Modern silicone breast implant was introduced in 1963, by Cronin and Gerow [1]. They report this implant as a “natural feel” implant. Since the appearance of silicone gel, this has been a most common implant that is used in breast augmentation and reconstruction. As the use of silicone gel-filled implant increased, this implant evolved in several steps [1]. The implant that is used from 1964 to 1972 is classified as 1st generation implant. It had a thick gel and shell. It had Dacron patches on the posterior wall that helps to adherent to the chest wall. However, this patch contributed to make a severe distortion in capsular contracture. To improve the texture and to avoid the severe complication such as distortion of breast, 2^nd generation implant had soft gel and wall. Although it overcame the weakness of 1st generation implant, soft shell and gel contents have a provisional risk of gel bleeding. In spontaneous rupture condition, less cohesive soft silicone gel is easily escape from the capsule. Leakage of silicone gel made a problem like a siliconoma. From the 1982, 3^rd generation implant choose the cohesive silicone gel. This gel has a cross-linked silicone with long polymer side chains [2]. It is more cohesive than former generation gel and the leakage is decreased. It is clearly more durable than 2nd generation gel, however, the degree of consistency is little different among the manufacturers.

Foam stable silicone implant is categorized in 4^th or 5^th generation implant. The classification between 4^th and 5^th generation implant is obscure. In 3^rd~5^th generation gel, all the generation has cohesive gel filling and smooth texture, and polyurethane covered shell types. Only with the degree of cohesiveness, the generation is divided. Regardless of the generation, foam stable silicone gel implant came into the market since 1993 in Europe. Foam stable silicone implant is also called in “cohesive gel”, “highly cohesive gel”, “tear drop gel” or “form stable anatomic shape gel.” Recently, in breast implant market, these neologisms make surgeons confused. In this article, we expect to look over the accurate definition about new generation silicone breast implant, and discuss about the effectiveness and adequate use of thisimplant.

As mentioned earlier, cohesiveness is important characteristics in recent generation of silicone implant. Cohesive means intermolecular attraction between like-molecule. Water particle (H₂O) is composed of molecules of hydrogen and oxygen. Oxygen molecules can have a viscosity with two-hydrogen bond. With cohesive characteristic of hydrogen bonding, water can make a drop. Closely, saline implant also can be a kind of cohesive implant. Main issue of cohesive implant is not a composition but a degree of form stable nature. As the polymer chain become longer, viscosity and stability of the gel becomes higher [3]. Generally, cohesive gel implant means cross-linked silicone gel. The texture of cohesive gel is soft and natural, and the form of cohesive gel is easily deformed by its position or circumferential pressure. Highly cohesive gel implant 4 implies that has longer cross-linked chain than the general cohesive gel implant. It is firmer than usual cohesive gel and easily maintains its shape in any condition. For the reason, this implant is also called “form stable” implant. Form stable implant means the implant that has capability to sustain the constant shape.

Up to this day, each corporation that produces breast implant has made such breast implant in name of “cohesive gel” and “highly cohesive gel”. However there are delicate differences in firmness and density between the products.

Understanding the importance of form stability of a gel implant is closely related to the shape of an implant. Implants can be made in a round shape orin a teardrop (anatomic)shape.

All four possibilities (Cohesive round, cohesive anatomic, non-cohesive round and non-cohesive anatomic) exist, but cohesive anatomic and non-cohesive types are the most common way that implants are manufactured. Anatomic implants are usually made out of form stable gel, round implants are made with a non-form stable gel.

Form stable round implants are used as“salvage implant” for thin tissues or loose pockets. However, these implants are used limitedly because they make breast lookssimilarto overinflated round saline implant,so called “Baywatch” look.

In United State, many people and surgeons call this implant “gummy bear implant”. Gummy bear is soft, elastic and flexible. At the same time, it retains consistent shape in any condition. When we cut the gummy bear, we can observe the each portion of this cut bear retain its original shape. Alike this delicious bear, this implant is sustained a constant form after being cut into small portions.

The biggest difference between non-form stable implant and form stable implant is the characteristic to sustain the constant shape under the breast parenchyma. This difference is showed in following simple test (Fig. 1). With loosely grasping the nonstable implant with hands, we can easily observe that implant upper pole filling is easily sunken. On the other hand, with tightly grasping, upper pole bulging can be observed.

