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The Biology and Evolution of Cohesive Gel and Shaped Implants

Calobrace, M. Bradley MD; Capizzi, Peter J. MD

Plastic and Reconstructive Surgery: July 2014 - Volume 134 - Issue 1S - p 6S–11S
doi: 10.1097/PRS.0000000000000347
Cosmetic: Original Articles

Summary: The introduction of the silicone gel implant in 1962 began the modern era of breast augmentation, and over the past 50 years, breast implants have evolved to encompass a wide array of commercially available implants today. In April 1992, the moratorium on silicone gel breast implants began, and much of the experience and development of the cohesive shaped implants occurred outside the United States. During this time, saline implants were used almost exclusively in North America, whereas silicone implants continued to be used throughout the rest of the world. In 2001, cohesive gel implants became available in the United States under an Investigational Device Exemption Study. Three companies have US Food and Drug Administration approval of fifth-generation implants: Sientra, Allergan, and Mentor. These implants are unique due to the texture of the shell, implant dimensions, gel-shell interaction, gel fill ratio, degree of cross-linking, and form stability. Although form stability is a relative term, it may best be reserved to describe the increased ability of the implant to maintain its basic, intended shape. The cross-linking, form stability, and cohesiveness of these fifth-generation implants provide surgeons with an innovative tool to more closely create a natural breast in both shape and softness.

Lousville, Ky.; and Charlotte, N.C.

From the Calobrace Plastic Surgery Center; Division of Plastic Surgery, Department of Surgery, University of Louisville; and Stillwater Plastic Surgery.

Received for publication December 2, 2013; accepted January 22, 2014.

Disclosure: Drs. Calobrace and Capizzi are clinical study investigators for Sientra and received standard research support for conducting their studies.

M. Bradley Calobrace, MD, Calobrace Plastic Surgery Center, 2341 Lime Kiln Lane, Louisville, KY 40222, drbrad@calobrace.com

Silicone gel used in breast implants has been labeled “cohesive” since the 1980s,1 although it is only recently that the term cohesive has been used to describe the solid characteristics of implants used today compared with the more liquid consistency of the gel used in the earlier generation of implants. Cohesiveness is a relative term based on the level of cross-linking, firmness, and form stability. To understand the cohesive nature of implants and how this affects the look, feel, and performance of highly cohesive shaped implants, a review of the evolution and biology of the available silicone breast implants is necessary.

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EVOLUTION OF GEL IMPLANTS

The introduction of the silicone gel implant by Cronin and Gerow2 in 1962 began the modern era of breast augmentation, and over the past 50 years, breast implants have evolved to encompass a wide array of commercially available implants today. Peters et al3 originally described 3 generations of silicone breast implants, which was further refined and described as 5 generations by Maxwell and Baker.4 The first-generation implants (1962–1970) were characterized by a dense, viscous silicone gel surrounded by a thick implant shell. As a response to requests from surgeons for a less firm implant with more natural feel, the second-generation implants (1970–1982) were round, less cross-linked gel implants covered with a thinner, slightly permeable shell. These implant shells proved unreliable with increased rupture rates and gel leakage. The subsequent third-generation implants (1982–1992) were created with a more viscous gel and a thicker smooth or textured shell, with a less permeable low-bleed barrier elastomer, in an attempt to reduce capsular contracture.5 The fourth-generation implants (1993 to present) are the current standard cohesive gel round implants available in the United States today and are differentiated from the previous generation of premoratorium implants by refinements in the manufacturing process.

The fifth-generation implants (1993 to present) were initially created to fill the market need for a more natural result than the full upper pole created by the previous round gel implant. The fifth-generation implants have the same silicone elastomer and low-bleed shell present in previous generations, filled with a silicone gel of greater cohesiveness that better retains its shape in varying positions (Fig. 1). This led to the creation of an array of anatomically shaped devices to accommodate a variety of different breast forms and chest dimensions. It is, however, the increased cohesiveness of the gel and not the shape that defines this generation of implants. In the United States, Sientra’s round and shaped gel implants were the first of the fifth-generation implants to gain approval in 2012, followed by Allergan’s and Mentor’s shaped implants in 2013.

