There is a commonly held belief that surgical repair of the ACL should be done when the knee is calm. Active inflammation leads to increased scar tissue. Doing surgery on top of an already acutely injured joint has been avoided.
Some studies have supported this thinking by their results. Some experts have advised against surgery after ACL injury for at least three weeks. Loss of motion and poor results with early surgery are the reasons given for waiting. But other studies don't support these findings.
In a recent study of active duty military personnel with ACL injuries, two groups who had surgery were compared. The first group had surgery withi21 days. This is considered to be during the acute phase. The second group delayed surgery for at least six weeks (sometimes longer).
Patients were tested and measured before the operation and again every six months for up to three years. There was no difference in results between the two groups. The concern that motion would be less and scar tissue more in the early surgery group was unfounded.
Strength, motion, and function were equal between the two groups. Most of the patients in both groups were still making improvements during the first six months. After that, later results were about the same as what they had at the end of six months. It appears that with proper rehab after surgery, the timing of the procedure may not matter.