Hurts when bending back. physical therapist thinks it's a disc tear. ct scan showed bone spur on facet joint. what could it be?

Hard to say: Determining the exact cause of pain is one of the most challenging issues in the field of spine surgery. It is a generalization, but facet pain is often reproduced by extension or rotation of the back, where disc pathology is usually worsened by flexion. Not every case follows this. Consider being seen.

It is really neither: The pain is coming from the arthritic facet joint. It is the arthritis that is causing both the spur and the pain. The spur is the result of the problem, not the cause of it. Any disc pathology would be causing leg pain, not back pain.