How do plastic surgeons know how deep to inject fat? I’m getting BBL done in September.

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.
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.
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.

A good candidate has enough fat available for transfer in the procedure. The amount of fat that can be transferred in a BBL depends on how much fat the patient has to harvest and the amount of space there is to move the fat into. It is recommended for the patient to have a BMI of 30 or less. If you are too thin, a Brazilian Butt Lift may not be possible.

hello thank you for your question and provided information as well.. based in your question its difficult to evaluate you if the case dont have a case.its recommend get a consultation in person with a plastic surgeon to evaluate you.