Do you remember Iam Harrison IFBB pro from England. He had serious knee trouble and he discoved his quads grew very well using high rep leg extensions to failure followed by leg press. If you do a search on Ian Harrison's training you should be able to find the article. I believe it was on anabolicextreme.com. I have knee trouble also and I can no longer squat heavy, but I have found that high rep pre-ex with leg ext/ sissy sqt prior to ass to grass squats has done wonders for my quads and actually helped my knee to feel much better. I do the following. Leg ext 20-30 reps to failure, 15 breaths, go again same wieght 10-15 reps, 15 breaths, go again same wt. 8-10 reps, 15 breaths, squats 8-15 reps to failure. OUCH!
Second, I'll bet your back situation could be improved with the proper rehab. There is a tool called the "Wobble Chair tm" that I have found to consistantly rehabilitate(re-inflate/re-hydrate) the lumbar spinal disks. Also you can use weights to align posture and decrease scoliosis. For more info go to www. pettibonsystem.com to see the wobble chair and body weights.
PS. I am not selling anything nor am I affialted with the above company in any way.