Put one foot in front of the other, the song goes, and soon you’ll be “walking ’cross the floor.” It seems simple, yet the act of moving your body forward requires a complex set of muscles, joints, and bones to work in a precise fashion so that you can get from point A to point B. For millions of American, this movement is extremely difficult and painful, the result of wear and tear or an acute injury to the body’s largest joint – the knee.