Technology and composition


dr. Pierre Blais

General Considerations

Silicones constitute a large family of products and very few of these entities are used in medicine. Contrary to frequently expressed views from the medical community, "silicone" is not a single product. It is a large family of heterogeneous substances. Even "medical silicone" is not a single class of materials. Numerous chemically different variations of these compounds exist. Even within the same sub-class or the same brand of silicones using similar components, there can be significant variations in molecular structure and properties. In addition, these properties can drastically affect the risks and the durability of the product. Processing conditions also affect the architecture of the finished material and impact drastically on properties.

To understand the significance of this technology, it is necessary to examine the main constituents and the methods used to prepare materials of this kind. Medical silicones are "compounded" with numerous additives which introduce further variations in their properties. The area is highly specialized and much of it is not published in conventional journals. Instead, patents and internal documents describe the sector. A brief overview is provided. It consists of excerpts from Chapter 35 in Biomaterials in Reconstructive Surgery, edited by Leonard R. Rubin, published by The C.V. Mosby Co. (1983).

These include a review of techniques and problems encountered in the fabrication of materials used in breast prostheses, the biological processes involved in the deterioration of such products and an outline of the chemistry and assembly technologies used to make shells and filling materials. The catalytic processes used during such preparations considerably widen the range of impurities present in the finished product.

It is also necessary to review terms and definitions used in this technology as many are incorrect. A typical example include inappropriate use of the term "gel" to define a class of substances used to fill breast prostheses. Materials used for this application are not gels. They are complex and uncharacterized liquid mixtures of oils, gums and reactive substances. Oils make up more than 85% for most formulae and the products remain as thick liquids with large amounts of impurities and an ability to release its components continuously over the service life of the product.

Other misconceptions surround the belief that shells and rubber-like substances made of silicones such as materials used for common saline implants are "solids". This is also incorrect. Shell formulae require significant amounts of oils to ensure processablity and resistance to water and tissue. These oils are gradually leached out of the shells over time to enter into tissues. As the oils are depleted, the materials embrittle leading ultimately to failure of the shell membranes with deflation of the prostheses. It is also important to note that there are major differences in the chemistry of silicones used for various prostheses made at different times. Specifically, implants associated with the early-seventies and nearly all products made by the 3M/McGhan group, used phenyl-based silicone intermediates with special toxicological problems and different profiles of impurity from other contemporary products.

Overview of Silicone Elastomer Technology

For historical and cost reasons, empirical compounding, a kind of cut-and-try approach, evolved as the main basis for preparing medical silicones. It prevails to this day. This technology parallels that used by the rubber industry for current industrial and consumer products.

Such technologies have their roots in the late 19th century. They are based on the "vulcanization" or cross reaction of complex mixtures of reactive polymer "gums" with a large number of secondary components which include solid mineral fillers, reinforcing agents, oil extenders, reactivity moderators, lubricants, solvents, visual and tactile properties modifiers, stabilizers, catalyst and many other substances and impurities. The result is generally an unstable mixture where the properties are more often controlled by the methodology used to mix the components and the processes employed to perform the curing reactions.

Unlike primary material-dependent industries such as aerospace, transport, defence and food industries which have generally developed and optimized materials for their specific applications, the medical implant industry has habitually depended on derived or "shelf" materials commercialized for other applications. This explains that industry’s preoccupation with empirical and speculative "testing" of plastics and elastomers for medical application.

That industry's dependency on "qualifying" materials with largely unknown "formulated" compositions as opposed to performing scientifically-based pharmacokinetics and materials engineering studies on well characterized substances designed for specific medical applications has been the source of many of the industry's problems with product failure, adverse reactions, litigation and product approval by regulatory agencies.

Silica in Silicone Elastomers

It is not widely known and understood that silicone formulations used in medical implants include from 20-30% "fumed silica" (silicone dioxide) and that this silica is not just a solid "filler"; it participates covalently and physico-chemically in the reactions that lead to the finished elastomer. Thus a large part of the silica becomes integrated chemically as ultrafine aggregate particles in cross-linked networks that constitute the finished parts .

After several years in vivo, poorly formulated, incompletely cured (vulcanized) and inadequately homogenized silicone products undergo water and lipid absorption. As a result, hydrophilic filler particles lose their cohesivity and breakup, allowing more permeability and further accelerating the physical deterioration of the material. Drastic changes in mechanical properties and grossly visible swelling take place in later stages of this process.

