Hard fall on stairs on left buttock with dent and hematoma. Will these resolve or surgery be necessary?

This is a very common condition on te buttocks and breast as result of  blunt trauma (hit, falling, etc) which cause the fat cells to die and in response creates internal scar tissue with an indentation as result. Unfortunately nothing to do to prevent the indentation after the event (trauma, injection or infection). You will know how severe (in your case I consider will be minimum) the damage was until couple of months after.  The options to cosmetically improve the area may be fat grafting, fillers (if it is very small) or if it is very large and deep, then a reconstructive procedure will be needed. Dr. Cardenas 

Without seeing any photos it sounds as though you might want to visit a plastic surgeon now to see if in fact you do have a hematoma and/or intervention is recommended sooner than later. Glad to help. 

Dear Lorena D,it is hard to tell for sure without an examination. In case of fat necrosis, fat transfer is usually required to address the deformity. 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