This is a question that I answered for someone who had a procedure by another physician.
Aging many times involves a volume loss. For the lower eyelids, this means that you lose volume in the cheek and under eye area. This reveals your lower eyelid fat and also exposes your muscle that surrounds your eye. With the more exposed orbital rim under your eye, you have many reasons for bags under the eyes. The fat protruding creates a shadow under your lower eyelid. The exposed muscle creates a darker appearance to the lower eyelid. The exposed orbital rim creates a visible rim under the eye as well.
Most surgeons treat this by removing the fat under the eye. This can help. But this solution may make you look hollow and something about it leaves you wanting to look younger! Volumizing this area is the key and fillers or fat injections are the answer to this. Because fat injections last longer and look better in my opinion, it is the treatment of choice for me.