Anterior Cruciate Ligament injuries to the knee are usually caused by a traumatic event/force upon the knee at an angle. They are considered a very serious injury as the ligaments are integral to the proper structural foundation for the knee. They can be surgically addressed with generally good results. I know that yoga offers a good opportunity to re-enter a physical routine however some of the more extreme positions of the knee need to be avoided initially. One can also use a sock or small hand towel rolled into the back of the knee space to assist the knee in folding without going into an extreme position. Much caution and proper alignment is required to begin however I have seen students recover quickly and fully through yoga and go onto create a healthier functioning knee for years of good fun.