Concurrently, the interior of the previously impermeable "composite" materials becomes accessible to waterborne reactants such as metallic ions, acids, bases and small organic molecules. The swollen, internally stressed materials thus become vulnerable to frank chemical attack. The exposed silica-siloxane linkages at the interface of the filler/reinforcing particles are particularly susceptible to attack from base (alkaline) media.

Separation of the silicone polymer molecules from the silica at or near this surface destroys the original properties of the material. The phenomenon is aided when there is improper mixing with clustering of solid additives and fillers such as silica. Clustered, loosely aggregated filler particles introduce water-vulnerable, brittle sites which act as frangible defect centers on deformation. Inadequate curing which leave unreacted cross-linking sites such as unsaturation also invite long term cross reactions with other residuals in the mixture as well as with adventitious biological substances.

Processes such as repeated elastic extension, flexion, friction and swelling further contribute to the loss of properties through material fatigue, crack propagation and deeper fluid penetration. This further causes separation of the siloxane polymer chains from the reinforcing silica aggregates. Stress, deformation, abrasion and excoriation alter the surface and cause it to open. Microscopic crevices develop perpendicular to the surface and thus a significant part of the surface mass becomes brittle and unlike the flexible interior. Deformation of the part allows the interior to comply but the surface has now become brittle and the microscopic parts simply break off.

As a consequence, the degraded surface material spallates to fine particles which may become entrained in biological fluids, embed in surrounding capsule tissue or remain loosely bound to the original surface until dislodged by movement. These particles are no longer chemically identical to the starting material. They expose an outside layer of chemically active silicon dioxide which has comparable immunochemical properties to ground silica. They exhibit a much greater surface area and are thus of considerably greater immunomodulating potency. The matrix of elastomer which originally held the reinforcing particles together is also changed; oxidation and cleavage of the molecules has introduced reactive groups which may have immunochemical characteristics more closely approximating that of reactive latex particles which are used in the diagnostic industry.

Gel Filling Technology

Gel filling according to the then prevalent technology was a conceptually faulty approach to implant fabrication. It entailed widely known risks and required unreliable technology to implement. Furthermore, it led to products with risks that far outweighed their benefits in cosmetic surgery.

Silicone gels used in plastic surgery products are proprietary formulae which can vary in composition from brand to brand. All are multi-component mixtures with potential for migration and adverse reaction. Their biocompatibility characteristics are much worse than that of "medical" silicone oils typically used to lubricate syringe barrels or "solid" silicone rubbers included in many other classes of medical implants.

Gel-filled prostheses require the introduction of a liquid precursor to the gel; the gel itself in the finished state is too viscous and intractable to be pumped into a shell. This liquid precursor is reactive and consists of chemicals mixed immediately prior to use. Another process, usually heat, subsequently causes this mixture to form a gel-like substance. The precursor has a working time which is comparatively brief. To circumvent the problem, the prevailing formulae incorporated "moderators" which artificially extended this working time. This was done primarily for economic reasons. In compensation, such moderating compounds adversely impacted on the properties of the cured material and rendered the process particularly unreliable. It also introduced additional impurities potentially enhancing the toxic potential of the mixture.

The chemistry and the toxicology of the liquid filling mixture are widely different from the finished gel. In addition, the technology of filling, the level of internal cleanliness of the shell, as well as environmental factors such as humidity and temperature history, impact on the composition of the final gelled product. Thus the properties of gel in different implants may vary from batch to batch or possibly even from item to item in the same batch. In addition, the reactive mixture, which consists of pre-polymers, crosslinking agents, catalysts as well as diluents, moderators and additives, does not react completely. There are residual impurities and unwanted by-products in large quantities. Ideally, such impurities ought to be removed as they may have toxicological risks. In addition, they are undesirable as they may cause the mixture to continue reacting and possibly suffer in vivo degradation. They also introduce uncontrollable variables into the finished product. However, the technology used in the making of mammary prostheses does not allow separation and removal of these impurities. Once the gel is formed in situ it remains with all of the unwanted substances sealed within the shell. If the shells are permeable, then the impurities will reappear in the capsule and soon enter the host.

When the implant is fabricated, there is no method to remove the impurities. Even the gel cannot be purified from its mobile "impurities" because it consists mostly of fluids loosely held together into an unstable mixture. It is not a well-defined compound. Its structure and properties are closer to those of an oil-soaked "sponge". This technology is comparatively old and was originally derived from military electronic device fabrication practices such as circuit potting for shock resistance and weatherproofing.

