As Dr. Pearlman pointed out, using fillers for crows feet is not a very rewarding procedure because of the lack of skin thickness in this area. However, the secondary reason is that these lines are primarily causes by tonic and dynamic muscle activity. This also applies to lines in the forehead. The forehead also contains what are described as end arterioles. There is a small but real chance that some of these vessels could be occluded by a filler and lead to skin slough (the forehead has a small but higher incidence of this occurrence than other areas of the face).
The appropriate use of filler in the forehead area can contribute to a smooth forehead with a slight lift of the eyebrows. For the crow's feet, I always begin with a neuromodulator to limit muscle movement in this area. I reserve filler to this area, only in severe cases.