Bony Pelvis

Introduction

Introduction to the Pelvic Walls and Floor

The pelvic walls and floor form the boundaries of the pelvic cavity, providing structural support and housing vital organs. These structures are composed of bones, ligaments, muscles, and fascia, working synergistically to maintain pelvic integrity. Understanding their anatomy is crucial for comprehending pelvic organ function and dysfunction.

Functional Significance

The pelvic walls and floor play critical roles in supporting abdominal and pelvic viscera against gravity and intra-abdominal pressure. They are essential for maintaining continence of urine and feces, facilitating defecation and micturition, and supporting reproductive functions. Their intricate muscular arrangement allows for both stability and dynamic movement.

Study

Bony Pelvis as a Framework

The bony pelvis, formed by the sacrum, coccyx, and two hip bones (ilium, ischium, pubis), provides the rigid framework for the pelvic walls. The sacrum and coccyx form the posterior wall, while parts of the ilium and ischium contribute to the lateral walls. These bony landmarks serve as crucial attachment points for the muscles and ligaments that complete the pelvic boundaries.

Lateral Pelvic Walls

The lateral pelvic walls are primarily formed by the obturator internus muscles, which originate from the inner surface of the obturator membrane and surrounding bone. These muscles exit the pelvis through the lesser sciatic foramen to insert on the greater trochanter of the femur. They are covered by the obturator fascia, which forms a canal for the pudendal nerve and internal pudendal vessels.

Posterior Pelvic Wall

The posterior pelvic wall is formed by the sacrum and coccyx, along with the piriformis muscles. The piriformis muscles originate from the anterior surface of the sacrum and exit the pelvis through the greater sciatic foramen. They contribute to the posterolateral wall and are important landmarks for neurovascular structures, including the sciatic nerve.

Pelvic Floor (Pelvic Diaphragm)

The pelvic floor, or pelvic diaphragm, is a funnel-shaped muscular sheet that closes the inferior outlet of the bony pelvis. It consists primarily of the levator ani muscles (puborectalis, pubococcygeus, and iliococcygeus) and the coccygeus muscles. These muscles attach to the pubic bones, ischial spines, and the sacrum/coccyx, forming a sling that supports pelvic organs and controls the passage of urine and feces.

Perineum and its Fascia

The perineum is the diamond-shaped region inferior to the pelvic diaphragm, extending from the pubic symphysis to the coccyx and bounded laterally by the ischial tuberosities. It is divided into an anterior urogenital triangle and a posterior anal triangle. The perineal membrane, a strong fascial layer, spans the anterior part of the urogenital triangle, providing attachment for external genitalia and muscles.

Summary

Key Takeaways

The pelvic walls are formed by bony structures (sacrum, coccyx, hip bones) and muscles (obturator internus, piriformis). The pelvic floor, or diaphragm, is a muscular sling primarily composed of the levator ani and coccygeus muscles. These structures collectively support pelvic organs, maintain continence, and facilitate various physiological functions.

Clinical Correlate: Pelvic Floor Dysfunction

Weakness or damage to the pelvic floor muscles, often due to childbirth, aging, or chronic straining, can lead to pelvic floor dysfunction. This may manifest as urinary or fecal incontinence, pelvic organ prolapse (e.g., cystocele, rectocele), or chronic pelvic pain. Physical therapy and surgical interventions often target these structures.

Clinical Correlate: Pudendal Nerve Block

Understanding the anatomy of the pelvic walls and floor is crucial for procedures like a pudendal nerve block. The pudendal nerve, which innervates much of the perineum and external genitalia, can be anesthetized by injecting near the ischial spine, where it crosses the sacrospinous ligament. This block is used for pain relief during childbirth or perineal procedures.