You can do sit-ups isolating the one leg which you have injured. Planks might help you a little but they are not all that challenging for the abs area after one moment onwards.
The only exercise that I can think of right now that will completely isolate your knee(and is not very advanced) is the dragon flag. It got popular thanks to bruce lee who introduced it to the world in his active years. The exercise primarily targets the abdominal muscle area of your body, it will also work out your arms a tad bit and completely leave the legs to rest. Below you can see snapshot of Bruce Lee performing it.
There's a video tutorial that I had released couple years ago explaining the progression methods that you can use in order to learn the exercise. What's great about this exercise is that no matter which progression you start with, it will provide your abs with an immense workout from the very beginning. Here's a link to the tutorial I am talking about, if you have any questions fire away at me! http://www.youtube.com/watch?v=zf0Skpkag50 (Disclosure: The video materials are mine)