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The iliolumbar ligament is a tough, dense band of fibrous tissue made up of bundles of collagen fibers connecting the spine to the pelvis. It is one of several ligaments in the region linking the lumbar vertebrae to the hipbone and to the adjacent sacrum. Like any ligament, its job is to connect bone to bone and maintain stability in the surrounding structures.
One on end, the iliolumbar ligament attaches to the transverse process of the fifth lumbar vertebra. The transverse process is a pointed, narrow bony protrusion emanating, wing-like, from each side of the vertebra. From there it runs horizontally across the gap between the spine and the ilium, the large, butterfly-shaped hipbone, and attaches on its other end to the iliac crest. The iliac crest is the topmost edge of the “wing” of the ilium, easily felt at the tops of the hips on either side. Specifically, the iliolumbar ligament connects to the inside lip of the iliac crest at its most posterior point, to either side of the tailbone.
Representing a border between the back and the hips, the iliolumbar ligament forms the bottom edge of the thoracolumbar fascia. The thoracolumbar fascia is a large membrane layered between and above the muscles of the low back, visible in anatomy drawings as an area of gray fibrous tissue separating the back muscles from the hipbones. It attaches medially along the thoracic and lumbar vertebrae, laterally to the transversus abdominis muscle on the sides of the trunk, and along its lower border to the iliac crest, ending at the iliolumbar ligament.
This ligament features two separate bands, with the upper band forming the border of the fascia and attaching to the iliac crest just anterior to or in front of the sacroiliac (SI) joint. The lower band stretches beneath it from the crest to the base of the sacrum, the stacked fused vertebral bone just below the lumbar spine, where it converges with the anterior sacroiliac ligament. This broad, short ligament horizontally connects the medial or inside edge of the ilium to the sacrum.
Several muscles come into contact with the iliolumbar ligament as they cross from the trunk of the body to the pelvis. The psoas major, the largest of the hip flexor muscles, descends from its origin along the thoracic and lumbar vertebrae and passes the anterior side of the ilium just in front of the ligament, inserting on the femur bone below. To the rear of the iliolumbar ligament are the erector spinae muscles, which run vertically along a groove to either side of the vertebral column and attach at various points along the sacrum. Finally, this ligament is bordered above and slightly laterally, or to the outside, by the quadratus lumborum muscle deep in the trunk, and spans the space between the iliac crest and the rib cage.