I have bad knees that prevent me from doing the things that I want to do. I have severe patella maltracking in each leg. The patellas in each knee are laterally tilted to the extent that the lateral edge of the patella has bone spurs on it from rubbing against the femur. Surgery is planned at the end of the year to fix it and hopefully I can get back to 75%.
Over the years I have had to learn to train legs an entirely different way which has lead to learning the correct way of training them.
The truth is that with your condition, you will never have bodybuilder-esque leg development. Depending on where you are in your development, if they are not a dominate bodypart, you will never make them a dominate bodypart. Chances are pretty good that to the bodybuilding eye your legs will be under developed. But to the mainstream world, they will be big. Just keep in mind that Arnold had small legs for his frame in the bodybuilding world, but ask anybody that is not a bodybuilder and everyone will say they are huge.
My patella maltracking gives me so much pain that I have had to completely overhaul and rethink my leg training. I can't do certain things because my knees can't handle it. Those things include items, bent over rows, deadlifts of any sorts, dumbbell rows, jumping, running and most leg exercises. But I have found ways to train them and actually make them a body part that gets complements. To the bodybuilding eye are my legs small? YES. But compared to most people in my gym are they big? YES. Do most general people think I have big legs? YES. Last summer I was walking up to a group of people wearing shorts and the very first thing said to me was "your legs are huge."
The point is you can build great legs. But it will take a lot of effort, discipline, patience.
This is what I do:
1) Take a super high quality Hyaluronic Acid supplement and Turmeric Supplement. I personally take Hyaluronic Acid, Turmeric, Glucosamine, MSM, Chondrition, Cissus, and Boswellia.
2) I also take 2 Aleve and 2 Tylenol every morning and night.
3) Your hips are extremely important. Your legs originate in your hips (Duh) which makes it insanely important to work them and stretch them. 3 times a week I spend an hour stretching my hips with long stretches. I hold the position for 3 minutes then do the next leg.
4) I train legs 2 times a week. When I do, I wear neoprene fat loss shorts. This will keep the heat in your hips and allow them to warm up.
5) When it comes to training you have to rethink how your train legs. It will be a blessing in disguise because you will actually learn how to properly train your legs for growth. Slow down all your reps. They should take 3 seconds minimum on the eccentric. Also boost up your reps. I literally never ever do less than 20 reps for quads. What you have to figure out to do is stress the quads/hammies without stressing the joints. Joints respond to load. The heavier the load the more stress on the joint. Muscles on the other hand respond to load and other things like time under tension and lactic acid build up. What you have to do it make the weight feel heavy without actually being heavy.
I do things like:
50 unbroken reps on the leg press. 3 seconds down, 1-2 seconds up. 50 reps. (trust me, if you can do this for 3-5 sets with 3 plates on a leg press, no way your legs will be small)
Regular Squat or Belt Squat. 10 sets of 5 reps, with 3 seconds down, 5 seconds in the hole. Set a timer for exactly 1 minute in between.
10 sets of 30 on leg press (unbroken). 3 Seconds down, 1-2 seconds up. Set a timer for 1 minute rest.
Trust me, those above are no joke.
6) WARM UP!! If I don't warm up correctly, my knees are fucked. My warm up includes 20 minutes of light biking. 3 sets of 50 each knee of TKE's supersetted with banded abduction for 20-30 reps. Followed by sled drags focusing on the lock out of the leg for about 20 yards, supersetted with sled pushes. For the pushes, keep the hips low and push through. Then it is on to hammies then quads last.
7) Figure out what exercises you can do pain free and do a fuck ton of those. You said 135lb squats hurt. Try something else. When I first started back I could only do TKE's. That was it. Do those for 5 sets of 50, slow and squeeze, 3 times a week and you will get some where. You mentioned 135lbs squats. Can you do them with 95lbs or just even the bar? If you can try to do 3 sets of 20-30, 3 seconds lowering, 3 seconds in the hole.
The point is to find an exercise or weight that doesn't hurt and make it heavy. I can't squat more than 135lbs without knee pain. I don't leg press more than 3 plates a side and it took 2 years to build that. If I do machine squats, it is 1 plate and it took 2 years to get there.
8) Most importantly, BE SMART!!!!! This has taken me a while to figure out. What I mean is, say you can squat 95lbs without pain. DO NOT instantly thing you can do 115 without pain. Take small jumps. Don't try to pr or do more because you are feelin froggy. Literally every time I have done that, I have paid the price by not being able to train legs for weeks afterwards. That is more of a detriment that training them lightly. If you think you can do more, do more next week on the last set with only a 5lb increase. Not training legs because of waiting for knee pain to heal slows down you more than sticking to the same weight.
9) Lastly, don't let a doctor tell you that you can't rebuild cartilage. That is a fallacy. You can. HGH injections in the knee have been shown to increase cartilage. Also there is MACI Implementation surgery which works, but the recovery is 1 year.
Any ways that is my blue print. It has worked for me considering that the weights I use are so small but I do I have decent legs. It is possible, but it does take work.