Why Would My Orthopedic Surgeon Ask For A Urine Sample

Why would my orthopedic surgeon ask for a urine sample when hes been prescribing me the same pain medications for 13 years? I've had multiple back surgeries all of which have caused more pain than I started with. The last surgery I had was an axiel lift not knowing it's high failure rate and the fusion didn't take. I now have broken facet screws and a large screw that they can not remove because it would cause more damage to my spine. I am extremely hesitant to have this surgery they say will alleviate my pain because the past surgeries were supposed to help and only made things worse. He's never asked for a urine test before and my prescription hasn't changed other than 120 pills last for 20 days because I take one every 4 hours where as before they would last 30 days.