The pelvic brim is the rim of the pelvic inlet, the large rounded opening formed by the upper inner surfaces of the bones of the lower pelvis. Delineated by those bones’ internal borders, the pelvic brim is the circumference of the pelvic inlet. It comprises the edges of three bones: the sacrum at the base of the spine; the ilium, the butterfly-wing-shaped bone to either side of the sacrum; and the pubis, the paired C-shaped bone forming the center portion of the lower pelvis. This circumference is broader from side to side and narrower from front to back in the female pelvis than in the male pelvis.
Higher along its posterior border, the pelvic brim tips downward from back to front. Its rear border is formed by the prominence of the sacrum, which is the broad front edge of the top of the sacrum, the tapered stacked bone of the lower spine. On either side of the sacrum, the brim is formed by the medial or inner edge of the ilium bone. This edge is known as the arcuate line, so named for its curved shape. The arcuate line indicates where the wing or anterior portion of the ilium bone ends and the body or posterior portion begins.
Where the arcuate lines of the ilia converge is the lower border of the pelvic brim, as formed by the edges of the paired pubic bones. The upper half of each bone is known as the superior ramus of the pubis. Along the rear border of the top of each ramus is a ridge known as the pectineal line. This line is named for the pectineus, a muscle of the hip that finds its origins on the superior ramus just adjacent to the pectineal line. Where the two pubic bones meet is a joint called the pubic symphysis, which forms the lowest border of the pelvic brim.
Several tissues pass through the pelvic inlet and thus are contained by the pelvic brim. These include the reproductive organs of both sexes; several urinary and digestive organs, such as the urethra and the sigmoid or pelvic portion of the colon; the nerves and blood vessels supplying them; and the muscles that ensure their function, such as the levator ani muscles of the pelvic floor. In women, the uterus is contained here, and the pelvic brim is shaped differently in the female pelvis to accommodate the uterus during pregnancy.