Can someone provide insight on knee animation

maytreejane

I don't aim to be an animator but I rig my models to see if such and such geometry deforms correctly so I can produce animation friendly models in the future. I don't have any trouble with the knees if I just make them straight however when they follow a more natural curve they don't deform good at all. I don't know how many game riggers add the third bone to the knee that would solve the problem so I'm trying to do my best without it. Is this result acceptable? How can I fix it? The resting position bones are straight except from the side view.