What Causes A Horizontal Line On The Thigh?

Hello. I would like to consult about a medical case that I faced several years ago. I have a horizontal line like a dent, which is quite deep and is about 10cm long, on my upper left thigh. I searched in the Internet and found that the medical name for such case is called lipoatrophy (or fat/muscle atrophy). I am not sure what causes this problem and when it appeared. I would like to get rid of line as it looks ugly. I would like to know what are the possible treatments, their cost range, and the type of doctor that can treat.

Thanks.