How do surgeons keep from hitting a blood vessel when transferring fat from the body to the buttocks?

Superficial injection with larger diameter cannula will reduce this risk to the lowest possible risk.

Brazilian Butt Lifts are complicated and delicate procedures.  Harvest and transfer of fat are done using blunt instruments called cannulas.  They do hit and traumatize blood vessels which results in bruising.  Tumescent fluid is used prior to harvest of fat with liposuction to constrict the blood vessels which helps some.  Knowing your anatomy is also very critical.  Surgeons should stay in safe areas to inject the fat away from "major" blood vessels of large diameter.  Injecting fat around major blood vessels can lead to large fat emboli.

There are a few steps that need to be taken to avoid fat embolism, which is the result of trauma to large veins with injection of high volumes of fat, that can then travel to the lungs, potentially resulting in a fatal complication:1) Using blunt, large bore cannulas (greater than 4.1 mm in diameter) - these are generally larger than the veins we are afraid of traumatizing2) Low pressure injection - using a pressure gauge to inject can help prevent high pressure injection into low pressure veins3) Avoiding direct puncture of deep gluteal veins - staying away from the inferior and superior gluteal veins will avoid trauma. This means avoiding injection deep to the gluteal muscles in the vicinity of these veins, which can be determined by the feel of the injection and the surgeon's angle of hand.Hope this helps!