Introducing and Sealing the Filling Material into the Shell

Prostheses must be assembled starting from an empty shell. The gel precursor is later added by injecting it through the shell at a pre-selected site. Some devices with highly crosslinked gels did not have seals applied on these filling sites. It was reasoned that the gel would not backflow through the small hole. As the technology changed and the gels became more fluid, attempts were made at using self-sealing ports based on a technology similar to the puncture-resistant (self-healing) rubber-based aircraft gas tanks of World War II. The process did not work for silicones and prostheses with leaky holes resulted. It became ultimately necessary to seal the filling hole in a separate operation. This was done by adding a specially formulated elastomer on the injection site and allowing this elastomer to cure. Prostheses manufactured by nearly all firms required separate sealing of the injection site. As a rule, the procedure was effective and a highly durable bond resulted between the shell and this seal. Gross leakage of oil and gel was thus precluded for some time, at least when the product was new.

Occasionally, formulations of this sealing adhesive deviated from norms. Alternately, the bonding surfaces were inadequately prepared or else the adhesive did not cure because of contamination or improper handling. The result was avulsion of the seal and an unsealed hole ab initio. Habitually such defects were readily discernible by quality assurance personnel. It would lead to rejection of individual devices, or in some cases the discarding of complete batches. Occasionally, batches would be salvaged by solvent-cleaning the area, reapplying a new batch of adhesive and repeating the curing process.

Although it is rare to recover clinically used devices with absent or failed adhesive seals on the gel fill hole, such defects are nevertheless encountered in significant numbers. When such products are recovered, it implies that adhesives were not consistently formulated or applied. It also suggests that quality assurance was either incompetent or absent. Prostheses recovered in that condition tended to originate from the early and mid-seventies when manufacturing operations grew very rapidly and when staff turnover and training problems became acute.

Be that as it may, boldly visible external production defects in released, high risk health care products demonstrate major anomalies in manufacturing culture and quality assurance practices. The defects not only have obvious long term safety implications for users but their occurrence also strains the creditability of the manufacturing community with respect to their claim of technical competence. It also raises more fundamental questions regarding reliability of the selected assembly technology, the validity of shell, adhesive and gel formulations, the selection of materials and the efficacy and/or commitment of the quality assurance processes.

Finally, if obvious material failures such as grossly defective gel formulations which can be seen or palpated easily, visibly defective and defectively patched shells and incorrectly sealed gel fill holes can be released for sale, then more subtle defects such as incorrectly vulcanized gel batches, contaminated materials and faulty shell processing/compositions can also be encountered. The results of small studies on limited numbers of items recovered from the field supports a very pessimistic view on the commitment of this industry to user safety, product excellence and the long term durability of these products in vivo.

Texturing Processes:

Basic techniques for texturing silicone-based prosthetic surfaces include the making of shells on mechanically rough molds and the addition of supplemental layers of shells made from silicone elastomer solutions mechanically mixed with sacrificial compounds such as particles of sodium chloride or other water soluble entities which are subsequently dissolved during a final water wash. The patent literature describes several equivalent processes for making porous polymer surfaces but all are fundamentally based on the same manufacturing philosophy. Primary shell-making operations are completed to produce the basic shell layer, much in the same way as a conventional prosthesis. Supplemental steps are added to create the additional layers which will later become porous by a secondary treatment. The textured surface is not expected to contribute significantly to the mechanical properties of the shell and has no capacity to retain effusates or debris from the interior of the prosthesis. Elastomers used in such processes are identical to conventional prostheses; most depend on the use of RTV technologies.

Problems associated with these techniques include debonding of the porous layer in vivo, assembly problems resulting from the incompatibility of the porous layer with certain sealing and bonding operations and visual assessment of defects during the quality assurance inspection of the finished product. The items are inherently more difficult to process and easier to contaminate. Conversely, if contaminated they are virtually impossible to clean. Moreover, they are opaque. They do not allow easy visualization of inhomogeneities and molding defects, a problem which does not exist for conventional shelled systems. The texturing process involves the use of more intermediates and additional curing steps than regular shells. Thus it has more scope for errors. Large amounts of impurities generally remain entrapped within the porous layers and there are major difficulties in removing all of the sacrificial compounds which are used to create the pores.

The removal of sacrificial material leads to zones of thin, frangible elastomer than can further detach in vivo to create additional problems. The prolonged washing and the necessity for breaking elastomer in thin areas produces large quantities of entrapped debris. The open and retentive properties of the textured surface facilitates the retention of impurities. It protects inoculae and biofilm which originate from the water and/or solvent washing solutions. The burden of proteinaceous matter on such shells is more elevated than for smooth shells and much of it is in the form of pyrogenic material. Another part contributes to the bioburden of the product and is made up of viable micro-organisms. This dictates supplemental care during the final sterilization of the products.

