A summary from The New York Times:
I chose to have ACL surgery in 2007. I don't think I would have been able to continue karate training without a stable knee. I was also in a hurry to have my knee "fixed". However, I do worry about the long term health of my knee. Early onset arthritis is a common occurrence in a knee with an ACL reconstruction.
Over two years, the injured knees were assessed using a comprehensive numerical score that rated pain, function during activity and other measures. At the time of the original injury, the knee also had been scored. At the end of the two years, both groups showed considerable improvement. The scores for the surgically repaired knees had risen by 39.2 points. The scores for the more conservatively treated knees also had risen, by 39.4 points. In other words, the outcomes were virtually identical. Despite a widespread belief that surgery leads to a stronger knee, the results showed that surgically reconstructing the A.C.L. as soon as possible after the tear “was not superior” to more conservative treatment, the study’s authors wrote. The findings suggest, the authors concluded, that “more than half the A.C.L. reconstructions” currently being conducted on injured knees “could be avoided without adversely affecting outcomes.
....By one estimate, as many as 1 in every 556 fit, active people will tear an A.C.L. — particularly if they participate in sports that involve frequent pivoting and landing, like soccer, football, tennis, skiing and basketball. ”