Can you tell me the function of the facet joint at 5-1?

Spine Joints. Facet joints are places on the spinal bones where two bones actually meet and articulate and limits extension and rotation of that spinal segment. You have facet joints at every level of your spine from your neck to your low back. Many times arthritis or inflammation can arise here and cause pain.
Weight bearing/ROM. These joints bear load and allow for a certain extent of rom (range of motion) to occur at this level of the spine. As well as the other levels where they are found in pairs, left and right sided..
Support & stability. The facet joints at l5s1 are very important. They allow movement, but help prevent the L5 bone from shifting forward on top of the s1 bone, a condition called spondylolisthesis. In doing this, they also protect the l5s1 disk.