What are the causes of the dimples commonly found above the buttocks? They seem to be in line with the sacroiliac joints. Do the joints have anything to do with them?
Back dimples or Dimples of Venus are created by a short ligament stretching between the posterior superior iliac spine and the skin. They are thought to be genetic.
Source and further reading: Kumar, Kanojia & Saili (2014) - Skin dimples