There are a number of low tension, higher float systems out there that are definitely worth a try.

Because of a wonky knee, I use Speedplays (the X2's). They have 25 degrees of float. There is almost no tension in the system. They're very easy to get in and out of but you do have to turn your foot more than with the SPDs. Since using them I haven't had any knee problems.

They take some getting used to, since there's so much float. When you first use them you feel like you're standing on an ice cube. But you get used to that very quickly.

If you need lots of float I would recommend trying Speedplays.