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Detailed Notes on the Anatomy of the Pelvis

Objectives

  • Understand the structure of the bony pelvis and its ligaments.
  • Describe the walls of the pelvic cavity.
  • Describe pelvic measurements in obstetrics.
  • Describe the pelvic diaphragm.

Structure of the Bony Pelvis

  • Composition: The bony pelvis consists of 4 bones:

    • Two Hip Bones (Coxal bones).
    • Sacrum.
    • Coccyx.
  • Main Functions:

    • Transmits body weight from the vertebral column to the femurs.
    • Supports and protects pelvic viscera.
  • Hip Bone Anatomy:

    • Each hip bone is comprised of three fused bones:
    • Ilium: Superior portion, articulates with sacrum.
    • Ischium: Posteroinferior part.
    • Pubis: Anterior-medial portion.
    • Acetabulum: Concave socket where femur articulates, forming the hip joint.

Bony Landmarks of Pelvis

  • Sacrum:

    • Base: Articulates with 5th lumbar vertebra.
    • Apex: Connects to the coccyx.
    • Auricular Surfaces: Articulate with ilium's auricular surface.
    • Ligament Attachments: For pelvic ligaments and muscles.
  • Coccyx:

    • Comprises apex, base, and multiple surfaces for articulation with sacrum.

Pelvic Joints

  1. Sacroiliac Joints:

    • Synovial joints between sacrum and ilium, enabled by ligaments, allowing limited movement.
  2. Symphysis Pubis:

    • Cartilaginous joint between two pubic bones, minimal movement possible.
  3. Sacrococcygeal Joint:

    • Joins sacrum and coccyx, allowing flexion and extension.

Pelvic Divisions

  • The pelvis is divided into:

    • True Pelvis (Lesser): Below the pelvic brim, containing pelvic organs.
    • False Pelvis (Greater): Above the pelvic brim, supports abdominal organs.
  • Pelvic Inlet:

    • Oval in females, measurements critical for delivery.
  • Pelvic Outlet:

    • Wider in females; boundaries defined by the pubic arch and ischial tuberosities.

Types of Pelvic Inlets

  1. Gynecoid: Normal female pelvis, circular inlet.
  2. Android: Heart-shaped, challenging for childbirth.
  3. Anthropoid: Anteroposterior diameter exceeds transverse diameter, common in males.
  4. Platypelloid: Very wide transverse diameter compared to anteroposterior.

Pelvic Cavity Contents

  • Lies between pelvic inlet and outlet; contains ureters, bladder, rectum, genital organs, blood vessels, lymphatics, and intestines. Size assessment important for delivery.

Clinical Relevance

  • Diagonal Conjugate: Measurable during vaginal exam; at least 11.5 cm.
  • Obstetric Conjugate: Available space for fetal head passage, estimated from diagonal conjugate; normal measurement ≥10 cm.

Blood Supply to Pelvis

  • Common Iliac Artery branches:
    • External Iliac Artery: Supplies lower limbs.
    • Internal Iliac Artery: Divides into anterior and posterior divisions, supplying pelvic organs and muscles.

Nerve Supply of Pelvis

  • Somatic: From sacral plexus, controls pelvic muscles and sensory functions.
  • Autonomic:
    • Parasympathetic (S2-S4): Relaxation of sphincters, erection, bladder contraction.
    • Sympathetic (T10-L2): Vasoconstriction, sphincter contraction.

Muscles of the Pelvic Wall

  1. Piriformis: Lateral rotation of thigh; innervated by S1-S2.
  2. Obturator Internus: Leaves pelvis through lesser sciatic foramen; lateral rotation of thigh; innervated by the nerve to obturator internus.

Pelvic Floor (Diaphragm)

  • Formed by levator ani and coccygeus muscles; supports pelvic organs.

  • Functions:

    1. Seal pelvic outlet.
    2. Support organs.
    3. Control material movement through urethra and anus.
    4. Resist increases in intra-abdominal pressure.
  • Levator Ani:

    • Supports prostate or vagina; maintains fecal continence.
  • Coccygeus:

    • Supports pelvic viscera and assists in pulling coccyx forward after childbirth.

Conclusion

  • Understanding the pelvic anatomy is crucial for facilitating childbirth and identifying any structural or functional abnormalities that may arise.