What happens with the eyes if the surgeon removes ALL the orbital fat?

Facial fat is essential to a youthful appearance.  Think of a young child's plump full face.  When the fat thins, which happens naturally as we age, the face begins to resemble more of a skeleton.  This look would be accentuated if too much fat was removed during surgery.  This is considered an older appearance.  Through techniques and treatments available today we can add volume to areas that are lacking through fat grafting or fillers and/or remove excess fat and tissue from areas where needed.

Thank you for your question.  From an aesthetic standpoint, facial and periorbital fat is necessary for a youthful appearance, among other factors.  Humans typically lose facial fat with age (i.e. imagine a baby's face, a 20 year old, and then the face of an elderly individual).  The removal of too much fat from the periorbital region will result in an unwanted hollow and  "skeletonized" appearance, which is the opposite of a youthful and well-rested one. 

If a surgeon removes too much fat from the upper or lower eyelids the region will take on a "skeleton-like" appearance. Imagine what a typical 90 year old woman looks like. You can see skin on bone all around the eyes. The upper eyelid appears "longer" and "sunken." If too much fat is removed, the upper eyelid demonstrates a "superior sulcus deformity," which is not ideal.

Not all surgeons will mark the fat pre-operatively. Dynamic adjustments to the amount of fat removed can be performed during surgery. 

There are many types of lower eyelid blepharoplasty. Older techniques just "sucked the fat out." Newer techniques involve using the eyelid fat to smooth the eyelid-cheek junction by repositioning it. This so-called "transposition lower eyelid blepharopalsty" is often a better option than simply removing the fat.