The pelvic girdle is a bony ring that connects the lower limbs to the axial skeleton. It's made up of two hip bones, the sacrum, and the coccyx, all held together by strong ligaments. This structure provides stability and support for the body's weight.

The pelvis has two main regions: the greater pelvis above and the lesser pelvis below. These areas house important organs and serve as attachment points for muscles. The pelvis also shows differences between males and females, with the female pelvis adapted for childbirth.

The Pelvic Girdle and Pelvis

Bones and ligaments of pelvic girdle

• Bones of the pelvic girdle consist of the right and left hip bones (os coxae), which are formed by the fusion of three bones: ilium, ischium, and pubis, along with the sacrum and coccyx
• Ligaments of the pelvic girdle include:
  • Sacroiliac joint ligaments, which are the anterior sacroiliac ligament, posterior sacroiliac ligament, and interosseous sacroiliac ligament, that stabilize the sacroiliac joint
  • Sacrospinous ligament extends from the sacrum to the ischial spine and helps support the pelvic organs
  • Sacrotuberous ligament runs from the sacrum to the ischial tuberosity and resists shearing forces at the sacroiliac joint
  • Anterior pubic ligament, posterior pubic ligament, superior pubic ligament, and inferior pubic ligament reinforce the pubic symphysis (joint between the pubic bones)

Regions of hip bone

• Ilium is the largest region of the hip bone and includes:
  • Body of ilium, which forms the acetabulum (hip socket)
  • Ala (wing) of ilium provides attachment for abdominal muscles and gluteal muscles
  • Iliac crest is the superior border of the ilium and serves as an attachment site for abdominal muscles and latissimus dorsi
  • Anterior superior iliac spine (ASIS) and anterior inferior iliac spine (AIIS) are important landmarks for muscle attachments (sartorius, rectus femoris)
  • Posterior superior iliac spine (PSIS) and posterior inferior iliac spine (PIIS) serve as attachment points for the sacroiliac ligaments and gluteus maximus muscle
  • Greater sciatic notch is a large opening that allows passage of nerves and blood vessels to the lower limb
  • Iliac fossa is a concave surface on the internal aspect of the ilium that supports the iliacus muscle
  • Auricular surface articulates with the sacrum to form the sacroiliac joint
• Ischium forms the posterior-inferior portion of the hip bone and includes:
  • Body of ischium forms part of the acetabulum
  • Ischial spine serves as an attachment point for the sacrospinous ligament and the superior gemellus muscle
  • Lesser sciatic notch allows passage of the tendon of the obturator internus muscle
  • Ischial tuberosity is a weight-bearing surface when sitting and serves as an attachment site for the hamstring muscles and sacrotuberous ligament
  • Ischial ramus joins with the inferior pubic ramus to form the pubic arch
• Pubis forms the anterior portion of the hip bone and includes:
  • Body of pubis forms part of the acetabulum and the pubic symphysis
  • Superior pubic ramus connects the body of the pubis to the ilium
  • Inferior pubic ramus joins with the ischial ramus to form the pubic arch
  • Pubic tubercle is an attachment site for the inguinal ligament
  • Pubic symphysis is a fibrocartilaginous joint between the right and left pubic bones
  • Obturator foramen is a large opening in the hip bone that allows passage of the obturator nerve and blood vessels

Structure and function of pelvis

• Greater (false) pelvis is located above the pelvic brim (linea terminalis) and:
  • Bounded by the alae of the ilia laterally and the lumbar vertebrae posteriorly
  • Contains lower abdominal organs (sigmoid colon, loops of small intestine) and provides attachment for abdominal muscles (transversus abdominis, internal oblique)
• Lesser (true) pelvis is located below the pelvic brim (linea terminalis) and:
  • Bounded by the bodies and rami of the ischium and pubis anteriorly, the sacrum and coccyx posteriorly, and the hip bone laterally
  • Contains reproductive organs (uterus, ovaries in females; prostate in males), bladder, and rectum
  • Pelvic inlet (superior pelvic aperture) is bounded by the sacral promontory, linea terminalis, and upper margin of the pubic symphysis and allows passage of the fetus during childbirth
  • Pelvic outlet (inferior pelvic aperture) is bounded by the pubic arch, ischial tuberosities, sacrotuberous ligaments, and coccyx and allows passage of the fetus during childbirth

Pelvic Characteristics and Function

• Pelvic diameters are important measurements used to assess the capacity of the pelvis, particularly in obstetrics
• Sexual dimorphism is evident in the pelvis, with females typically having a wider and more circular pelvic inlet to facilitate childbirth
• The obstetrical conjugate is the shortest anterior-posterior diameter of the pelvic inlet, crucial for determining the feasibility of vaginal delivery
• Pelvic floor muscles, including the levator ani and coccygeus, form a supportive sling for pelvic organs and play a role in continence
• The acetabular labrum is a fibrocartilaginous rim that deepens the hip socket, enhancing joint stability and distributing pressure within the hip joint