FBS refers to chronic back pain and/or leg pain which is experienced 'after' surgery.
Pain can be dull, persistent and aching and in the back or at the incision site, and or legs, it can be sharp, pricking or stabbing.
It is not a single disease but a collection of conditions that come after surgery.
Some pains are common after surgery but problems could arise from fusion failure or a transfer lesion to another level after a spine fusion.
What can happen is that the next level from the fusion degenerates and becomes a pain generator, which has happened to me. But, they also say that spinal fusion for multiple level spinal degeneration is also far less likely to be successful in reducing a patients pain after surgery.
A joint could also become irritated or inflamed due to the way the surgery has altered the patients posture, gait and ways of moving.
And of course scar tissue can also cause pain.