Joint laxity after ACL repair occurs in about 10 to 30 percent of the patients who have a hamstring transplant. It usually has nothing to do with what the patient did or didn't do.

To keep this from happening the surgeon tries to "tighten up" the reconstructed ligament during the operation. The problem is that the amount of tension to apply to the graft is unknown at this point.

Some surgeons warn that too much tension will increase the pressure on the knee too much. But studies show that the tension on the tendon graft tends to decrease by more than 50 percent with repeated use.

To prevent this from happening the graft is "preconditioned" before being used to repair the torn ACL. The tension is preset and the graft is kept at a lower temperature than normal body temperature. There may be a problem with this preparation. According to a recent study, once the tendon graft warms up to body temperature, it relaxes and loses some of its tension.

It seems there may be several variables at work here that the patient doesn't have control over. More study is needed to find the right amount of tension to use with ACL repair and to find ways to hold it.