Tiger Woods revealed last week that he'd been playing golf on a bum left knee for nearly a year. And he hadn't been doing badly: Recently, he finished second at the Master's and won the U.S. Open after forcing a playoff last week.

We'll never know if perfect knee health would have meant another green jacket. What we do know is that he winced in pain after every shot and caused more damage to his knee.

Woods had torn the anterior cruciate ligament (ACL) in his left knee about 10 months ago. One of four rubber band–like ligaments that connects the thigh bone (femur) to the tibia (the bone that makes up your shin) the ACL is the primary stabilizer of the knee. It extends from the back of the femur to the front of the tibia and basically keeps the lower bone from rotating all the way around when you pivot on your foot, allowing you to make sharp turns.

Such tears are a common injury in basketball, soccer, football and skiing, in which cutting, turning and pivoting are important parts of the game. For a college basketball player an ACL tear is a season-ending injury. People who injure the ligament typically hear a pop as it ruptures and the joint seems to give out.

It's exceedingly rare for anyone to tear an ACL playing golf, and in fact Woods says he did it while running. Not only are ACL tears rare in golfers, but for a typical golfer, the injury won't keep you from playing.

So why did Woods decide to have his injury repaired, meaning he would need to miss so much golf?

It's probably because in Woods's case, his whole golf swing is all about rotation. He gets much more hip and lower-back rotation than any other golfer I have seen. The ACL is the only thing holding that rotation back, so his knee is under more pressure than the average golfer's. That means more pressure on the cartilage (the whitish tissue found at joints between bones) in his knee.

Without the ACL preventing it, repeated stress can soften and weaken the cartilage, which can break off. That's probably what happened in Woods's knee, and it squares with the fact that he had surgery in April to remove broken cartilage in that joint. The reason he didn't have the ACL repair done at that time, according to a statement by Woods, was that he wanted to play in June at the U.S. Open at the Torrey Pines Golf Course outside San Diego near where he grew up. Recovering from the cartilage surgery was only supposed to take several weeks, whereas the ACL surgery would have put him out through October or so.

He was able to play, but his knee may have held him back—and certainly kept him in pain. The rotation required to execute a Tiger Woods's swing is immense, and if Tiger can't rotate like Tiger, he can't play like Tiger. Tiger Woods at 90 percent can still win tournaments—as he showed last week—but being less than perfect falls short of his own demanding standards.