Sacrum

Where is the sacrum?

Its upper end, the sacral promontory, meets the body of the fifth lumbar vertebra at the lumbosacral joint. Its lower end meets the coccyx. (StatPearls, Sacral Vertebrae)

Its alae, or wings, meet the ilia of the hip bones at the sacroiliac joints. The promontory forms the back ridge of the pelvic inlet. (StatPearls, Sacral Vertebrae)

What does the sacrum look like?

The sacral canal inside it continues the vertebral canal and ends at the sacral hiatus. The spinal cord ends higher, at L1 to L2, but the dura mater reaches S2. (StatPearls, Sacral Vertebrae)

Four pairs of sacral foramina open on the front and the back and pass the S1 to S4 spinal nerves. S5 leaves through the sacral hiatus instead. (StatPearls, Sacral Vertebrae)

The back carries crests from the fused processes: the median crest from the first three spinous processes, the intermediate crests from the articular processes and the lateral crests from the transverse processes. On the front, four transverse ridges mark the fused discs. (StatPearls, Sacral Vertebrae)

What does the sacrum do?

The sacroiliac joints connect the axial skeleton to the lower limbs and pass force from the legs to the vertebral column. They also carry the load of the trunk down to the lower limbs. (StatPearls, Sacral Vertebrae and Sacroiliac Joint)

The fused vertebrae and large size of the sacrum make it a strong, weight-bearing base. The sacroiliac joint moves only 2 to 4 mm in most people. (StatPearls, Sacral Vertebrae and Sacroiliac Joint)

What attaches to the sacrum?

On the front, the piriformis arises from sacral segments 2 to 4 and some iliacus fibers from the ala. The coccygeus inserts on the border of the lower sacrum and coccyx. (StatPearls, Sacral Vertebrae)

On the back, the gluteus maximus, multifidus lumborum and erector spinae attach. (StatPearls, Sacral Vertebrae)

The interosseous sacroiliac ligament forms the main link between the sacrum and the ilium. StatPearls calls it the strongest ligament in the body. (StatPearls, Sacroiliac Joint)

The median sacral artery, a branch of the abdominal aorta, runs down its midline, and lateral sacral arteries come from the internal iliac arteries. (StatPearls, Sacral Vertebrae)

How does the sacrum vary?

The sacral vertebrae begin to fuse around puberty and finish by age 25 to 30. The female sacrum tends to be shorter and broader, with a less prominent promontory. (StatPearls, Sacral Vertebrae)

In lumbosacral transitional vertebrae, L5 fuses to the sacrum, giving six sacral vertebrae, or S1 separates as a sixth lumbar vertebra, leaving four. A meta-analysis put their prevalence as high as 12.3%. (StatPearls, Sacral Vertebrae)

Where the sources differ

The sources differ on the bone below the sacrum. StatPearls’ review of the sacral vertebrae gives the coccyx 3 to 5 fused vertebrae; its review of the vertebral column gives 4 coccygeal vertebrae. (StatPearls, Sacral Vertebrae and Vertebral Column)