In same situation with hand grasping, form stable implant has a tendency to keep a consistent contour (Fig. 2). Form stable implant is easily predictable in the aspects of final contour and durability.

Fig. 3 shows this difference between form stable and nonform stable gel. With standing position, non-form stable gel is easily dropped in influence of gravity. In contrast, form stable gel implant keepsitsshape in upstanding position.

As for these findings, we suggest the change of implant after that is inserted under the breast. In long term follow up (2 years) of magnetic resonance image study of augmented breast, non-form stable anatomic shape implant gets out of its initial shape (Fig. 4) [4]. Non-form stable implant cannot overcome the weight of breast. The implant is molded by breast parenchyma. In this MRI finding, upper pole filling was collapsed and the upper border of implant changed in wavy shape. Because of the collapse of implant upper pole, the shape of upper breast is sunken. Correspondingly, lower breast appears overfill shape. Collapsed and folding implant may cause spontaneous implant rupture. Also, silicone leakage from the non-form stable implant provokes inflammatory reaction that brings about capsular contracture [5]. By contrast, with form stable implant, the implant shape did not change in long term follow up period. Form stable implant is stable and strong to overcome the breast weight and to mold the breast in constantshape. In other words , breast molds the non-form stable implant shape and the form stable implant moldsthe breast shape.

In cross-section of form stable implant, when examiner applied the finger press the portion, gel was bulging but when the finger press was removed, gel shape returned to initial shape without deformation (Fig. 5).

Even entire shell of form stable implant is peel off, the contour of gel is not changed (Fig. 6). Because form stable gel has a highly cohesiveness, it has ability to keep its original shape without thick implant cover. With this result, we predict that the gel bleeding phenomenon is extremely rare although implant rupture. Regardless of confused terminology like “Cohesive” and “Form stable” implant, the form stable nature is most important characterin these new generation silicone implant.

Natrelle TM style 410(Allergan Inc., Irvine, CA, USA) is the first highly cohesive form stable implant. Since Allergan introduce this implant in market in 1993, many corporations have manufactured similarimplant. MEMORYGEL TM CPG^TM (Contour Profile Gel) (Mentor Worldwide LLC, Santa Barbara, CA, USA), Replicon® (POLYTECH Health & Aesthetics, Dieburg, Germany), Naturgel TM (Sebbin, Boissy-l’Aillerie, France) et al. are classified asform stable implant.

In style 410, there are 12 types of gel according to the degree of implant height and projection (Fig. 7). In traditional round gel implant, breast width is most important factor for selecting suitable implant size. Because non-form stable gel shape is eas ily molded in the body depending on patient’s tissue tension, the projection of breast after augmentation is hardly predicted. The factors that decide the non-form stable device shapes are i) tissue envelopes tension surrounding the implant, ii) volumes that fills the implant, iii) degree of cohesiveness of silicone gel. However, in form stable implant, the anatomic shape of this implant is strong enough to endure the tissue tension; we can anticipate postoperative breast projection with ease. Implant projection and dimension is more important than implant volume in choosing the form stable implant. Considering patient’s desire,surgeonsshould select an implant type [6].

To obtain a good result with form stable implant, surgeon must keep in mind following core points.

Appearance of form stable highly cohesive gel silicone implant makes a migration to next generation in breast implant market. Someone still point out that this implant has several weaknesses [13]. Usually, to insert this implant, longer incision is demanded than traditional axillary incision. Also, to make a precise pocket for anatomical implant, learning curve period is needed in young surgeons, and it has a risk of rotation and is hardly used in re-operation.

Although these disadvantages of form stable implant, it provides an excellent result and gives a new opportunity in implant choice. This newer implant can make a predictable result with a high patient and offers superior result than previous generation silicone implant and it can makes a precise and safe result [7,11]. Bengtson [9] called this implant in “these next generation, highly cohesive implant” and he said this implant lowers complication rates and elevates patient’s satisfaction. As mentioned above, highly cohesive form stable silicone implant is more advanced generation silicone implant. To achieve a successful transition from previous generation to next generation of silicone implant, surgeon’s education, discretion and effort to avoid the problems are needed.