I found this NY Times article, The Uneven Playing Filed, by Michael Sokolove on ACL injuries in girls and young women. It describes the ACL, the injury, and the recovery pretty well.
The A.C.L. is a small, rubber-band-like fiber, no bigger than a little finger, that attaches to the femur in the upper leg and the tibia in the lower leg and stabilizes the knee. When it ruptures, the reconstructive surgery is complicated and the rehabilitation painful and long. It usually takes six to nine months to return to competition, even for professional athletes.
An ACL does not tear so much as it explodes, often during routine athletic maneuvers — landings from jumps, decelerations from sprints — that look innocuous until the athlete crumples to the ground. After the A.C.L. pulls off the femur, it turns into a viscous liquid. The ligament cannot be repaired; it has to be replaced with a graft, which the surgeon usually forms by taking a slice of the patellar tendon below the kneecap or from a hamstring tendon. [I've read that most ACL replacement come from cadavers these days.] One reason for the long rehabilitation is that the procedure is really two operations — one at the site of the injury and the other at the donor site, where the tendon is cut.
It takes 2,000 Newtons (a measure of force) to rip an A.C.L. apart. (Researchers know this from intentionally snapping cadaver knees.) The mystery is why a knee works properly for many years — through game after game, practice after practice, long season after long season, for tens of thousands of repetitions — and then, without warning, a tiny but crucial component suddenly malfunctions.
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