Where do surgeons typically inject fat for a BBL procedure and with what size cannula? Above the muscle, into the muscle, other?

The risk will not be zero with any technique or instrument, but superficial injections with larger blunt cannula will have the lowest risk.  I perform the fat transfer and BBL procedures with the subdermal
transfer method.  The one overarching theme in this is to use a cannula that is much larger than the vessels to be avoided. In addition, injection should occur into the areas where the vessels are the smallest possible in diameter. This only occurs at the level directly under the skin.

We always injected fat grafts exclusively into subcutaneous plane, between muscles and skin. This is the safest area for injection considering size and density of blood vessels; injury of blood vessel(s) with cannula and injection of the fat under pressure, would allow fat to enter blood stream and be carried into vital organs like kidneys, lungs, brain and possibly cause death. Injection in subcutaneous tissue, seem to significantly decrease chance for fat embolisms (10 to 16 times) in comparison to injection into muscles or, in particular, between the muscles where significantly larger blood vessels are located. It is now the low in State of Florida that fat injections in the buttocks can be performed only in subcutaneous plane. This technique may decrease amount of fat we can inject in a single session but significantly decreases frequency of fat embolism. In some patients multiple surgeries (staged fat transfer) may be needed to achieve significant augmentation. 

Dear Letitiacf,
in case of BBL, fat should be injected above the muscle in order to avoid possible complications. Fat viability depends a lot of surgeon technique. The fat has to be placed back into the buttocks as soon as possible so that it has a greater chance of survival. It has to be placed carefully so that not too much fat is placed in only one area. 
If you are considering a surgery, I would suggest you to consult a board-certified plastic surgeon. Only after a thorough examination you will get more information and recommendations.
Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery

Hello and thank you for your question. There are some safety precautions that most plastic surgeons agree on, which include superficial injection of fat only (NO intramuscular injection), avoidance of large volume fat grafting and use of closed system to harvest and inject fat. Please make sure your surgeon is board-certified by the American Board of Plastic Surgery and follows safe BBL practices, as outlined by the American Board of Plastic Surgery. Best,
Pejman Aflaki, M.D.

Johns Hopkins-trained double board-certified plastic surgeon

The fat is strategically placed between the skin and the muscle. Injecting into the muscle can increase the risk of fat embolus and should be avoided.If you are a good candidate for a Brazilian Butt Lift (BBL), the optimal outcomes are achieved with: 1) use of VASER liposuction to harvest the fat, 2) a closed looped system such as Puregraft to process the fat, and 3) strategic micro-packet transplant technique. These three processes have demonstrated over 95% fat graft take. This means that your results will be stable as long as you maintain your weight. As an official VASER trainer, I am routinely referred unhappy BBL patients. One of the most common reasons for disgruntled BBL patients is that patients are not able to accurately communicate their desired buttocks goals to their surgeons.  A buttocks assessment tool can be an invaluable tool to communicate to your surgeon the exact buttocks size and shape you are desiring. A Skype or FaceTime consultation with a BBL specialist can help you get an accurate recommendation and pricing. 
Respectfully,
Arian Mowlavi MD,FACS
Realself 100 Hall of Fame Inductee