Fig. 1

Fig. 1

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EVOLUTION OF SHAPED COHESIVE IMPLANTS

The current fifth-generation implants can conceptually be traced back to the Replicon (Surgitek) implants of the 1980s with an anatomic shape and a textured shell, giving the breast a “one breast feel” where the implant is adherent to the breast tissue and is no longer a mobile structure, which is easily discernible in the underlying breast mound. The polyurethane shell on these implants was an attempt to reduce capsular contracture,5 but it was prone to degradation, resulting in a thin, pliable shell that was more susceptible to visible folds, rupture, silicone leakage, and late capsular contractures.6,7 In the early 1990s, Tebbetts8 introduced the concept that both implant shell and the contents of the shell contribute to the final shape of the implant. The manufacturers could alter the softness or firmness of the implant by the amount of cross-linker added to the silicone gel. The significant advancement in development provided sufficient cross-linking and thus stiffness to the implant to allow it to maintain its shape in different positions and resist the inherent deforming pressure and forces of the surrounding breast tissue. These concepts led to Tebbetts’9 aid in the design of the original Style 410 implant for McGhan/Inamed in 1993.10

Concurrently, silicone implants came under increased scrutiny in the United States. In April 1992, the Food and Drug Administration (FDA) placed a moratorium on silicone implants. Multiple forces—including the paucity of medical literature, new medical device regulations, and the concern for medical claims related to gel implants—threatened access to the world’s most commonly used prosthesis, the silicone gel breast implant. The moratorium established by the FDA required manufacturers to demonstrate the safety of silicone implants and restricted the use of these implants to clinical studies.5 With the exception of 2 adjunct studies that allowed access to gel implants for breast reconstruction and revision patients, there was no commercial access to gel implants from 1992 to 2006. As a result, saline breast implants dominated the US market.

During this time, saline implants were used almost exclusively in North America, whereas silicone implants continued to be used throughout the rest of the world. Although saline implants proved to be an acceptable breast implant, several performance issues were identified. These included a propensity for surface irregularities (rippling and palpable edges), a “rounder” and unnatural firmer result as compared with gel implants, and the potential for leaking and complete deflation. Saline implants were also considered to be a mobile/nonfixed implant, which rendered them completely distinguishable from the overlying breast tissue.6,11

Because of the suspension by the FDA, much of the experience and development of cohesive shaped breast implants occurred outside the United States over the next decade. Cohesive gel implants—referred to as fifth generation—became available in Canada through a special access program of the Medical Device Bureau of Health Canada in November 2000.12 Evaluation of the Allergan Style 410 implant, with more cohesive gel, began in the United States under an Investigational Device Exemption Study in February 2001.13 Investigational Device Exemption studies of both the Mentor Contour Profile Gel (CPG) and the Sientra HSC+ (High-Strength Cohesive) were initiated within the next year. The current regulatory approval status of silicone gel breast implants in the United States is shown in Table 1.

Table 1

Table 1

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COHESIVENESS AND CROSS-LINKING

All 5 generations of silicone breast implants are made from polydimethylsiloxanes (PDMS), a group of polymers that make up the silicon-oxygen backbone, and appropriate levels of gel consistency are created by varying the degree of cross-linking of the silicone gel. Substituting methyl groups from the PDMS chain with vinyl and hydride groups allows cross-linking to be achieved where vinyl groups react with hydride groups through platinum-catalyzed hydrosilylation reaction at high temperatures to form a 2-carbon bridge (cross-linkage) between other polymers.14 As the number of cross-link sites increases, the gel gets firmer and cohesiveness increases, providing more form stability. Although the physical properties are altered with increased cross-linking, the molecular structure of the cured gel is fundamentally the same as that of the uncured gel; both types include long PDMS polymer chains, whereas the cured gel has cross-linkages between the chains. Thus, the gross physical properties are altered far more as increased cross-linking occurs.15 The difference of the observed firmness between the implants from different manufacturers is essentially due to the amount of cross-linking in gels (Fig. 2).

Fig. 2

Fig. 2

The physical properties of increased cohesiveness and elastic memory have allowed for the creation of shaped-implant designs to accommodate a variety of chest wall and breast forms.15 The highly cohesive gel has allowed for the creation of an anatomically designed shaped implant that provides a more stable form that persists despite breast position or restrictive forces and pressures placed on the implant.9,12,16

To further demonstrate the cohesiveness of fifth-generation implants and how the increased cross-linked gel may perform in vivo, gel deformation testing evaluates the ability of the gel to retain its shape under force. A portion of the implant is placed under negative pressure inside a cylindrical chamber elastic deformation is measured. Results from independent testing performed on the cohesive gel implants offered by all 3 manufacturers are represented in Figure 3. Higher values represent “softness,” and lower values represent “firmness.”

