Illustration showing transverse sections at different levels of the lumbar region.

<p>(a) At the upper level of the lumbar spine, three cleavage planes within the paraspinal muscles were found: ML, SQ and LI, which are the entries used in the Wiltse, Watkins and Weaver approaches, respectively. (b) At the lower level of the lumbar spine, only ML was found, and its superficial location lied more laterally to the midline than the location at the upper level. The Wiltse approach through ML may be the best choice for protecting muscle integrity and its neurovascular supply. <i>Arrow</i>: incision site; <i>M</i>: multifidus; <i>L</i>: longissimus; <i>I</i>: iliocostalis; <i>IS</i>: iliac spine; <i>QL</i>: quadratus lumborum; <i>PS</i>: psoas major.