I had back surgery done about 1 month ago and the pain in the back is gone, but numbness in the leg still?

I have had back surgery done on my back. It has been about a month now but I still have numbness in my left lower leg. What is causing this? I can walk on it fine, it is just that below the knee cap to the foot area is still numb. Has anybody went through this before. What is causing this in my leg?