Because of a shouder injury I can't do barbell squats either, so I've been using db squats, and the results aren't that bad. You could also try using bodyweight squats holding a 45 plate tight against your chest. If you go low enough and slow enough, you should get a good burn off them, although you may have to shoot for 20 or more reps. O.M.G Just remember that any form of doing a squat is better than none at all!