Which Of The Following Is Associated With The Lesser Pelvis

The lesser pelvis, also known as the true pelvis, is a critical anatomical region in the human body, especially significant in obstetrics and gynecology due to its role in childbirth. Understanding which structures and functions are associated with the lesser pelvis is essential for medical professionals, students, and anyone interested in human anatomy. This article delves into the detailed anatomy of the lesser pelvis, its boundaries, contents, and associated structures, to provide a comprehensive overview.

Defining the Lesser Pelvis

The lesser pelvis is the region of the pelvis inferior to the pelvic brim (also known as the pelvic inlet). The pelvic brim is an oblique plane that extends from the sacral promontory posteriorly to the superior border of the pubic symphysis anteriorly. Everything below this plane is considered part of the lesser pelvis.

Boundaries of the Lesser Pelvis

The boundaries of the lesser pelvis are formed by bony structures and ligaments:

Anteriorly: Pubic symphysis, pubic bones, and superior pubic rami.
Laterally: Ischium and ilium below the arcuate line.
Posteriorly: Sacrum and coccyx.
Inferiorly: Pelvic outlet, which is bounded by the pubic arch, ischial tuberosities, sacrotuberous ligaments, and the tip of the coccyx.

Key Differences Between Greater and Lesser Pelvis

It's important to differentiate between the greater (false) and lesser (true) pelvis:

Greater Pelvis: Located superior to the pelvic brim. It supports the abdominal viscera and is not directly involved in childbirth.
Lesser Pelvis: Located inferior to the pelvic brim. It contains the pelvic organs, including the bladder, rectum, and reproductive organs, and forms the bony birth canal in females.

Contents of the Lesser Pelvis

The lesser pelvis houses several vital organs, blood vessels, nerves, and muscles. Understanding these contents is crucial for comprehending the function and clinical significance of this region.

Organs

Urinary Bladder: The urinary bladder is located in the anterior part of the lesser pelvis, behind the pubic symphysis. Its primary function is to store urine before it is excreted from the body. The bladder's size and position vary depending on its fullness.
Rectum: The rectum is the terminal part of the large intestine and is situated in the posterior part of the lesser pelvis, anterior to the sacrum and coccyx. It stores feces before defecation.
Reproductive Organs:
- Females: The female lesser pelvis contains the uterus, ovaries, fallopian tubes, and vagina. The uterus is a muscular organ responsible for nurturing the developing fetus during pregnancy. The ovaries produce eggs and hormones like estrogen and progesterone. The fallopian tubes transport the eggs from the ovaries to the uterus. The vagina connects the uterus to the external environment.
- Males: The male lesser pelvis contains the prostate gland, seminal vesicles, and part of the vas deferens. The prostate gland contributes to the seminal fluid, which protects and nourishes sperm. The seminal vesicles produce a fluid that mixes with sperm to form semen. The vas deferens transports sperm from the epididymis to the ejaculatory ducts.
Ureters: The ureters, which carry urine from the kidneys to the bladder, pass through the lesser pelvis along its lateral walls.

Blood Vessels

Internal Iliac Artery: The internal iliac artery is the main blood supply to the pelvic organs, perineum, and gluteal region. It arises from the common iliac artery and divides into anterior and posterior divisions, each giving off numerous branches.
Internal Iliac Vein: The internal iliac vein drains blood from the pelvic organs, perineum, and gluteal region. It joins with the external iliac vein to form the common iliac vein.
Ovarian/Testicular Vessels: In females, the ovarian arteries and veins supply the ovaries and part of the fallopian tubes. In males, the testicular arteries and veins supply the testes. Although these vessels originate in the abdomen, they pass through the lesser pelvis to reach their target organs.
Middle Sacral Artery and Vein: These vessels run along the midline of the sacrum, providing blood supply to the sacrum and coccyx.

Nerves

Sacral Plexus: The sacral plexus is a network of nerves formed by the ventral rami of the L4-S4 spinal nerves. It provides innervation to the lower limb, pelvis, and perineum. Major nerves arising from the sacral plexus include the sciatic nerve, pudendal nerve, and superior and inferior gluteal nerves.
Coccygeal Plexus: The coccygeal plexus is a small network of nerves formed by the ventral rami of the S4-S5 spinal nerves and the coccygeal nerve. It provides sensory innervation to the skin around the coccyx.
Autonomic Nerves: The lesser pelvis receives sympathetic and parasympathetic innervation from the pelvic splanchnic nerves and the superior hypogastric plexus. These nerves regulate the function of the pelvic organs, including bladder control, rectal function, and sexual function.

