In ST:TNG, Ethics (Season 5, episode 16), Worf sustains a serious back injury and undergoes experimental back-treatment by implantation of a replacement spinal cord. We see that this works, but do we ever hear of Worf experiencing back problems after this procedure?
Worf never complains of back problems again throughout the course of both TNG and DS9, nor does he have back troubles during the films. In fact, in the DS9 episode Strange Bedfellows, Worf and Ezri Dax are hung upside-down in a cell. Ezri states: "I hate to admit it, but this is doing wonders for my back." Worf, who, as you said, has a history that includes a shocking spinal injury, seems to be experiencing far less discomfort than she is.
Within TNG and DS9, Worf never complains of any further back trouble. This is hardly surprising since the very essence of the ending of TNG: Ethics was that Worf only accepted Russell's experimental treatment (over suicide) because it offered him the chance to completely repair the damage caused by his accident;
RUSSELL : That's what this is really about, isn't it? Lieutenant Worf. I'm offering him the chance to recover fully -- a chance you can't give him.
BEVERLY : I'm delighted that Worf is going to recover. You gambled. He won. Most of your patients aren't so lucky.