Pelvis Anatomy and Muscles

Posture and Pelvis

• Relaxation of sacro-iliac ligaments can lead to "swayback" posture, allowing greater rotation of the pelvis.

Muscles of the Pelvis

• Two muscles in the pelvic wall:
  • Piriformis
• Two muscles in the pelvic floor:
  • Levator ani
  • Obturator internus
• Levator ani and coccygeus form the pelvic diaphragm, which forms the pelvic floor.
• The part of the pelvis above levator ani is the pelvic cavity.
• The part of the pelvis below levator ani is the perineum.

Pelvic Fascia

• Piriformis fascia:
  • Part of parietal pelvic fascia
  • Anteriorly related to internal iliac vessels
  • Posteriorly related to sacral nerves
• Obturator fascia:
  • Covers the pelvic surface of obturator internus
  • Fuses with the periosteum at the margins of the muscle, except at the obturator groove
  • Thickens to form the tendinous arch (white line) between the lower border of the pubic body and ischial spine, which gives origin to levator ani muscle.

Muscles of the Pelvic Floor

• Puborectalis:
  • Passes inferomedially to become continuous with the opposite ones behind the anorectal junction, forming a U-shaped sling
  • Inserted into the anococcygeal body
• Pubococcygeus proper:
  • Passes medially to be attached to the side of the coccyx and anococcygeal body
• Iliococcygeus:
  • Arises from the posterior half of the white line and ischial spine
  • Fibers pass medially inferior to the pubococcygeus proper and have the same insertion into the side of the coccyx and the anococcygeal raphe
• Coccygeus muscle:
  • Origin: Ischial spine
  • Insertion: Lower end of the sacrum and into the coccyx
  • Nerve supply: A branch of the 4th and 5th sacral nerves
  • Action: Assists the levator ani in supporting the pelvic viscera

Functional Significance of the Pelvic Floor in the Female

• Helps in head rotation during the second stage of labor
• Injury to the pelvic floor can occur during a difficult childbirth, leading to:
  • Uterine and vaginal prolapse
  • Herniation of the bladder (cystocele)
  • Alteration in the position of the bladder neck and urethra, leading to stress incontinence
  • Prolapse of the rectum

Anatomy of the Pelvis

• The bony pelvis is formed of 4 bones: Right and left hip bones, the sacrum, and coccyx
• The pelvis is united by 4 joints:
  • 2 Sacro-iliac joints (plane synovial)
  • Symphysis pubis
  • Sacrococcygeal joints (cartilaginous joints)
• The pelvis is supported by 4 ligaments:
  • Iliolumbar
  • Lumbosacral
  • Sacrotuberous
  • Sacrospinous
• Normal position of the pelvis:
  • In erect posture, the pelvis lies with the anterior superior iliac spine and pubic tubercles in the same vertical plane
  • The ischial spine and upper border of symphysis pubis are in the same horizontal plane
• The pelvic brim:
  • An oblique plane extends from the sacral promontory to the upper margin of symphysis pubis