How "rare" is fat embolism leading to blindness with facial fat transfer?

The incidence of blindness with filler injection is very rare.The essential precautions to be taken, Choose a board certified Plastic Surgeon, who is familiar with the face anatomy and specially the vascular anatomy.The use of a cannula instead of a needle decrease the possibility of filler embolism.The pressure of injecting the filler is important.Certain areas of the face are more prone to filler embolization.So the person doing the injection is the most important.

first of all, embolisms to the ophthalmic artery are very rare.
 First of all, embolisms to the ophthalmic artery are very rare.
All the things you mentioned are true.
Plastic surgeons are very well acquainted with facial anatomy.
If someone needs volume in the region where an embolism could occur then there's always a chance of this complication.
The arteries leading to the ophthalmic artery are fairly predictable.
For example fat grafting to the lips would never cause an ophthalmic archery occlusion.
The outcomes of fat embolism's did the ophthalmic artery or worse than those of fillers such as Juvaderm.
Using sharp needles increases the risk.
Most blunt cannulas will not penetrate the artery but they can and it's happened.
If this is something particular that you're very worried about then you should discuss it with your hopefully board-certified plastic surgeon, before the procedure.
The surgeon can show you the general anatomy of where the arteries are that lead to the ophthalmic artery.
The ophthalmic artery is a branch of the internal carotid artery.
An embolism anywhere proximal to where the internal carotid artery he enters the school can lead to an embolism to the ophthalmic artery.
The embolisms can also go to the brain causing strokes.
My guess is your chance of developing blindness, and if so it would most likely only be on one side from facial fat transfer it's by far less then developing blindness of all other causes during a lifetime.
One well published study from Korea showed 44 cases of ophthalmic artery embolisms out of approximately 110,000 facial injection treatments.
That makes a chance of having an ophthalmic artery embolism four patients per 10,000 injections.
For reference the chance of being struck by lightning in a lifetime is approximately one out of 12,000 people.
All medical procedures have risks and in the end the decision to have a procedure performed comes down to a risk versus benefit ratio.
Best,
Mats Hagstrom M.D.

I fastidiously avoid injection of any fillers to the corrugator area (11s) since I have especially heard of fat injections causing partial or complete blindness in more than one patient when this area was injected.  I think that filler is safer in the form of hyaluronic acid but I also do not use it between the brows or in the forehead for the same reason.  Embolism from injections of fat in other areas have been reported but are extremely rare.  I have been using fat injections to the cheeks and nasolabial areas as well as the jawline for approximately 10-20 years and have never seen a single complication.
Jon A Perlman M.D., FACSDiplomate, American Board of Plastic SurgeryMember, American Society for Aesthetic Plastic Surgery (ASAPS)ABC-TV Extreme Makeover SurgeonBeverly Hills, CaliforniaIG: jonperlmanmd

there is no way to guarantee that a complication does not occur except to do the procedure. The odds of this happening varies with the location and delivery method of the fat. Using a needle seems to be related to a higher rate of complications and cannulas are generally thought to be safer.  The top of the nose and forehead is associated with more complications, Which in any event are extremely rare.  The incidence is unknown, estimates go from 1 to a million  to one in fifty thousand. The lifetime odds of dying in a car accident is about one in 80.