This is a situation where the height of the nipple is similar to (or lower than) that of the tissue adjacent to the nipple.
As an inverted nipple is aesthetically bad, it can make the depressed area unhygienic, and make breast-feeding hard after a child birth; those with an inverted nipple receive the surgery. The cause of the inverted nipple is congenital. It is known that less connective tissue beneath the nipple or the underdeveloped muscles in the areola cause it.
Various methods are used to correct an inverted nipple, but the basic principle is to fill the area beneath the nipple with the tissue of the nipple and push the nipple out by raising the tissue of the nipple.
A surgery method is decided depending on how severely the nipple is depressed. If it is severely depressed, the mammary duct must be incised, which means that breast-feeding can become impossible. If it is not severely depressed, a method not correcting the mammary duct is used.