.

    • Interosseous Sacroiliac Ligament: The strongest component, located deep to the posterior ligament, filling the space between the tuberosities of the sacrum and ilium.

  • Sacrotuberous Ligament: A strong, broad ligament extending from the sacrum and coccyx to the ischial tuberosity. It helps stabilize the sacrum and converts the greater sciatic notch into the greater sciatic foramen.

  • Sacrospinous Ligament: A triangular-shaped ligament extending from the lateral part of the sacrum and coccyx to the ischial spine. It also contributes to reinforcing the pelvic outlet and converting the greater sciatic notch into the greater sciatic foramen, as well as the lesser sciatic notch into the lesser sciatic foramen.

  • Pelvic Foramina:

    • Greater Sciatic Foramen: Formed by the greater sciatic notch, the sacrotuberous ligament, and the sacrospinous ligament. It is a major passageway for structures leaving the pelvis to enter the gluteal region, including the sciatic nerve, superior and inferior gluteal vessels and nerves, pudendal nerve, and internal pudendal vessels.

    • Lesser Sciatic Foramen: Formed by the lesser sciatic notch, the sacrotuberous ligament, and the sacrospinous ligament. It serves as a passageway for structures entering or leaving the perineum, such as the pudendal nerve and internal pudendal vessels, which loop around the ischial spine.

Abdominopelvic Cavity

  • The abdominopelvic cavity is a continuous space, extending from the diaphragm to the pelvic floor. It is divided into two continuous cavities: the abdominal cavity (superior) and the pelvic cavity (inferior).

  • Pelvic cavity borders:

    • Superior: Defined by the pelvic inlet, which separates it from the abdominal cavity.

    • Inferior: Bounded by the pelvic floor (pelvic diaphragm), beneath which lies the perineum.

    • Lateral/Anterior/Posterior: Surrounded and protected by the pelvic bones (ilium, ischium, pubis, sacrum, coccyx) and associated muscles and ligaments.

  • The abdominopelvic cavity is further conceptually split into:

    • Greater (false) pelvis: The superior part, bounded by the iliac fossae laterally and the sacral promontory posteriorly. It primarily contains abdominal organs (e.g., parts of the ileum and sigmoid colon) and is considered part of the abdominal cavity.

    • Lesser (true) pelvis: The inferior part, enclosed by the pelvic bones and sacrum, extending from the pelvic inlet to the pelvic outlet. It contains the pelvic organs (bladder, reproductive organs, rectum) and is the obstetric canal in females.

Boundaries of the Pelvis

  • Pelvic Inlet (Superior Pelvic Aperture):

    • The plane defined by the sacral promontory, the arcuate line of the ilium, the pectineal line of the pubis, and the pubic symphysis. This opening marks the division between the false and true pelvis and is crucial for measuring obstetric dimensions.

  • Pelvic Outlet (Inferior Pelvic Aperture):

    • The plane defined by the pubic symphysis (anteriorly), the ischiopubic rami (anterolaterally), the ischial tuberosities (laterally), the sacrotuberous ligaments (posterolaterally), and the coccyx (posteriorly). This opening is broader in females to facilitate childbirth.

  • Shape:

    • The pelvic inlet is typically heart-shaped in males (android pelvis) due to a more projecting sacrum and narrower anterior segment. It is usually more oval or round in females (gynaecoid pelvis), which is the most common type and optimal for childbirth.

    • The diamond shape mentioned in the original note usually refers to the perineum, which separates the pelvic outlet into the urogenital and anal triangles.

Pelvic Cavity Pouches

  • These peritoneal reflections are clinically significant for fluid accumulation and disease spread.

  • Female:

    • Vesicouterine pouch: A shallow peritoneal cul-de-sac located between the anterior surface of the uterus and the posterior wall of the urinary bladder.

    • Rectouterine pouch (Pouch of Douglas): The deepest peritoneal pouch in the female, located between the posterior surface of the uterus (and upper vagina) and the anterior surface of the rectum. This is a common site for fluid accumulation (e.g., ascites, blood from ectopic pregnancy) and can be accessed via culdocentesis through the posterior fornix of the vagina.

  • Male:

    • Rectovesical pouch: The peritoneal reflection between the posterior surface of the urinary bladder and the anterior surface of the rectum. It is the lowest point of the peritoneal cavity in males, hence a site for fluid collection.

Pelvic Organs

Both Sexes – Primarily Retroperitoneal

  • Organs described as primarily retroperitoneal are those that lie posterior to the peritoneum and are only partially covered by it on their anterior surface. This includes parts of the urinary system and the rectum.

  • Urinary Bladder: Rests on the pelvic floor.

  • Ureters: Descend along the posterior abdominal and pelvic walls.

  • Rectum: The terminal part of the large intestine, largely retroperitoneal except for its most superior aspect, which can be partially intraperitoneal.

  • Application: Rectal exam is a vital clinical tool to assess the posterior aspect of several pelvic organs due to their anatomical proximity:

    • Females: The posterior aspect of the vagina, uterus (specifically the cervix), and indeed the rectouterine pouch can be palpated.

    • Males: The prostate gland and seminal vesicles, which lie directly anterior to the rectum, are readily palpable. Any enlargement or abnormality of these structures can be detected.

Ureters

  • The ureters are muscular tubes, typically 25−3025-30 cm long and 3−43-4 mm in diameter, that transport urine from the kidneys to the urinary bladder. They are retroperitoneal throughout their course.

  • Course: They descend from the renal pelvis, enter the pelvis by crossing the bifurcation of the common iliac vessels, then course inferiorly and medially along the lateral pelvic walls, eventually turning anteromedially to enter the posterior-inferior aspect of the bladder.

  • Location: In females, the ureters pass inferior to the uterine artery (