How do doctors know where to inject the fat in a BBL so a fat embolus won't happen?

Fat embolus is a very serious and known complication of fat transfer (which has been recently spotlighted in the media). While it is true, injection of donor fat into the infra-muscular plane (within the muscle) increases fat viability this does come with a significant increase in risk. There are large vessels deep to and within the muscle which can be inadvertently injected while grafting leading to a fat embolus. The consequences of such a complication are very serious and life threatening.
At the recent American Society of Plastic Surgeons Hot Topics, data from AAAASF (ambulatory surgical certifying body) was presented, suggesting that BBL/gluteal lipoaugmentation demonstrates a dramatically higher risk profile than other surgeries. The Aesthetic Surgery and Educational and Research Fund is convening a task force as well. The hope is that we will have more concrete guidelines for addressing these very serious complications.
That being said, the safest way to avoid fat emboli is to stay in the subcutaneous plane (at the expense of fat viability)and avoid the deeper muscular plane. If grafting of the deep plane is planned, surgeons should consider blunt cannulas, aspiration prior to injection. Using a supra-gluteal approach to minimize the risk of inadvertent intravascular injection has also been proposed.
In summary, the procedure can be safely performed, however, they should be performed by a board certified (ABPS) plastic surgeon with experience. These risks should be discussed as well as the strategies being employed to avoid them.

You're asking the right questions and this is an appropriate conversation to have with your surgeon. Because Brazilian Butt Lift is a closed operation (i.e surgeon can't see where he or she is injecting), performing the operation safely requires intimate knowledge of anatomy and technical modifications to allow the surgery to be performed safely.In regards to anatomy, most or all of the reported cases of fat embolism after BBL are thought to be related to laceration of deep veins (superior and inferior gluteal veins) with high pressure fat injection into those veins, causing the fat to travel to the lungs. It's important that your surgeon knows where these veins are relying on surface anatomy and tactile feel to guide his injections.Technical modifications that promote safety include the use of blunt, large bore cannulas (>4.1 mm), low pressure injections (I used an automated pump with a built in pressure gauge) and angling the hand in an upward direction when injecting the fat to avoid deep veins.Hope this helps! Sam Jejurikar, MD

Hello and Thank you for question. It is normal to have some concern and feel nervous having a procedure done. It is important to always seek for a Certified Plastic Surgeon with experience.  Performing any procedure it is required to have knowledge of different techniques and anatomy. Also it is always important to speak with your Plastic Surgeon and have a complete understanding of the procedure.
Hope this can be of help and Good Luck.