Is it safe to get a BBL after hydrogel thigh injections?

The truth is that there is little evidence based medicine on the subject. The injections that have been performed will place you at a higher risk for infection and possibly fat necrosis.  I have removed certain substances in patients who have had issues such as wound healing problems and then reconstructed them at a later date.

Dear lovemyself2302,
it is hard to tell for sure without an examination. If you are considering a surgery, I would suggest you to consult a board-certified plastic surgeon. Only after a thorough examination you will get more information and recommendations.
Daniel Barrett, MD

You would benefit from an in-person office consultation with a highly-trained, board-certified plastic surgeon to discuss your history and see what options are available to you as far as liposuction and BBL goes.If you are a good candidate for a Brazilian Butt Lift (BBL), the optimal outcomes are achieved with: 1) use of VASER liposuction to harvest the fat, 2) a closed looped system such as Puregraft to process the fat, and 3) strategic micro-packet transplant technique. These three processes have demonstrated over 95% fat graft take. This means that your results will be stable as long as you maintain your weight. As an official VASER trainer, I am routinely referred unhappy BBL patients. One of the most common reasons for disgruntled BBL patients is that patients are not able to accurately communicate their desired buttocks goals to their surgeons.  A buttocks assessment tool can be an invaluable tool to communicate to your surgeon the exact buttocks size and shape you are desiring. A Skype or FaceTime consultation with a BBL specialist can help you get an accurate recommendation and pricing. 
Respectfully,
Arian Mowlavi MD,FACS

I had one patient who had hydrogel injected in this area several years before I met her. Because of the pressure on the area the hydrogel started tracking all over her back and buttocks. For sure it’s going to put you at higher risk for complications and lesser outcomes. May still be possible but your situation is complex, and exam is needed and you should do everything you can to get medical records from your previous treatments. It would be helpful to know where the injections were made and the volumes used.Best, Mats Hagstrom MD