hey this is brief info… under is when the implant is placed under the chest muscle – surgeons prefer this technique when you do not have enough breast tissue as if you had it over the muscle under the breast tissue you would be able to see it – howver if you have enough breast tissue surgeons would use the overs technique as the implant has sufficient tissue to cover it.
i hope this helps – another way to find out more accurate info is to google it and there are lots of websites that will give you info xxx