Cause of bilateral thigh pain after failed spinal fusion?

Thigh pain. My first answer is missed hip arthritis. This causes pain from groin to knee, worse getting up from sitting. Commonly missed by spine surgeons too focused on abnormal MRI and not performing a complete exam. Not intentional, just happens. If fusion has failed, this could also cause pain into groins/thighs. A good exam and studies including x-rays would be helpful.
L4-5 issue? Failed spinal fusions, or pseudarthrosis, can be the source of hip pain, especially due to the l4-l5 level. Sometimes diagnostic injections help, and wearing a bone growth stimulator can cause the spine eventually to stabilize. Maybe add a picture to your question for further details. Thank you very much.