Everyone agrees that ACL ligament repairs loosen up between six months and a year after the operation. It's not clear yet just what causes it. Likewise, no one knows how to tell how much it will loosen up.
Researchers report different results of studies on this topic. ACL repairs in animals and humans have been done. Some report more tension applied during the operation means a tighter knee after. Other studies show grafts with less tension tighten up more later. Some doctors advise minimal tension put on ACL grafts give the best results.
It's likely that joint tightness or looseness depends on many things. Graft tension is just one factor. The type of graft used, size of tunnel for the graft, and method used to hold the graft in place may be just as important. More studies are needed to find out the best graft tension and how to get it for each patient.