L5-S1 is the exact spot where the lumbar spine ... a bone area in the back that connects the adjoining parts of the facet ...

A CT scan of a 32-year-old ballet dancer who had intermittent low back pain radiating to the back of her left leg during leg extension movements. It reveals arthrosis of the L5-S1 facet joints.

The L5- S1 refers to specific vertebrae in the spine ... the front and does not involve any kind of damage to healthy bone, ...