There is also a matter of efficacy. The biophysical and biological effects of textured surfaces, in particular in situations involving elevated impurity levels and inflammatory solubles, are not well characterized. The application of textured surfaces to prosthetic devices and the impact of the process on safety and efficacy of the technology is a matter of debate and research. A significant fraction of the research community does not perceive the process as having significant beneficial impact in terms of control of contracture. Most research suggests that textured surfaces simply bind more tenaciously to the periprosthetic tissue and further complicates the inevitable explantation surgery which users must periodically undergo in order to minimize complications and improve aesthetics.

Foam-Coating Processes:

In the early days of foam-coated prostheses, it was generally believed that the foam covering was a stable and permanent feature of these implants and that the rough surface would contribute to immobilizing and retaining the prosthetic object in the pocket. The early technologies addressed composite multi-layered prostheses assembled from bulk foam, fluid compartments, silicone gel and other systems assembled from different polymeric entities using assorted silicone-based adhesives. In the sixties, foam-coating was perceived as a feature that would aid "fixation" of the items in their biological environment. These views are reflected in early patents and papers and are credited to Pangman and other early investigators on breast augmentation.

Foam-coating using thin, compliant layers of foam is simply achieved by creating a prefabricated shell-like envelope of foam material. Preliminary assembly requires thermal fusing of the foam sheeting along seams to shape the covering. This process is extremely harsh and initiates the degradation of the foam. It further leaves large quantities of combustion products within the foam near the seams. An acid catalyzed silicone adhesive is then spread on a conventional prosthesis or a prefabricated uncontained gel core and the prepared foam cover is then pressed into the adhesive. The resulting acid from the curing process directly attacks the foam to yield significant quantities of low molecular weight substances, some of which are aromatic amine adducts. These entities remain with the product as there is no process to remove the impurities upon completion of the coating. Finally, the prefabricated coating parts are fused thermally along the perimeter to fully enclose the gel core. This bonding further increases the quantities of thermal degradation products which remain in the prosthesis.

In the final analysis, it would seem that the only permanent feature of foam prostheses was the rough surface created by impressing the porous foam layer onto its moldable adhesive layer. This perception reduces the foam-coated implants to a variant of textured surface prostheses. Contemporary patents document the concept of foam used as a "tool" to texture an adhesive substrate. Accordingly, they use manufacturing techniques which are identical to those employed in the making of composite foam implants but introduce an additional step to strip off the foam layer after final curing of the coated device. This approach is credited to Cavon and co-workers and had a brief popularity in the late-seventies.

Tissue fixation is the underlying philosophy for the more widely used fabric-based implants. The devices are dependent on bonding of separate layers of elastomer, adhesive and open fabric using reactive room temperature curing adhesives (RTV). Composite fabric implants are much older. They were originally investigated for cardiovascular repair in the forties. In the fifties, major studies were undertaken at Baylor Methodist in Houston in connection with cardiovascular reconstruction surgery. Complex fabric assemblies were fashioned from Dacron and were designed to encourage growth of tissue to become fabric-reinforced composites of Dacron and blood-compatible tissue. The systems were commercialized early. Typical products involved porous fabrics assembled using knitting and texturing technologies. In such situations, attention was paid to the cleanliness of the product and many manufacturing steps aimed at extracting impurities from the devices before sale. In vascular surgery, these composite fabric systems performed comparatively well and provide continuing service to this day.

Analogous fabric-based surfaces were introduced much later as tissue fixation systems by proponents of breast augmentation surgery but the resemblance to vascular repair products is only superficial. The breast implant technology was never designed to accept the type of exhaustive cleaning that must be used for vascular fabrics. Thus, it omits a key step that would have ensured cleanliness and stability of the finished product. It appears that the idea of fabric fixation for breast implants was simply borrowed. Its effects were not studied in detail in the context of non-vascular implants. Significantly, breast prostheses fixation via fabric surfaces was also associated with Baylor Methodist research facilities in Houston, specifically Cronin.

There are similarities between the cardiovascular approach and the patents credited to Cronin and associates but the essence of the technology for cardiovascular fabric devices was not retained. Cronin-related devices for composite fabric-based breast implants made use of open fabrics solely as fixation features. The technology embodied in the Cronin prosthesis was essentially the same. It used the same types of adhesives to bind porous fabric on the surface of conventional gel-filled prostheses.

