Early on in my martial arts practice it was repeatedly stressed the importance of protecting the knees. Not in terms of kyusho- although that is important, but in terms of movement, so one does not hurt them over the years of martial arts practice.
The knees only bend optimally in one way and even at that there is a limit to the range of motion, bending backwards or side to side is a no.
A few points to keep in mind.
The direction of the feet as one walks or moves, making sure they are in alignment with the direction one is moving.
When extending the knee, making sure it does not go past the toes of the foot, bending at the hips vs. bending at the knee.
Paying attention to take ukemi correctly when thrown or falling so the knees don’t bend sideways when one falls.
Not slamming the feed down on the ground during training, especially if working on solo-training that involves a good amount of repetitions like the san shin no kata.
Paying attention to the health of the knees in the martial arts.