bone problem

hello
i am trying to rig up this person, but i have a problem at the knee section. When I bend the knee, the bending makes the knee section really thin as if its going to be broken. I want the knee to bend but i don’t want the radius of the knee to get small, how would i do that?

You might be able to solve this with weight painting. I assume you’re familiar with weight painting to have gotten this far. You can pose your armature while you weight paint, which can be very helpful to get the painting right. I’d suggest playing around with this, painting while posing the joint and trying to get it to deform right. If your mesh geometry is good, you might be able to get pretty close this way.

If not, the next step would be to take a look at this guy’s tutorials:

http://kokcito.tk/tut/rig1.html