A very rare but severe complication of breast enhancement plastic surgery using breast implants is when the breast implants are positioned too much towards the middle of the chest. This usually occurs because the plastic surgeon is trying to get more cleavage than the woman's anatomy can naturally have. Synmastia can also occur because breast implants that are too large for the woman are put in. It may also occur because of the woman's anatomy and genetic makeup. The synmastia complication results in undue pulling on the midline tissues. When the breast implant pulls the breast bone skin and tissues off of the breast bone it makes the breasts come together. So there is essentially no cleavage when the synmastia complication occurs. Synmastia means 'one breast'.

The synmastia complication can occur at the time of breast augmentation plastic surgery, shortly after or within a few months of the breast enhancement plastic surgery. It is better to attempt to repair this problem as soon as it occurs. This helps in preventing stretching of the breast skin and diminishes the distortion or nipple asymmetries that can occur if the synmastia is allowed to last for a long time. If the synmastia complication does cause abnormal breast skin stretching and nipple deviations, it is usually very difficult to correct this. Some women will elect to have the synmastia complication corrected and not address the nipple areolar distortions that may have occured with long standing synmastia.

Correcting synmastia involves a secondary breast plastic surgery. It can be corrected using several different techniques. Which technique that is used depends on the woman's unique set of circumstances.