I had a knee issue also and couldn’t place any heavy vertical weight on my knee. In addition to leg ext, I added horizontal leg presses, pistol squats on a hack machine, and cable squats with a sissy squat machine. For the pistol squats, I put a calf raise bar on a hack and did rapid partial rep squats for 12 to 15 sets. These exercises didn’t bother my knee and I did those until my knee was ready to go again. U prob need to find what works for you.