Configurable joints rotating at a certain angle

So I have a config joint on a knee for my character it only rotates on it's Y axis giving it that knee swing effect. I set it at 90 degrees and currently it bends forward and backwards past where your knee should bend. I want it to rotate from its starting position and then max out backwards at 90 degrees like a normal knee. Not rotate forward because if your knee rotates that direction you need a doctor. Any ideas? I can't find tutorials or info on it.