What can cause back pain after knee surgery?

Alignment issue...: Recovery from knee surgery is problematic enough; but if you have a 'sensitive' back this raises the level of care. Any problem that throws off your gait(or how you walk) will cause instability in the muscles and dynamic of your back. Keeping strong abdominal muscles will help counteract this problem; and as your knee recovers your back issue should walk on by. Try some heat to help your back.

Altered walking: I see this fairly often and it is usually do to a change in the way a person walks after the knee surgery. They will tend to favor the operated leg and this will cause excessive strain on their other joints including their back . Usually when the knee gets better and the walking improved the back pain gets better too.