Large deep dent on my left buttock due to injection from when I was a kid. How can I reduce it?

Unfortunately, anything nonsurgical is highly unlikely to help much or be permanent. As far as surgical options, a couple rounds of scar tissue release and fat grafting may help some but a customized implant or cosmetic hip dip implants may be the best, most reliable and only permanent option. Glad to be of help.

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.
I understand your concerns about going under the knife. Typically to improve the appearance of this 'dent', you would need fat transfer.
I would definitely recommend doing an in person consultation with a board certified plastic surgeon who specializes in this procedure and who can suggest what is needed. I hope that helps! Best of luck!

Dear 21beans,
it is hard to tell for sure but you will probably get the best results with fat transfer. If you want to avoid surgery, you might also get some improvement with subcision and dermal fillers, but that is only a temporary solution. 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
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery

I’m guessing you may have had an anti-inflammatory steroid injection that caused fat atrophy.
Only surgical interventions will correct this.
I would still encourage you to exercise regularly.
There is no easy solution to correct this deformity.
There’s going to be scar tissue that will restrict the area from expanding with procedures like fat transfer though they may help. 
You could probably have a partial correction and should plan it that you may need a few procedures.
There is no easy fix for your problem but some improvements can be had.
Best, 
Mats Hagstrom MD