Treatment decisions for ACL tears are always individualized - tailored to each individual. The decision whether to offer surgery is based on the person's age, activity level, how unstable the knee is, and whether other structures in the knee have been injured.
It is important to keep in mind that surgery to reconstruct a torn ACL is not an emergency for most people. Many people with a torn ACL do not need surgery at all. Even though the chances for complete success from surgery are now excellent, surgery is not for everyone. This is because not everyone needs the ligament repaired to return to his or her pre-injury level of function. It is important to distinguish whether the work, recreational, and athletic activities of the person is light, moderate, or strenuous. Another important issue that needs to be understood by the individual considering ACL reconstruction is that it requires many weeks and months of hard work in rehabilitation following the reconstruction. This needs commitment and time.
The Ultimate Deciding Factors...
Whether the injury is a recent tear or an old ACL problem, individuals need to consider their present activity level and decide if their daily activities and livelihood would be affected by the injury. The question of whether to have surgery to reconstruct the torn ACL arises most frequently with less athletically inclined older persons. Generally in such people, if the instability is severe, and the knee is constantly buckling, the decision to offer surgery is intended to prevent further damage to the knee and stop the daily discomfort of the knee giving way.
On the other hand, if the knee instability can be controlled by avoiding activities that the individual doesn't really mind avoiding, then going the "conservative" route and avoiding surgery is often a very good choice - and many people are satisfied with it. Certainly, there are many older athletes who are willing to avoid basketball, soccer, or racquetball, and stick to jogging or biking for fitness. As long as their knees are stable and pain-free for these activities, they are happy. Also with the use of a functional brace, many of these people find they can do most of what they wish to do without significant problems.
For those people who choose not to have surgery, this does not mean going without any treatment at all. There is still a treatment program to be followed emphasizing strengthening the leg muscles and learning to better control the knee and to avoid those situations most likely to cause instability. Many people benefit from this kind of rehabilitation.
If athletics is a regular part of your life, or if your work is likely to be affected by mild instability of the knee (for example, construction workers or other non-sedentary type jobs), your physician will lean toward reconstructing the torn ligament.
In general, stronger and fitter is better - and this applies to operated and non-operated knees equally.