There are four ways a leg (hip,knee,ankle) can deviate from the postural plumb line of the body.
1. Knees that angle in toward each other, with the feet facing straight forward; this is called tibial torsion. You can also see this clearly if you sit on a table and let the calves and feet dangle over the edge. Here your knees are straight in front of your thighs, and the lower part of the leg turns out.
Compensation for this is understanding and using your turnout from the hips, as best you can, and never allowing pronation, or "rolling ankles".
It is easy for legs like this to get a good turnout in the foot positions, but it should be worked to get the leg as close as possible to postural plumb line.
2. Knock knees is when the knees face forward when the feet are parallel, but the inside of the knees touch and the feet are apart on the floor, a little turned out, and slightly pronated (rolled in).
3. Bowed legs. This where the knees turn in slightly and the outside of the calves bow outwards. The feet can rest comfortably close together. The feet may also pronate slightly, yet will come to a correct position, flat on the floor, when the turnout is held well in the hips and thighs. This may straighten out the whole leg in some cases.
4. Hyper-extended legs, where the knees go beyond straight and the calves sway backwards. This will pull the body weight back onto the heels, and the thighs will turn in as a result (which can lead to tears around the knee).
The correction of stacking the ankle, knee and hips above each other along the plumb line, strengthens the legs. It also corrects the weight on the whole foot, and keeps the body weight forward enough. Uncorrected, this will lead to other complexities of technical inaccuracies, especially in doing ballet on pointe, if they do not show up before that.
The knees are wonderfully engineered joints. The details are described well elsewhere. Suffice it to say they are held in place by muscles, ligaments and tendons, and when healthy, all the moving parts glide and move well. The knees bend and straighten a zillion times for dancers and sports enthusiasts, without mishap, if used correctly.
Turnout enables easy pivoting to change direction without straining the knees. Many athletes now study basic ballet and turnout to prevent knee injuries.
A sharp pain in the knee, a pop, any clicking or feeling of impeded movement around or under the knee warrants an immediate pause. Any dance teacher or sports coach will want you to get it looked at by a chiropractor or sports medicine practitioner right away.
Tears can occur in the tendons, ligaments and other supportive tissue around the knee. Usually ice and rest will reduce the inflammation and heal theses injuries. Sometimes tissue will tear off and go under the kneecap, and this must be removed.
Normal growth in kids and teens can cause imbalances in muscle flexibility and strength which can lead to injuries and inflammation from overuse. Regular stretching and relaxing efficiently with the help of a muscle roller stick rolled on tight muscles, can help this temporary condition.
Correct turnout, foot strength in landings, in fact all ballet position placement, helps protect the knee joints. A sprung floor is also essential, rather than dancing over concrete.
If you are a serious ballet student or athlete, take a look at the anatomy of the knee structure. It is brilliant, and you'll see clearly why you are taught the way you are, to prevent knee injuries in ballet shoes and pointe shoes, or on the fields and courts.