Fat transfer to the buttocks: why under the skin, not into the muscle?

Fat transfer to the buttocks is surgery that takes fat removed from the thighs or abdomen and moves it to the buttocks to shape them. Abroad it’s called a “Brazilian butt lift (BBL),” and it was once known as one of the cosmetic surgeries with a high risk of death. Knowing where this risk came from and how it was reduced shows you what to ask when choosing surgery.

In 2017, a task force of the US Aesthetic Surgery Education and Research Foundation (ASERF) gathered 198,857 cases reported by 692 surgeons worldwide. Deaths from pulmonary fat embolism, in which fat enters a blood vessel and blocks the lungs, were estimated at between 1 in 6,214 and 1 in 2,351 cases. That was 10–20 times higher than the average for all cosmetic surgery (about 1 in 55,000 cases).

The cause was the injection depth. Large veins run beneath the deep part of the buttock muscles. In the same survey, when surgeons reported injecting deep into the muscle, the risk of a fatal embolism was about 4 times higher. In 2018–2019, cadaver studies confirmed how this happens. Fat injected into the muscle flowed into the deep space beneath the muscle every time, while fat injected into the subcutaneous layer was not found there. When the fascia (the connective tissue covering the muscle) was intact, fat injected into the subcutaneous layer stayed within that layer. Based on these studies, the standard of “never beneath the fascia or into the muscle, only into the subcutaneous layer” was proposed, and it led to joint recommendations from several professional societies.

Changes after the recommendations were also confirmed. In a 2019 survey, the share of surgeons who said they inject deep into the muscle fell from 13.1% to 0.8%. Pulmonary fat embolism fell from 1 in 1,030 cases to 1 in 2,492, and estimated deaths showed a downward trend, from 1 in 3,448 cases to 1 in 14,952. However, these numbers were collected by survey, so they may be underreported.

Then does enough fat stay if it’s placed only in the subcutaneous layer? In a 2019 study that followed 50 people who had subcutaneous-only transfer with ultrasound for 1 year, the thickness of the fat under the skin increased by an average of 56.5% right after surgery and then decreased by 18.2% over the year. The researchers considered retention to be no worse than with injection into the muscle.

Fat transfer to the buttocks in Korea often uses smaller amounts than BBLs abroad. But even with smaller amounts, the blood vessels run at the same depth. So where the fat goes comes before how much goes in. If you’re considering this surgery, be sure to ask, “Which layer will the fat go into?”