Options for Correcting Sagging Buttocks?

Butt Lift Options for best results
There are many option to have a more shapely butt. They include a Brazilian Butt Lift if there is adequate fat available for fat transfer, buttock implants and a butt lift. Fat transferred to the Buttocks is generally called a Brazilian Butt Lift. Some surgeons, including myself harvest fat in particular from the lower back to give it an S shaped contour that defines and sculpts the top of the buttocks giving it the illusion that it is bigger and definitely more shapely before the fat transfer is actually done. Similarly, sometimes fat is taken from the upper back of the thighs to also define the edge of the buttocks while taking care of what many women call the "banana rolls" as well as the "diamond" shape space that is attractive at the highest point of the inside of the thighs when viewed behind. Also the amount of fat that takes is highly technique dependent and experience dependent.
Implants are generally reserved for those that want more fullness but have inadequate fat available for transfer. It is more involved and has a longer recovery. Lastly Butt Lifts generally refer to using "flaps" of redundant soft tissue that are tucked inside the but while tightening up the skin envelope. This requires the longest time for recovery and results in the largest scars which visible.
In other words there are many subtleties that make all the difference between a good result and a great result.  My suggestion is to go to a board certified plastic surgeon with significant experience in these techniques. His exam will determine which course if right for you.
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Hello. Without pictures or examination it is difficult to determain which procedure would be best for you. However, from how you are describing what bothers you an option could be the brazilian butt lift. The “Brazilian Butt Lift,” or gluteal lift/augmentation is a procedure designed to improve the shape and appearance of the gluteal region. This surgery uses a combination of liposuction and fat grafting (i.e. fat injections with the patient’s own fat) to produce a fuller, firmer, more youthful-appearing buttocks. Consult with a board certified plastic surgeon who is experienced in performing this procedure. Best of luck.