Is fat injected into the muscle when performing a BBL?

The most important aspect of the procedure is how to prevent the occurrence of fat embolus. Though it has never been considered malpractice or negligence, many surgeons who perform this procedure are doing everything possible to prevent this complication from occurring.  Performing the procedure only by injecting fat under the skin avoids any of the larger vessels. The vessel diameter of the vessels feeding the gluteal skin are much smaller than the 4+ mm, blunt, remote single hole cannula used for injection. Thus, the fat cannot gain entrance into the vessels and no fat embolus can occur. Consult with an American Board of Plastic Surgery board certified PS who performs hundreds of these procedures annually, read patient reviews, and view before and after photos on his/her website.  
Kenneth Hughes, MD, Board Certified Plastic Surgeon

There are worldwide recommendations from the Task force for Safety in Gluteal Fat Grafting to reduce the risks in BBL surgeries.Fat embolism during surgery can happen when there is injury to deep veins. They are deep to the gluteal muscle fascia. So buttocks lipoinjection should be performed carefully and it should be avoided to inject into the muscle. There are also other tools and techniques to reduce the risks like ultrasound mapping, special patient positioning during the surgery, canullas that are resistant to bend during injections. I hope this answer can help you.
Kind regardsAlexander T. Hamers MD

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.
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.