How do surgeons ensure they are staying within the subcutaneous plane and not accidentally injecting into the infra-muscular pla

It can be done under control of surgeon's hand; while one hand is injecting fat the other hand is constantly controlling position of the tip of cannula. Other method is intraoperative ultrasound imaging where assistant runs US device in front of cannula and surgeon visualize plan where to inject. This is technically (and financially) very demanding and most experienced surgeons do not see the need for such an intraoperative imaging. Hope this helps. Good luck.

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, ensuring that you are in the appropriate (i.e. subcutaneous plane) is a function of technique. The surgeon must be aware of where the cannula tip is at all times. Using a rigid cannula which does not bend can be helpful. Some will employ real time imaging such as ultrasound to visualize planes. Injections should also be performed under low pressure aspirating before injecting to prevent accidental cannulation of a vessel.
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.

Hi dear, thank you for your question! A previous evaluation is made and this is precisely the reason why you should be seen by a board certified plastic surgeon who can ensure your safety and guarantee certain knowledge and experience doing the job.Hope this helps,Dr. Jimenez ToribioPlastic and Reconstructive Surgeon.