Hi Chris,
I hope you are feeling better. Am just wondering if you had resolved your knee problem already. If not, I almost feel like it was a hamstring strain that accompanied your knee hyperextension. Soft tissue injury could take upto 6 weeks or some even 3 months, depending on the extent of the injury, before it fully heals.
If your ACL or any other lig. was torn, it would have been easily picked up with special tests. You could have a patellofemoral syndrome as well if going down the stairs had become difficult or painful.
anyway, please let us know how you're doing....
cheers,
R