Fig. 3

Fig. 3

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COHESIVENESS AND FORM STABILITY

As cross-linking increases, gel cohesiveness and form stability of the implant increase. The most important consequence of form stability is the ability of the gel to maintain its appropriate distribution within the implant shell and its designed shape in varying positions, especially the upright position (Fig. 1). However, the term “form-stable” is not an absolute term but rather a relative term.9 No breast implant is completely form-stable, and this term has created some misconceptions across plastic surgeons and patients. Form-stable implants can contain folds and ripples but with a lower frequency as compared with their less cohesive counterparts. Although some have proposed that form stability of the implant provides less rippling—due to the resistance to collapse—and folding when placed in any position,12 magnetic resonance imaging (MRI) studies have confirmed that form changes and shell rippling occur when the patient is in the prone position.17 Likewise, MRI scans in the Style 410 premarket approval studies have revealed distortions of the shell.9 Additionally, Hammond16 reports folds and distortions seen on upright MRI as well as on physical examination in varying positions.

Form stability as a relative term may best be reserved to describe the ability of the implant to maintain its basic, intended shape, with projection-to-height ratios and lower-pole projections maintained.16 Fifth-generation cohesive gel implants fulfill this definition of form stability best. These more cohesive attributes, a firmer, more stable gel, and textured shell have implications beyond the ability of the implant to maintain its shape. Although the ability to maintain shape is important, devices with greater cohesiveness and texturing have been reported to have lower wrinkling and capsular contracture rates when compared with their corresponding round implants.18,19

Similar reduction in rupture within the shaped devices has been observed. Form stability of the gel and its interaction with the shell have implications on shell longevity and rupture rates. It is theorized that more cohesive gel implants prevent gel collapse, producing less shearing forces and reduced wear and tear on the shell, leading to decreased rupture rates.10,16 It should be noted that device performance is not solely determined by gel cohesiveness but by many confounding factors, such as shell elasticity, gel characteristics, interactions at the gel-shell interface, and the influences of the implant/soft-tissue dynamics.9 Recent data suggest that the gel-shell interface may be most critical in maintaining shape and minimizing upper-pole deformity in the highly cohesive shaped implant.

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BALANCING COHESIVE STABILITY AND BREAST SOFTNESS

Unique to each manufacturer’s fifth-generation devices are the texture of the shell, the gel-shell interaction, the gel fill ratio, the degree of cross-linking of the gel, and the dimensions of the implant. The degree of cross-linking contributes to the degree of shape retention and may affect the firmness of the implant. Although some surgeons report that patients prefer the feel of a more cohesive gel over a liquid gel,10 the challenge in implant design is developing an implant with enough balance between form stability to maintain its shape and limit failure due to shell delamination or gel fracture20 and yet providing a soft natural feel to the patient. The development of the Style 410 Soft Touch was in response to the observations that the original Style 410 implant may sometimes create an overly firm result. However, the decreased cross-linking in the Style 410 Soft Touch implant reduces the form stability of the implant and perhaps the performance of the implant9 though it is reported to provide a softer result. Sientra’s HSC+ implant was designed to maintain softness while also preserving form stability without decreasing the amount of cross-linking, rather by increasing the gel-shell adherence. Altering attributes of the implant other than gel cohesiveness, such as gel-shell interaction, to create softness and yet stability may define the direction of future generations of implants.

In consideration of the fifth-generation implants available currently from Sientra, Mentor, and Allergan, the difference in the feel of the implant and the level of form stability are unique qualities that help distinguish these implants. The variation in the feel of the implants and the implant performance may be more than simply the level of cohesiveness as one considers other characteristics of these unique devices. The differences may likely be influenced by many variables, including unique gel characteristics, the gel fill volume/mandrel volume ratio, the shell elasticity and texture, the gel-shell interface, and the associated interaction of the implant with the soft-tissue envelope.9,21

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CONCLUSIONS

The history of silicone breast implants spans 51 years and 5 generations. This period has been marked by confusion and misperception regarding differences in silicone implant design, safety, and performance. The debate has further accelerated in the last year since the FDA’s approval of Sientra’s fifth-generation silicone breast implant in early 2012 and those from Allergan and Mentor in the following year (2013). Before incorporating the newest silicone implants into their practices, surgeons must thoroughly understand how fifth-generation implants differ from the previous fourth-generation implants. Although the term “form-stable” has become a part of our lexicon, the term should primarily be used to differentiate the fifth-generation implants from their predecessor. Form stability is not an absolute term. It is a relative term and should be viewed as a comparative description of the implant’s ability to retain its intended shape in all positions.

Importantly, greater form stability does not necessarily increase implant firmness, and likewise, greater form stability does not automatically translate to a more durable implant. It is clear that increasing cohesiveness may introduce a trade-off of characteristics, such as increased firmness, potential for gel fracture, and delamination. With increased understanding and objective exploration of the many attributes that contribute to the success of these implants, one must weigh the collective experience in light of the desired outcome. The stronger, highly cohesive gel implants are the latest in the evolution of the most investigated medical device of all time. This generation of implants allows surgeons increased ability to create a natural breast in both shape and softness now and into the foreseeable future.

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REFERENCES

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