The point of the brace is to keep your ACL from healing and remodelling in a lax state. The graft that is your new ACL is not a native ligament and has to remodel to become your new, functioning ACL. Currently, it is still structurally a patellar or hamstring tendon or someone else's old ACL, and has not seen the forces of your specific joint. It has the potential to remodel as an appropriately tight ligament, or as an inappropriately lax one, and will do so, based on the biomechanical forces that it sees. The brace ensures that it sees the appropriate forces; as does the rehab program. A lax ACL graft is akin to not having a graft at all. Nothing pisses a surgeon off more than a failed graft. So stick with the program
