When the hamstring tendon is used to repair the ruptured anterior cruciate ligament, the donor graft is harvested early and "preconditioned." Basically this means the graft is allowed to reach the same temperature of the operating room -- about 20 to 30 degrees lower than the body temperature.
The idea is to increase the tension and stiffness of the tendon graft. Hamstring tendon grafts tend to loosen up more than patellar-tendon grafts. Setting the proper level of tension to start is important. The problem is no one knows just exactly what that is -- and it may be different from patient to patient.
Most recently a study done on cadavers (bodies preserved after death for study) showed that cooled tendon grafts warm up once they are placed in the body. So maybe it's better to keep the tendon grafts warm until implanted. Until an answer is found to this dilemma, surgeons will continue to precondition temperature and tension of tendon grafts in hopes of getting the right amount of joint stiffness and stability.