There are risks--be sure to discuss these with your doctor. There are risks associated with the graft. These include failure of the graft, loosening of the graft, and a chance that the graft does not provide optimal stabilization of the knee following ACL surgery. All of these may require a revision reconstructive surgery.
There are complications associated with ACL surgery that are risks of all surgical procedures. The most significant is the risk of infection. This is especially concerning in patients who receive a donor graft. Recently, there have been several reports of patient deaths because of infected donor grafts after ACL surgery. These cases have been addressed, and the provider of the graft has been shut down, but this is a potential risk with a donor graft. There are also risks of anesthesia. These should be discussed with the anesthesiologist before ACL surgery.
All that said, the chance of a successful ACL surgery is about 90%. It is important, however, that patients understand this is a big procedure. Most importantly, for the ACL surgery to have any chance of success, a significant effort is required post-operatively by the patient. Without adherence to a proper rehabilitation protocol, the ACL surgery failure rate increases dramatically!
What if I choose not to have ACL surgery?
That's fine--ACL surgery can always be done in the future. Many patients opt to try physical therapy to strengthen the muscles around the knee in an effort to avoid ACL surgery. Many sports do not require an anterior cruciate ligament. The ACL comes into play when you are pivoting, or making lateral movements suddenly. For example, running in a straight line shouldn't be a problem without an anterior cruciate ligament. Many sports such as jogging, cycling, swimming, and others do not always require a functional ACL. Often people who participate in these sports, or those who are not physically active, choose to try rehabilitation alone. Some doctors also prescribe a sports brace to help support the knee. Most studies show these have only a small impact (if any at all), but they certainly don't hurt.
What are the risks of not having ACL surgery?
This is also very important to consider. While no one dies because they don't have a good ACL, there may be a significant downside to not having ACL surgery. One important factor is quality of life. Many patients rely on certain activities to maintain their mental health. If an individual cannot find satisfaction without being able to participate in sports that require a function ligament, then ACL surgery is an option. Also, new studies have shown that degenerative (arthritic) changes in the knee may be accelerated in patients without an ACL. This means that every time the knee "gives out," you may be damaging the cartilage. This is especially important for younger patients who will need good knees for many years. Many surgeons recommend that if a young patient attempts non-operative treatment, and they have a repeat episode of instability (after the initial injury), then they should have the reconstructive surgery.