Are you wearing a brace?
If leg stability is an issue you can always try roman crunches... basically place your legs on a chair to keep your knees at 90 degrees and then do a normal crunch. I lift until my shoulder blades are off the ground and keep the move slow.
An ab-wheel might be a good one too however put a lot of padding under your knee, like a pillow for support.
Also, think about taking Arnica 30CH... I recommend it all the time here as it helps promote recovery from tissue damage and bruising and has zero side-effects. It's the shit basically.
Take your time in your recovery, good luck.