As a sports physiotherapist and having treated numerous knee ligament tear patients for many years, I can tell you from the start that this is a big choice to make. Tearing the ACL is an unfortunately common injury in sports that involve cutting, pivoting and twisting at speed, tackling, and jumping and landing. For most people, it’s an end result of an alliance between a tired knee, combined with speed, bindings that didn't release and a sudden twist or deceleration that stops the motion very quickly and Bang! The entire force of the resultant torque tears the ligament apart.
If you've been diagnosed with an ACL rupture (this can be done with MRI scanning, or sometimes it's just pretty obvious with clinical testing), you will then have the choice whether to go ahead and have it surgically reconstructed, or not to have surgery and work on rehabilitating the knee. Contrary to popular belief, surgery is not always 100% needed, and there are actually even high-level sports people that continue to operate with an ACL-deficient knee. It is entirely possible to strengthen the knee and work on balance and control, enough so that the surrounding muscles and rest of the ligaments in the knee can cope with the function required of it.
Why is having surgery such a big choice? Let me tell you from experience as a rehabilitator, intensive rehabilitation is required following surgical repair of an ACL. It really is a good 9- 10 months for most people, before the knee, is functioning to a good enough level to return to most previous activities. Over that time, if you want a successful outcome, you have to be diligent with strengthening, balance and fitness exercises. A patient that is not committed to the rehab program, in general, will not have a successful outcome. You also have to consider other issues such as time off work (this can vary but for a sedentary person generally around 2 weeks is required, for non-sedentary, this can be much longer), social/family factors such as child care and home responsibilities, and the possible costs of rehabilitation.
What are the positives in that case of having the surgery done?
Firstly, if you want to return to any type of recreational activity/sport that involves twisting and change of direction, then, in general, a surgical repair is the best option to get back to this. However, if you are someone whose exercise regime generally doesn't involve this, then again you may wish to reconsider. The degree of instability after injury is also a factor; some people find that after an ACL rupture they have recurrent episodes of the knee giving way, even with often trivial movements. Others don't find this at all, so again this is an individual consideration. Another important factor to consider is whether any other structures in the knee are involved. It is not uncommon to have a concurrent meniscal tear in the knee with an ACL injury, and it can be tough for that injury to heal by itself, often requiring surgical intervention. Lastly and probably most importantly, there is the possible problem of joint instability as a result of ACL deficiency that may lead to early degeneration or osteoarthritis in the knee. True, just by having an ACL rupture you are many times more likely to develop osteoarthritis in the knee anyway compared to someone who hasn't had this injury. But the worry is that the excess joint motion or joint 'play' in an ACL-deficient knee may mean that this degenerative process develops even earlier.
At the end of the day, the final decision is up to you. Have a look at the evidence of surgical vs. non-surgical trials, think about your personal circumstances and personal goals, and get as much information as you can from your physiotherapist and surgeon. And whatever you decide, get working on that knee strengthening and control.