Pelvic Diaphragm

Introduction

Introduction to the Pelvis and Perineum

The pelvis is a basin-shaped structure formed by the sacrum, coccyx, and hip bones, providing a strong foundation for the trunk and protecting pelvic viscera. It transmits weight from the axial skeleton to the lower limbs and serves as an attachment point for muscles of the trunk and lower limbs. The perineum is the diamond-shaped region inferior to the pelvic diaphragm, containing the external genitalia and anal opening.

Overview of the Pelvic Floor

The pelvic floor, primarily composed of the pelvic diaphragm, forms a muscular sling that supports the pelvic organs and helps maintain continence. It separates the pelvic cavity superiorly from the perineum inferiorly. Understanding its complex anatomy is crucial for comprehending its physiological functions and common clinical dysfunctions.

Study

The Bony Pelvis

The bony pelvis consists of two hip bones (ilium, ischium, pubis), the sacrum, and the coccyx, articulating at the sacroiliac joints and pubic symphysis. Key landmarks include the pelvic inlet (pelvic brim) and outlet, which define the greater (false) and lesser (true) pelves. The shape and dimensions of the bony pelvis exhibit sexual dimorphism, with the female pelvis generally wider and shallower to facilitate childbirth.

Pelvic Walls

The lateral walls of the pelvis are formed by the obturator internus muscles, which originate from the inner surface of the obturator membrane and surrounding bone, exiting the pelvis via the lesser sciatic foramen to insert on the greater trochanter. The posterior wall is formed by the piriformis muscles, originating from the anterior surface of the sacrum and exiting through the greater sciatic foramen. Both muscles are lateral rotators of the thigh and contribute to the structural integrity of the pelvic cavity.

The Pelvic Diaphragm

The pelvic diaphragm is the primary component of the pelvic floor, comprising the levator ani and coccygeus muscles. The levator ani is a broad, funnel-shaped muscle group divided into the puborectalis, pubococcygeus, and iliococcygeus, originating from the pubis and ischial spines and inserting onto the coccyx and anococcygeal ligament. The coccygeus (ischiococcygeus) muscle extends from the ischial spine to the lateral sacrum and coccyx. These muscles support pelvic viscera, resist increases in intra-abdominal pressure, and play a vital role in fecal and urinary continence.

The Perineum and its Subdivisions

The perineum is the region inferior to the pelvic diaphragm, bounded by the pubic symphysis anteriorly, ischial tuberosities laterally, and the coccyx posteriorly. It is divided into two triangles by an imaginary line connecting the ischial tuberosities: the anterior urogenital triangle and the posterior anal triangle. The urogenital triangle contains the external genitalia and the openings of the urethra and vagina (in females), while the anal triangle contains the anal canal and its external sphincter.

Neurovasculature of the Pelvic Floor

The primary arterial supply to the pelvic floor and perineum is derived from branches of the internal iliac artery, including the internal pudendal artery, which supplies most of the perineum. Venous drainage largely parallels the arterial supply, emptying into the internal iliac veins. The main innervation is provided by the pudendal nerve (S2-S4), which supplies motor innervation to the pelvic diaphragm and perineal muscles, as well as sensory innervation to the external genitalia and perineal skin. Other nerves from the sacral plexus also contribute to the innervation of the pelvic floor.

Summary

Key Takeaways

The pelvis provides skeletal support and protection, while the pelvic floor, primarily the pelvic diaphragm, forms a muscular sling crucial for organ support and continence. The perineum is the superficial region inferior to the pelvic floor, divided into urogenital and anal triangles. Understanding the intricate bony, muscular, and neurovascular anatomy of these regions is fundamental to clinical practice.

Clinical Correlate

Weakness or damage to the pelvic floor muscles, often due to childbirth, aging, or chronic straining, can lead to pelvic organ prolapse (e.g., cystocele, rectocele) and urinary or fecal incontinence. Damage to the pudendal nerve can result in sensory deficits in the perineum and motor weakness of the pelvic floor muscles, impacting continence and sexual function. Pelvic floor physical therapy and surgical interventions are common treatments for these conditions.