The posterior pelvis normally refers to the bones that make up the rear aspect of the pelvis. At times, it also may refer to structures or tissues found within or attaching to these bones. Bones found here include the paired ilium bones of the pelvis and the sacrum and coccyx bones at the base of the spine. Ligaments of note include the sacroiliac ligaments joining the sacrum and ilium and the sacrococcygeal ligaments connecting the sacrum to the coccyx. Interior, or deep, to the posterior pelvis is the rear half of the circular space known as the pelvic cavity, which contains the colon and rectum. Exterior, or superficial, to the posterior pelvis are the muscles of the back of the hip, muscles like the gluteus maximus that link the pelvic bones to the femur bone of the thigh and thereby move the legs.
Consisting of three pairs of bones — the ilium, ischium, and pubis — the pelvis is the structure that absorbs the weight of the top half of the body when seated or standing upright. It also makes lower body movement possible as many of the leg muscles attach to these bones, and it houses several organs of the gastrointestinal, urinary, and reproductive tracts. The pelvis is an irregularly shaped structure. When viewed from the side, a large portion of the ilia is situated in the posterior half, as are the bones of the base of the spine.
At the topmost point of the posterior pelvis, felt on either side of the back of the hip just below the waist, is the iliac crest. This is the highest aspect of the wing-shaped portion of the ilium bone and is the bony ridge felt upon placing one’s hands on her hips. Sloping posteriorly and slightly downward from here is a continuation of this ridge known as the posterior superior iliac spine.
Where the two ilium bones meet in the middle are two palpable bony bumps. These are the horns of the sacrum. Made up of five fused vertebral bones, the sacrum is the large wedge-shaped bone found immediately below the lumbar portion of the spine and between the ilia and is responsible for transferring movement forces between the spine and lower body.
Just below the sacrum is the coccyx or tailbone, which is made up of four narrow, fused vertebrae. Considered the vestige of what was once a tail in early humans, it is not as relevant to movement as the other structures of the posterior pelvis, yet it still serves a purpose. The coccyx functions to maintain balance when sitting leaning backward and is a site of attachment for the muscles of the pelvic floor.