Back pain continues after l4/l5 fusion last spring. post-op i developed scarring & lumbosacral arachnoiditis. does this fall under the definition of "failed back surgery"? i can't stand long & have sensory ataxia so i'm not in a good spot. thank you?

It could be. But a good exam and an MRI/CT would help. In the meantime do PT under the care of a good Therapist and use local pain control with things like heat, pain patches or TENS. See your surgeon or get another opinion. Hope you feel better.

Your experience is consistent with post-operative scarring, leaving you with lumbosacral arachnoiditis, which will not depart nor dissipate, over time. Injections and/or future surgery will make things worse. Stay with conservative physical therapy, learn and practice McKenzie Back Exercises, learn meditation and biofeedback for pain control. HOPE THIS HELPS!