Anatomy of Pelvis - Everything you need to know

 

The pelvis is a complex, bony structure located at the base of the spine and serves as a connection between the axial skeleton and the lower limbs. It plays a vital role in supporting the weight of the upper body, providing stability during movement, and protecting the pelvic organs. Here, we will discuss the anatomy of the pelvis along with a brief description of the figures.

Bony Pelvis

The bony pelvis consists of four bones:

1. Two hip bones (also called os coxae or innominate bones) - one on each side
2. The sacrum
3. The coccyx

Hip Bones

Each hip bone is formed by the fusion of three bones during adolescence: the ilium, ischium, and pubis. The hip bones join together at the symphysis pubis anteriorly and with the sacrum posteriorly at the sacroiliac joints.

Ilium:

• The largest and most superior part of the hip bone
• Consists of a body and a wing-like ala
• The iliac crest forms the superior border of the ala
• The anterior superior iliac spine (ASIS) and the posterior superior iliac spine (PSIS) are important bony landmarks

Ischium:

• The posteroinferior part of the hip bone
• Consists of a body and a ramus
• The ischial tuberosity is a roughened projection that supports body weight when sitting
• The ischial spine is an important landmark for obstetric and gynecological procedures

Pubis:

• The anteromedial part of the hip bone
• Consists of a body and two rami (superior and inferior)
• The pubic symphysis is a fibrocartilaginous joint that connects the two pubic bones

                             Ilium
                       _______|______
                      /              \
                     /                \
      Anterior      /                  \
      Superior      |                  |  Posterior
      Iliac Spine-> |                  |  Superior Iliac Spine
                    \                  /
                     \                /
                      \______|______/
                             Ischium

Sacrum

• A triangular-shaped bone formed by the fusion of five sacral vertebrae
• Articulates with the hip bones at the sacroiliac joints
• Protects the spinal nerves and forms the posterior wall of the pelvic cavity

Coccyx

• A small, triangular bone formed by the fusion of four coccygeal vertebrae
• The vestigial remnant of a tail in humans
• Provides attachment points for several muscles and ligaments

Pelvic Cavity

The pelvic cavity is a basin-shaped space enclosed by the bony pelvis. It is divided into two regions:

1. The greater (false) pelvis - located above the pelvic brim, between the ala of the ilium
2. The lesser (true) pelvis - located below the pelvic brim, enclosed by the pelvic girdle

The pelvic inlet (superior pelvic aperture) and the pelvic outlet (inferior pelvic aperture) are the two main openings of the pelvic cavity.

Pelvic Joints

There are three main joints in the pelvic region:

1. Sacroiliac joint: The joint between the sacrum and the ilium, which is a strong, weight-bearing synovial joint
2. Pubic symphysis: The joint between the two pubic bones, which is a secondary cartilaginous joint
3. Hip joint: A ball-and-socket joint between the head of the femur and the acetabulum of the hip bone

Pelvic Muscles and Ligaments

Numerous muscles and ligaments support the pelvic region and contribute to the stability of the joints. Some of the key muscles and ligaments include:

• Pelvic diaphragm: A muscular sheet that forms the floor of the pelvic cavity, composed of the levator ani and coccygeus muscles
• Iliopsoas: A powerful hip flexor muscle that originates from the lumbar vertebrae and iliac fossa and inserts onto the lesser trochanter of the femur
• Piriformis: A muscle that originates from the anterior sacrum and inserts onto the greater trochanter of the femur, playing a role in hip joint stabilization and external rotation
• Obturator internus: A muscle that originates from the internal surface of the obturator membrane and inserts onto the greater trochanter of the femur, aiding in hip joint stabilization and external rotation
• Sacrospinous and sacrotuberous ligaments: Important ligaments that stabilize the sacroiliac joint and provide attachment points for muscles

Anatomy of Pelvis: Pelvic Organs, Vasculature, and Nerves

After covering the pelvic muscles and ligaments, it's essential to understand the pelvic organs, vasculature, and nerves. These components work together to support the functions of the urinary, reproductive, and gastrointestinal systems.

Pelvic Organs

The pelvic organs can be divided into three categories: urinary, reproductive, and gastrointestinal. The main organs in the pelvis include the bladder, urethra, uterus (in females), prostate (in males), rectum, and anus.

1. Urinary System

   • Bladder: A muscular, sac-like organ that stores urine before it is expelled from the body.
   • Urethra: A tube that transports urine from the bladder to the outside of the body. In males, the urethra also serves as a conduit for semen during ejaculation.

2. Reproductive System

   • Female
     • Uterus: A pear-shaped muscular organ that houses and nourishes a developing fetus during pregnancy.
     • Fallopian tubes: Two tubes that extend from the uterus to the ovaries, serving as a passageway for eggs to travel from the ovaries to the uterus.
     • Ovaries: Two almond-shaped glands that produce eggs and female sex hormones.
     • Vagina: A muscular, tubular canal that extends from the cervix to the vulva, serving as the birth canal and as a passageway for menstrual blood and sexual intercourse.
   • Male
     • Prostate: A walnut-sized gland that produces seminal fluid, which nourishes and transports sperm.
     • Seminal vesicles: Two small glands that secrete seminal fluid, which merges with sperm to form semen.
     • Vas deferens: Two tubes that transport sperm from the testicles to the urethra.

3. Gastrointestinal System

   • Rectum: The final portion of the large intestine that stores feces before it is expelled from the body.
   • Anus: The external opening through which feces are expelled from the body.

Pelvic Vasculature

The major blood vessels in the pelvic region include the internal iliac arteries and their branches, which supply blood to the pelvic organs, muscles, and ligaments.

1. Internal Iliac Arteries: Each internal iliac artery branches from the common iliac artery and divides into anterior and posterior divisions.
   • Anterior Division: Supplies the urinary bladder, rectum, reproductive organs, and external genitalia.
   • Posterior Division: Supplies the gluteal muscles, sacrum, and coccyx.

Pelvic Nerves

The pelvis contains several essential nerves that control the functions of the pelvic organs, muscles, and skin sensation. These nerves stem from the sacral plexus, which is formed by the ventral rami of spinal nerves L4, L5, and S1-S4.

1. Pudendal Nerve: Provides motor and sensory innervation to the external genitalia, anal and urethral sphincters, and perineal muscles.
2. Pelvic Splanchnic Nerves: Parasympathetic fibers that innervate the pelvic viscera, including the bladder, rectum, and reproductive organs.
3. Inferior Hypogastric Plexus: A network of sympathetic and parasympathetic fibers that innervate the pelvic organs.
4. Coccygeal Plexus: Innervates the coccygeus muscle and the skin over the coccyx.

Understanding the intricate anatomy of the pelvis is crucial for medical professionals to diagnose and treat various pelvic disorders and conditions effectively.