Can hyaluronic acid be used to fill in a small indentation on the butt cheek after BBL?

While this may not be unreasonable, the volume required to fill a dent is often more than may appear.

Hello and Great Question.  I appreciate the time you are spending researching this procedure.  My name is Dr. Matthew J. Nykiel.  I am a Board Certified Plastic Surgeon, a Fellowship Trained Cosmetic Surgeon, and a Butt Augmentation Specialist.
Yes!  That is completely reasonable.  I would recommend using a more long lasting filler than hyaluronic acid.  You want to wait until ALL of the swelling has gone down before putting in the filler.  Usually that is at least 6 weeks to 3 months.
I hope that helps!  Best of Luck!
Matthew J. Nykiel, MD#TopREALSELFDoctor#CoreREALSELFDoctorButt Augmentation SpecialistLiposuction SpecialistBoard Certified Plastic Surgeon

Hyaluronic Acid fillers are sometimes useful in this situation, but they are relatively expensive for small amounts, and are non-permanent. It's difficult to day without seeing the area, but touch up with additional fat transfer may be more effective long term.