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.

Learn about the structure and composition of the pelvis, including the bony pelvis, sacrum, and coccyx, and their articulations.