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.