Q: I have a ballet student who feels a click in her working knee every time she does a fondu in any direction. It happens just as she straightens it, and it seems to be relatively painful. What do you think is happening?
A: While the knee is a hinge joint that should only bend and straighten, sometimes, when the knee is bent, the tibia (lower leg) will rotate, causing discrepancies. I suspect that your dancer's tibia and femur are not straightening in the same range of rotation, which is creating a torque at the knee when she bends.
A simple way to check this is to have her sit on a table with the lower half of her legs hanging off. Stand in front of her and have her slowly straighten her entire leg in parallel—no turning out! Watch if the kneecap gets pulled to the outside at the end of straightening, or if the foot has changed its facing. This indicates some muscle imbalance or tibial torsion that may need attending to.
If they are not having any pain in straightening the leg in parallel, then you need to look more closely at how they are working their turnout when the knee is bent. If they have pain only when turned out, there may be a problem with over-rotating the lower leg as they are presenting their fondu.