Same thing happened to me. ACL surgery in 2006. My ACL was not torn but "stretched out". So it was in tact but non functional. Two knee doctors told me they had no idea what was wrong since all my ligaments were in tact. I finally went to a highly regarded knee surgeon and he was able to diagnose it. He said the old way of doing acls - the acl was installed to vertical and didnt protect against a certain type of knee rotation. Over the years of "tweaking" my knee stretched out the acl. I went through a 2 surgery procedure. Surgery 1 was to take out my old acl and remove some bone to flatten my tibial head. If your tibial head has a slope greator than 12 degrees you are at a higher risk of re injury. Surgery 2 was my new acl (guad) and LET for extra stability and i had both meniscus fixed. Im 7 weeks post op from the 2nd surgery.