Pelvis and Sacrum Anatomy

Pelvis

• The pelvis consists of the bony pelvis and muscles and tissue beneath it.
• The bony pelvis, also known as the pelvis skeleton, consists of the pelvic (hip) bones, sacrum, and coccyx.
• The pelvic bones articulate posteriorly with the sacrum at the sacroiliac joints and with each other anteriorly at the pubic symphysis.

Sacrum

• The sacrum is formed by the fusion of the five sacral vertebrae.
• It is pyramid-shaped with a base, an apex, pelvic (anterior) surface, dorsal (posterior) surface, and two lateral surfaces.
• The base of the sacrum articulates with vertebra L5, and the apex articulates with the coccyx.
• The sacrum has an anterior edge that projects forward as the sacral promontory.
• There are four pairs of anterior sacral foramina on the pelvic surface and four pairs of posterior sacral foramina on the dorsal surface.
• Five vertical crests are present on the dorsal surface, including the median sacral crest, intermediate sacral crests, and lateral sacral crests.
• The sacrum has an auricular surface on each lateral surface for articulation with the ilium of the pelvic bone.

Pelvis Regions

• The pelvis can be divided into two regions: the greater pelvis (false pelvis) and lesser pelvis (true pelvis).
• The greater pelvis is superior and serves as the abdominal part of the pelvis.
• The lesser pelvis is related to the inferior parts of the pelvic bones, sacrum, and coccyx, and has an inlet, walls, and an outlet.

Pelvic Cavity

• The pelvic cavity is a bowl-shaped space enclosed by the true pelvis, consisting of the pelvic inlet, walls, and floor.
• The pelvic cavity contains elements of the urinary, gastrointestinal, and reproductive systems.
• The perineum is inferior to the floor of the pelvic cavity and contains the external genitalia and external openings of the genitourinary and gastrointestinal systems.

Pelvic Inlet

• The pelvic inlet is completely ringed by bone.
• The inlet is bordered posteriorly by the body of vertebra S1, which projects into the inlet as the sacral promontory.
• The alae of the sacrum (wings-like transverse processes) contribute to the margin of the pelvic inlet.
• The margin of the pelvic inlet continues along the arcuate line on the pelvic bone and then along the pecten pubis to the pubic symphysis.