In yoga we try to avoid straining the knees at all cost, as it can be very difficult to resolve any problems with the knees once they occur. They are only really meant to bend in one direction and any sideways or twisting movement of the leg should come from the hips. That is one of the reasons why it’s essential to create flexibility in the hips, through standing poses such as Parsvakonasana and yin poses such as the Swan for example. Without flexible hips it would be too painful and harmful for the knees to sit cross-legged for any amount of time.
Please find below two brillant articles by Bernie Clark on the inner workings of the knees and what you can do if you suffer from a knee injury.