The lumbosacral joint, also called L5-S1, is a term used to describe a part of the spine. L5-S1 is the exact spot where the lumbar spine ends and the sacral spine begins.

Axial MRI of the Lumbar Spine - T2 weighted image at the L4 level. On this view centered over the L4-L5 disk, one can see well the cauda equina surrounded by CSF (note that CSF is bright on ...