There are many illegal and unapproved materials that has been used for buttock and other areas for augmentation and enhancement. All these underground injections are not medical grade materials and have many unusual bacteria. Though you may not have infection, ant surgery or fat injection, buttock implants may stimulate the dormant bacteria and cause severe infections.
MRI or CT Scans can determine the distribution of the material injected for possible removal.
Unclear substance injected in the buttocks ?
With pre-existing substance in your buttocks there's a significant risk of infection. When fat is injected in the area of the pre existing substance capsule is opened and entrapped bacteria can infect the fat grafts and your entire buttocks. Your AMA or AOA board certified surgeon in office consult will give you the best advise.
I wish you well with your surgery and recovery,
James S McAdoo DO FACOS, AOA Dual Board Certified General, Plastic / Reconstructive, and Microsurgery Surgeon