In the seventies, it was found that the widely used polyurethane foams were not stable. They dissolved and disintegrated rapidly in vivo and ultimately disappeared over a few years. It was further found that the debris was inflammatory and created additional tissue abnormalities in the prosthetic interface area. Parallel work documented that the fabric fixation systems of the Cronin type prostheses had marked adverse effects and were unnecessary. Both systems had large quantities of associated impurities from adhesives and processing of the composite layers of fabric or foam. Polyurethane-coated prostheses introduced additional problems with respect to safety and efficacy because of the soluble by-products of the foam. Typical degradation yielded low molecular weight aromatic amines and other products with documented toxicologic risks. Adhesives stimulated the formation of scar tissue and inflammation.

By the early-eighties, the industry was aware that fabric fixation and foam-coating technologies were illogical and ineffective processes. This information was not widely disseminated. Fabric fixation systems were discretely abandoned. Paradoxically, the foam-coated counterparts gained popularity in spite of mounting adverse reactions. Increasing promotion led to larger numbers of users and a broadening of the population at risk from degrading implants. It took nearly ten additional years for the clinical community to accept that composite breast prostheses based on frangible and biologically unstable foams provided no supplemental benefits and greatly enhanced long term risks of the product.

Shelf Life of Reagents and Working Time Problems:

Reactive substances used in silicone product fabrication lose some of their potency with time and with increasing contamination of the mixtures. Adhesives are particularly sensitive to these effects. Thus, as the raw materials age, they produce less of the desired materials and more impurities as well as more residues of unreacted intermediates. The probability of adverse reactions from incompletely reacted intermediate therefore increases rapidly with the age of the reagents and with delays in curing the prepared mixtures.

Silicone elastomers such as those used for shells, patches and other mechanical parts are thermoset systems. That is, they are compounded as separate reactants which upon heating or reacting undergo irreversible changes that affect the mechanical properties, usually converting semi-liquids or liquids into objects of fixed shape or gels. Such products cannot be reheated to be restored back to the original condition. Their molecular structure is changed as a result of the mixing and curing.

The manufacturing of silicone elastomers and certain classes of gels depends on a technology which is more empirical than scientific. Complex mixtures of reactants are created shortly before use and may be cast into molds or processed to yield the final product within a brief delay after mixing and/or initiating the reaction.

Secondary processing steps which may include assembly, bonding or finishing of subassemblies can also be conducted using these technologies. All require catalysis of mixed hetogeneous compounds and subsequent curing. The age of the intermediates, their level of purity, the method and chronology of mixing and the time elapsed between mixing, compounding and curing can affect not only the mechanical properties of the product upon completion, but may also affect the susceptibility of the material to secondary reactions such as radiation resistance, susceptibility to degradation, uptake of moisture and hydrophobic substances, gas and liquid permeability and, in most instances, durability in vivo.

Problems of biodeterioration are known to vary with these conditions of preparation and can sometimes motivate engineering revisions in the assembly/fabrication protocols. This is why product investigations and pre-clinical studies must include materials research on explanted items and preferably pre-implantation studies which make use of elastomer coupons subjected to extended aqueous/simulated biological fluid contact.

Unprocessed elastomeric materials, in particular silicones and silicone adhesives, have very limited shelf lives which are normally indicated on the labels. The notation of "five years" is very generous and seldom realized; on average, shelf life of unreacted silicone intermediates is less than 2-3 years under refrigeration. The rationale for these "expiry dates" or "recommended use" dates is that the substances become increasingly difficult to polymerize (react) as they age.

Towards the end of their shelf life, the mixtures do not even yield "solid" or coherent objects; instead, sticky or partly solidified entities result, in some cases nothing more than the original liquids can be obtained from attempted polymerization. At any rate, even marginally successful polymerizations often yield substandard products which behave differently under chemical and/or biological environments. This problem is widely encountered in breast and other plastic surgery implants which deteriorate much more rapidly when they are made from intermediates used near the end of their ultimate shelf life.

Because of shelf life problems and unpredictable behavior of intermediates towards the end of their useful life, such mixtures must be tested immediately prior to use. The resulting "sham" coupons or pieces must then be evaluated to determine the suitability of the products for the application; completion of the polymerization reaction is normally evaluated by measuring the visual attributes (color, transparency, porosity, etc. ), mechanical properties (tensile strength, tear strength) and most importantly, surface quality and ratio of solvent extractables (surface "tack", percentage of solubles, etc.).

