XLIF CANNOT be done at the L5-S1 level because there is no access to the area due to the hip being in the way. That is why he suggested the PLIF for you at L5-S1 and the XLIF for the L4-5. Hope this helps you understand why he is doing two different procedures. Good Luck with your surgery.
I saw a spine specialist 2/8/13 for a second opinion regarding my severe DDD back issues and have done extensive research this weekend. For me, he recommended an XLIF as well as Axialif for my back fusions L2 thru S1. This compares to my local ortho spine surgeon who recommended ALIF and PLIF at the L5-S1 ONLY. I have allot to look into. This in NOT an urgent matter as I have dealt with back and leg pain since my last surgery 2007.
I still need a CT Scan this week to better understand my issues at the L3-4 level. I am just joining this message board today and will continue to do research this week re: doctors trained for the recommended procedures and getting in touch with reps in my area who sell the needed medical devices/equipment for the surgery.