When you have ACL reconstruction, a new ligament is created and held in your knee with screws or other fixation devices. The ACL then has to heal in this position in order to withstand repetitive stresses associated with sports activities. The problem with any type of ACL graft fixation method is that, if repeatedly stressed before the graft has healed in position, the fixation will eventually fail. That means the new ACL graft can become loose.
Furthermore, there are other risks of overly accelerated rehabilitation. For example, former NFL wide receiver Jerry Rice famously made a comeback 3 ½ months after undergoing ACL reconstruction in 1997. In his first game back, he fractured his kneecap (where the ACL graft was taken from). This new injury ended his season.
What Is the Minimum Time for ACL Tear Rehabilitation?
Exactly how long the ACL graft takes to heal sufficiently is open to debate. Furthermore, some sports and activities will place a higher demand on the ACL and more healing may be necessary prior to participation. Most orthopedic physicians agree that a minimum of 6 months is needed before returning to competitive sports. Because of potential risks of reinjury, however, most recommend waiting 7 to 9 months.You should always discuss returning to sports with your surgeon, as there are variations to standard ACL rehabilitation. Variations in rehabilitation may be due to the type of graft used, any procedures performed along with the reconstruction (such as meniscus repairs or cartilage repairs), or surgeon preference.
Lastly, ACL rehabilitation is not only an issue of time. ACL rehabilitation involves a progression of therapeutic and sport-specific activities. In order to progress from one step of rehabilitation to the next, you must be able to complete certain activities. If a patient is unable to continue progression, overall rehabilitation may be delayed. That is why working with a physical therapist is so important after ACL surgery.
Sources:
Larson RL and Tailon M "Anterior Cruciate Ligament Insufficiency: Principles of Treatment" J. Am. Acad. Ortho. Surg., Jan 1994; 2: 26 - 35.

