This is called capsule formation. The body is probably attempting to extrude the implant which it perceives as a foreign body. As the implant is compressed it changes in shape from flat to spherical, which is the smallest surface area for that volume. Not all capsule formation is clinically significant and plastic surgeons grade the amount of contraction using several different systems. The cause of capsular contracture is unclear.
Many surgeons feel it is more likely after infection or bleeding.
Some surgeons have tried to reduce the chances of capsule formation by using steroids either in or around the implant. It is more common when the implant is placed above the pectoral muscle and when smooth implants are used. It is less common with textured implants. Most contractures develop within the first six months. Although late contractions do occur they are usually preceded by an injury or infection.