Is injecting fat under the muscle safe with a BBL?

Hello and Great Question.  I appreciate the time you are spending researching this procedure.  My name is Dr. Matthew J. Nykiel.  I am a Board Certified Plastic Surgeon, a Fellowship Trained Cosmetic Surgeon, and a Butt Augmentation Specialist.
The recommendation for a BBL is to place the fat above the muscle because injecting it into the muscle puts you at higher risk of this complication.  I would have a discussion about this risk with your plastic surgeon and confirm where he/she likes to inject the fat.  
I hope that helps!  Best of Luck!
Matthew J. Nykiel, MD
#CoreAesthetic
Butt Augmentation Specialist
Liposuction Specialist
Board Certified Plastic Surgeon

Thanks for your question. Injecting fat under the muscle or into the muscle is dangerous as it can cause pulmonary fat enbolism. With Brazilian butt lift we transfer fat from one area of the body to the other.  These fat cells are transferred in microinjections throughout multiple planes in the buttock tissue so that they can gain new blood supply and live in their new home, providing increased volume and shape to the buttocks.  

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.
BBL/gluteal lipoaugmentation demonstrates a dramatically higher risk profile than other surgeries. The Aesthetic Surgery and Educational and Research Fund has convened 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 and aspiration prior to injection. Using a supra-gluteal approach to minimize the risk of inadvertent intravascular injection has also been proposed. While this will likely impact graft viability negatively, it will also lower the risk of a serious complication.
According to AAASF data the risk of death from BBL will range from 1:3000 (subcutaneous grafting should reduce this risk relative to intramuscular grafting). By comparison the risk of fatal complication from abdominoplasty is 1:13,000. It is unclear how the relative risk falls with subcutaneous injection vs. intramuscular injection (however, in all likelihood, it is less).
No result is worth risking one's life for. I find that the patients who focus not on the volumization but rather the contouring benefits of the liposuction are happier than those whose sole goal is size. Relatively speaking, the subcutaneous plane is the "safe" plane.
With regards to your specific question, I would discuss your concerns with your surgeon. This will him/her to alleviate your concerns.
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.