I have a back injury as well. I only squat with the Smith machine and for 3 years I didn't squat at all. I still made serious progress on my legs and, in fact, some of the big squatters in my gym thought that I must be able to squat a shitload of weight. So I disagree that you must squat to get big legs. My workout partner, who has lifted his entire life, has made more progress since training with me for 2 years than all of the squatting that he has previously done in the past.
What I do is isolate the quads by changing foot position on the hack, Smith and leg press. I also do weighted walking lunges to hit the stabilizers. I do a lot of circuit and drop sets, rest pauses and slow reps. I also do ball squats and sissy squats.
One thing to note is that if you have a choice, use the Smith machine with vertical rails on it. The ones that are slanted suck ass IMO.