Muscles

Pelvic Diaphragm: The pelvic diaphragm is a muscular sheet that forms the floor of the lesser pelvis. It is composed of the levator ani and coccygeus muscles. The levator ani is further divided into the pubococcygeus, puborectalis, and iliococcygeus muscles. The pelvic diaphragm supports the pelvic organs and helps maintain continence.
Obturator Internus: The obturator internus muscle originates on the internal surface of the obturator membrane and the surrounding bony margins. It passes through the lesser sciatic foramen and inserts on the greater trochanter of the femur. It functions to laterally rotate the hip.
Piriformis: The piriformis muscle originates on the anterior surface of the sacrum and passes through the greater sciatic foramen to insert on the greater trochanter of the femur. It also functions to laterally rotate the hip.

Ligaments

Several ligaments support the pelvic organs and maintain the integrity of the pelvic girdle:

Sacroiliac Ligaments: These ligaments connect the sacrum to the ilium and provide stability to the sacroiliac joint.
Sacrotuberous Ligament: This ligament extends from the sacrum and coccyx to the ischial tuberosity. It helps prevent upward rotation of the sacrum.
Sacrospinous Ligament: This ligament extends from the sacrum and coccyx to the ischial spine. It divides the greater and lesser sciatic foramen.
Pubic Ligaments: These ligaments surround the pubic symphysis and provide stability to the joint.
Ovarian Ligament: Connects the ovary to the uterus.
Round Ligament of the Uterus: Connects the uterus to the labia majora, traversing the inguinal canal.
Cardinal Ligament: Supports the uterus and cervix, containing the uterine artery and vein.
Uterosacral Ligament: Connects the uterus to the sacrum, providing posterior support to the uterus.

Clinical Significance

Understanding the anatomy of the lesser pelvis is crucial in several clinical scenarios:

Childbirth

The lesser pelvis forms the bony birth canal, and its dimensions are critical for successful vaginal delivery. Obstetricians assess the size and shape of the lesser pelvis to determine whether vaginal delivery is feasible.

Pelvic Organ Prolapse

Weakening of the pelvic diaphragm and ligaments can lead to pelvic organ prolapse, where the pelvic organs descend into the vagina. This condition can cause urinary incontinence, fecal incontinence, and pelvic pain.

Pelvic Inflammatory Disease (PID)

PID is an infection of the female reproductive organs, including the uterus, fallopian tubes, and ovaries. Understanding the anatomy of the female lesser pelvis is essential for diagnosing and treating PID.

Prostate Cancer

The prostate gland is located in the male lesser pelvis, and prostate cancer is a common malignancy in men. Knowledge of the prostate's anatomy and its relationship to surrounding structures is crucial for surgical and radiation treatment planning.

Bladder Cancer

The urinary bladder is located in the anterior part of the lesser pelvis, and bladder cancer can affect its function and surrounding structures. Understanding the bladder's anatomy and its blood supply and innervation is essential for surgical management.

Rectal Cancer

The rectum is located in the posterior part of the lesser pelvis, and rectal cancer can require surgical resection. Knowledge of the rectum's anatomy and its relationship to the pelvic diaphragm and surrounding structures is crucial for surgical planning.

Nerve Entrapment

The nerves of the sacral plexus can be compressed or entrapped as they pass through the lesser pelvis, leading to pain, numbness, and weakness in the lower limb and perineum. Piriformis syndrome and pudendal nerve entrapment are examples of nerve entrapment syndromes that can occur in the lesser pelvis.

Vascular Issues

The internal iliac artery and its branches provide the main blood supply to the pelvic organs. Vascular issues, such as aneurysms or blockages, can compromise blood flow to these organs and lead to ischemia or infarction.

Associated Structures in Detail

To further clarify the associations with the lesser pelvis, let's examine several key structures in more detail:

The Pelvic Diaphragm

The pelvic diaphragm is a crucial component of the lesser pelvis. It's primarily composed of the levator ani muscle group, which includes:

Pubococcygeus: Arises from the posterior aspect of the pubic bone and extends posteriorly to insert into the coccyx and anococcygeal ligament. In females, it surrounds the urethra and vagina.
Puborectalis: Arises from the pubic bone and forms a sling around the anorectal junction, contributing to fecal continence.
Iliococcygeus: Arises from the ischial spine and tendinous arch of the obturator fascia and inserts into the coccyx and anococcygeal ligament.

The coccygeus muscle, which lies posterior to the levator ani, also contributes to the pelvic diaphragm. This muscle arises from the ischial spine and inserts into the sacrum and coccyx.