If the product deviates from what is obtained from fresh reactants, individuals skilled in the art will either discard the unprocessed, outdated raw material or else will "boost" the mixture extemporaneously according to prior art or nomographs. Such steps may include the use of additional reactants such as fresh catalysts or alternately, the procedures for use will be modified. For example, the "curing" of the filled molds will be differently programmed so as to compensate for changes in the activity of the polymerizing mixtures. Such procedures drastically alter the attributes of the end products. They also change their composition. The biocompatibility properties of the modified product can therefore be drastically altered.

On Shelf-life and Product Deterioration:

Shelf life considerations affect suitability for use in the context of sterility. Sterile packages deteriorate during storage and the "sterile status" of their content may be compromised by the development of small openings in the package enclosure. Thus the protective enclosure of a product may be lost during storage. Even if the item is deemed otherwise suitable for clinical use, its uncertain microbiological status renders it useless. This is not always a technical issue. Some countries place mandatory expiry guidelines on packaged medical products.

Their approach is predicated on the belief that package integrity is frequently lost after 36 months of storage. This requirement is not applicable in the U.S. Some institutions nevertheless adhere to similar guidelines on shelf life of sterile goods. For example, "sterile" items would not generally be deemed suitable for use 5 years later in North American institutions. Most European countries would explicitly forbid its use after about 36-48 months and a resterilization procedure would have to be performed. This is assuming the product was not technologically outdated or deteriorated to the point where the properties had changed and that it could withstand resterilization.

Failure Mechanisms for Mammary Prostheses

The implant shell is an inadequate containment system doomed by design to early in vivo failure. Failure modes are highly injurious. Such information has been known since the mid-sixties. Shells from silicone elastomer breast prostheses have a high tear susceptibility. Mechanical properties of the envelopes decay with time, in particular for users who calcify early.

Prostheses of the seventies and eighties were of variable quality. Their durability was not predictable. Some retained shell integrity for about 10-12 years but many failed before. More recent ones are somewhat less vulnerable but early leakage from crease-fold perforations, deterioration, crazing and progressive fatigue-induced failure are still commonplace. The gel filling material of such devices varies widely in viscosity and in content, some being very fluid and susceptible to distal migration, others contained known toxic entities. In addition, the bioavailable, extractable oil content of a gel can be significant.

The Capsule Is Not A Barrier For Prosthetic Effluents

Contrary to opinions generally held by plastic surgeons, tissue capsules are not effective barriers against gel effluents and components of the degrading mixture as it ages. Capsules that form about implants are not stable entities. They rarely close completely to a fluid-tight membrane. For most users, capsules remodel into complex, partly open structures with porous walls and fenestrations. After 7-10 years, in particular when compression capsulotomies are performed, they become lace-like and lose their ability to contain prosthetic debris.

For many patients, they eventually become mineralized. The capsular tissue loses its elasticity. Sharp mineralized plaques erode through the capsule wall and the implant shell. Openings develop and solid prosthetic material with silicone oils and denatured tissue, as well as calcified particles escape the prosthetic area. Much of this finely suspended material is entrained by extracellular fluid and enters the lymphatic system. Fouling, hyperplasia and inflammation of the proximal lymph nodes and drastically reduced lymphatic fluid flow generally follows. Damaged prostheses can break-up into discrete pools of solid and gel debris if there is a long delay between rupture and removal. Both factors create an environment which is propitious for aberrant biochemical and biophysical processes and unusual radiographic presentation.

Silicone gels used in plastic surgery products are proprietary formulae which vary in composition from brand to brand. All are multi-component mixtures with potential for migration and adverse reaction. Their biocompatibility characteristics are much worse than that of "medical" silicone oils typically used to lubricate syringe barrels or "solid" silicone rubbers included in many other classes of medical implants. The prolonged presence of uncontained silicone gel/oil in the breast is generally acknowledged as a serious health risk. The direct injection of such substances into soft tissue is contrary to State and Professional legislation in at least two U.S. States (California and Nevada). The release of free gel components in tissue is equivalent to gross oil injections of adulterated silicone oils. It will cause granulomatous tissue development and markedly contribute to fibrosis even in the absence of inflammation. It also alters the healing characteristics of the implant site. Hyperplasia, adhesions and/or periprosthetic contracture generally worsen in the presence of oils originating from leaky prostheses. Burst prostheses left in situ for long periods correlate with chronic local phenomena and systemic adverse effects in most users.