My surgeon has mentioned that I will have to be n back brace for twelve weeks following my lumbar fusion, why is this?

According to the longest prospective study to date on lumbar fusion due to degenerative spondylolisthesis, the best long term outcomes occur when the result is a solid fusion. The body needs time without movement to fully fill in the spaces from the surgery around the bone graft and or instrumentation in order to complete the solid fusion. If there is too much movement during this critical healing phase this biological process may not run its full course and the result may be incomplete fusion. Even though twelve weeks seems like a long time, it will be over before you know it, and will greatly enhance your likelihood of a successful result.