I blew my ACL while palying for the junior national rugby team and I had an ACL repair on the right knee using fiber carbon ligaments.
Best method I used to build up the legs was in the swimming pool doing the following exercise progression;
1)I would put on a life vest and some old converse sneakers and go in and do front/rear scissor kicks, and sided to side scissor kicks.
2) Once strength built up, I had a pair of special strap on shoe soles made that had 3 inch plates (one running down the middle of the sole to the heel, and another the bisected in the middle. I would strap these on and do the scissors exercises again.
3) Once strength built up, I would strap on some ankle weights or iron shoe weights and do scissors again.
4) Every other workout I would get in the pool and put on some scuba fins. I started by just doing lengths over and over again using a flutter board held out in front flat on the surface while swimming on my stomach, and held flat under my head while on my back.
5) Once I built up strength I would strap on the scuba fins only this time I grabbed the ends of the flutter board and held it straight out in front of me under the water as if I was trying to push a giant squegee. I also held it this way under my neck when I was on my back.
6) Once I built up strength this way, I used a spinal board they had at the pool doing the same exercise both front and back. When you get to doing this resistance, your legs will be incredibly strong.
Each training session in the pool I would do as many lengths as I could for 1 hour (30 minutes on front front and 30 minutes on back )
The best way I found to train the legs was in the water using the scuba fins doing the aforementioned exercises. It will strengthen all of your leg muscles, tendons, and ligaments while giving you a great anaroebic workout.
I was also strapped to a Russian EMS (Electro muscular stimulation) machine twice a week, and tortured myself using varying intensities and time durations. This also can help you develop some strength if you have access to one.
Another great tool that I used was a machine called a fitter, which I would balance on using one leg while moving front to back and side to side.
Hope this helps.
P.S. After doing this training, I have had no problems with full squats, deadlifts, running, etc.