Iliolumbar ligament

The iliolumbar ligament is a strong ligament passing from the tip of the transverse process of the fifth lumbar vertebra to the posterior part of the inner lip of the iliac crest (upper margin of ilium). 

It forms the thickened lower border of two of the layers of the thoracolumbar fascia. It radiates as it passes laterally and is attached by two main bands to the pelvis. The lower bands run to the base of the sacrum, blending with the anterior sacroiliac ligament; the upper is attached to the crest of the ilium immediately in front of the sacroiliac articulation, and is continuous above with the lumbodorsal fascia. In front, it is in relation with the Psoas major; behind, with the muscles occupying the vertebral groove; above, with the Quadratus lumborum.