As everyone else has said, there is no replacement for the squat, lower your weight and concentrate on form. Squats involve not just your quads, hams, calves, ligaments, tendons, buttock, spinal erectors, traps, shoulders, I mean it virtually works the whole body.
Not only that it takes coordination and balance. Anything else such as a machine like leg press, extensions, etc. isolate the muscle, its not the same.