Function:

Supports the pelvic organs, preventing prolapse.
Maintains urinary and fecal continence.
Contributes to sexual function.
Provides support during increased intra-abdominal pressure (e.g., coughing, sneezing).

The Internal Iliac Artery

The internal iliac artery is a major blood vessel within the lesser pelvis, supplying blood to many pelvic structures. Its branching pattern is complex and can vary between individuals, but the general pattern is as follows:

Anterior Division:
- Umbilical Artery: In the fetus, this artery carries deoxygenated blood to the placenta. After birth, the distal portion obliterates to form the medial umbilical ligament.
- Obturator Artery: Supplies the adductor muscles of the thigh.
- Inferior Vesical Artery: Supplies the bladder (males).
- Uterine Artery (Females): Supplies the uterus and gives off the vaginal artery.
- Vaginal Artery (Females): Supplies the vagina.
- Middle Rectal Artery: Supplies the rectum.
- Internal Pudendal Artery: Supplies the perineum, including the external genitalia.
- Inferior Gluteal Artery: Supplies the gluteal region.
Posterior Division:
- Iliolumbar Artery: Supplies the iliacus and psoas muscles.
- Lateral Sacral Artery: Supplies the sacrum and spinal meninges.
- Superior Gluteal Artery: Supplies the gluteal region.

Understanding the branching pattern of the internal iliac artery is crucial for surgeons performing pelvic procedures, as it helps them avoid damaging important blood vessels.

The Sacral Plexus

The sacral plexus is a network of nerves that arises from the ventral rami of the L4-S4 spinal nerves. It provides innervation to the lower limb, pelvis, and perineum. Key nerves arising from the sacral plexus include:

Sciatic Nerve: The largest nerve in the body, it supplies the posterior thigh, leg, and foot. It is formed by the L4-S3 nerve roots.
Superior Gluteal Nerve: Supplies the gluteus medius, gluteus minimus, and tensor fasciae latae muscles.
Inferior Gluteal Nerve: Supplies the gluteus maximus muscle.
Pudendal Nerve: Supplies the perineum, including the external genitalia and the external anal sphincter. It is crucial for urinary and fecal continence, as well as sexual function.
Posterior Femoral Cutaneous Nerve: Supplies the skin of the posterior thigh and leg.

Damage to the sacral plexus can result in a variety of neurological deficits, including weakness, numbness, and pain in the lower limb and perineum.

FAQ About the Lesser Pelvis

What is the primary function of the lesser pelvis?

The primary function of the lesser pelvis is to house and support the pelvic organs, including the bladder, rectum, and reproductive organs. In females, it also forms the bony birth canal.
What are the main differences between the male and female lesser pelvis?

The female lesser pelvis is generally wider and shallower than the male lesser pelvis. The pelvic inlet is more oval-shaped in females, while it is more heart-shaped in males. The subpubic angle is wider in females, allowing for the passage of the fetus during childbirth.
What is pelvic floor dysfunction?

Pelvic floor dysfunction refers to a range of conditions that occur when the muscles and ligaments of the pelvic floor are weak or damaged. This can lead to pelvic organ prolapse, urinary incontinence, fecal incontinence, and pelvic pain.
How is the lesser pelvis assessed during pregnancy?

Obstetricians assess the size and shape of the lesser pelvis during pregnancy to determine whether vaginal delivery is feasible. This can be done through a physical exam, including palpation of the bony landmarks of the pelvis. In some cases, imaging studies such as X-rays or CT scans may be used to assess the pelvic dimensions.
What are some common surgical procedures performed in the lesser pelvis?

Common surgical procedures performed in the lesser pelvis include hysterectomy (removal of the uterus), prostatectomy (removal of the prostate gland), cystectomy (removal of the bladder), and rectal resection (removal of the rectum). These procedures require a thorough understanding of the anatomy of the lesser pelvis to avoid damaging surrounding structures.

Conclusion

The lesser pelvis is a complex anatomical region that plays a crucial role in supporting vital organs and facilitating essential functions like childbirth, urination, and defecation. Its boundaries, contents, and associated structures are intricate and require a detailed understanding for medical professionals and anyone interested in human anatomy. By exploring the organs, blood vessels, nerves, muscles, and ligaments within the lesser pelvis, this article provides a comprehensive overview of this critical area of the human body. A thorough understanding of the lesser pelvis is essential for diagnosing and treating a wide range of clinical conditions, from pelvic organ prolapse to cancer.