It is rather difficult to pinpoint the exact cause of an itch, especially when there is no visible problem with the affected area of skin. The presence of inflammation, scaling, pustules, or sores would indicate a problem quite clearly, but when these are not present, diagnosis is a bit difficult. On the other hand, since these symptoms are not present, the diagnosis is quite positive, as it is quite certain that there is no infection of any sort, and no problematic skin condition. Most likely, it is just some minor, harmless problem such as dry skin. It would be rather unusual and strange for dry skin to strike only in a single place, particularly when that place is the back of the knees, but dry skin is one cause of itching. Unless you are sure that this is not the problem, try using a little moisturizer in this area a couple of times through the day, and before going to bed at night.
Another possibility is the stubble that is left by shaving. If you shave your legs, this is a very strong possibility. Shaving does not of course remove the entire hair, but leaves behind a little bit of the shaft, which we call stubble. Stubble can be quite itchy sometimes, especially when the weather is bad. And this is particularly troublesome on parts like the underarms and the backs of the knees, where skin rubs against skin, and the stubble is very likely to cause adjacent skin to itch. If you shave your legs, try shaving more carefully or using a different technique behind your knees. It may also be that this is a spot you are unable to shave as well as the rest of your leg, leaving more itchy stubble than usual. If these tips do not work and the itch is particularly bothersome, you should visit your doctor.
answered by G M