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
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
Sacroiliac Joints:
- Synovial joints between sacrum and ilium, enabled by ligaments, allowing limited movement.
Symphysis Pubis:
- Cartilaginous joint between two pubic bones, minimal movement possible.
Sacrococcygeal Joint:
- Joins sacrum and coccyx, allowing flexion and extension.
Pelvic Divisions
Types of Pelvic Inlets
- Gynecoid: Normal female pelvis, circular inlet.
- Android: Heart-shaped, challenging for childbirth.
- Anthropoid: Anteroposterior diameter exceeds transverse diameter, common in males.
- 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
- Piriformis: Lateral rotation of thigh; innervated by S1-S2.
- Obturator Internus: Leaves pelvis through lesser sciatic foramen; lateral rotation of thigh; innervated by the nerve to obturator internus.
Pelvic Floor (Diaphragm)
Conclusion
- Understanding the pelvic anatomy is crucial for facilitating childbirth and identifying any structural or functional abnormalities that may arise.