Rupture

Summary

• Rupture is the development of a split or hole in the shell of a breast implant. Ruptures can arise because of weakness in the implant shell material or as a result of damage to the implant. Rupture should not be confused with gel bleed, in which low molecular weight components migrate through the intact implant shell.
• There is no doubt that some breast implants will rupture. It is not possible to obtain an accurate estimate of either the rate of rupture nor its incidence at any particular point in time for breast implants in general, different manufacturers or any particular model.
• The literature on rupture is confounded by differences in the definition of rupture, differences in the accuracy of detection methods and limited identification of the implants.
• Rupture can only be definitively detected following surgical removal of breast implants, so estimates of rupture rate in the implanted population as a whole must rely on the less accurate non-invasive imaging methods.
• The published information regarding rupture rates relies mainly on the performance of the earlier generations of breast implant designs. There are differences in shell structure resulting from the barrier layer, which could affect its mechanical properties in addition to decreasing gel bleed. Caution must be exercised in any extrapolation of their performance to current designs.
• Patients and surgeons require more information on the performance of current implant designs in vivo to enable them to make informed decisions.

Introduction
The first silicone gel breast implants were marketed in the early 1960s. After more than 30 years of clinical use, it is now estimated that in excess of 2 million women in the US and 100,000 in the UK have had silicone gel breast prostheses implanted. Many issues concerning the safety and performance of silicone gel breast implants remain unresolved. In particular the rates of occurrence and the precise mechanism of implant rupture are not fully known. Although a number of studies of the incidence of rupture rates have now been published, the range of rupture rates proposed is wide. Manufacturers have reported incidence of rupture as low as 0.2 to 1.1% and while some investigators describe rupture as being uncommon, - other authors cite rates of rupture up to 71 %.

Definition of rupture
The direct comparison of rupture rates published in the literature is complicated by each individual author's precise definition of rupture. For the purposes of this review rupture is defined as the development of a tear or hole in the shell of the implant. Gel bleed describes the migration through the intact implant shell of low molecular weight components of the gel and should not be confused with rupture. A range of rupture characteristics have been reported in the literature; from focal rupture involving very small holes with only a small amount of silicone gel being present outside of the shell, to large visible tears and complete disruption with the implant shell surrounded by silicone gel/fluid. Following rupture the silicone gel normally contained within the silicone shell may either be contained within the fibrous capsule, termed intracapsular rupture, or may breach the capsule, termed extracapsular rupture.

Terminology
The description of results relies on the three terms which are defined below for the purposes of this review:

Rate: percentage failed per unit time

Incidence: the percentage of the total number of implants studied which were observed to have failed per unit time (without an accompanying time-scale only provides an estimate of the incidence of failure)

Survival curve: cumulative plot of incidence against time

Detection of rupture
Part of the difficulty in identification of true rupture rates is obtaining a reliable diagnosis that the implant shell is ruptured. Clinical evaluation including physical examination can fail to detect even advanced stages of intracapsular rupture where little or no silicone gel is observed to have leaked outside of the fibrous capsule. The use of appropriate imaging techniques is therefore essential to determine the status of an implant in vivo.

Determination of implant integrity, especially intracapsular rupture, by imaging techniques alone is difficult. Rupture is often asymptomatic; the fibrous capsule contains the silicone gel and causes no perceptible change in the external cosmetic appearance of the breast. Several imaging methods including mammography, ultrasonography, computed tomography (CT) and magnetic resonance imaging (MRI) may therefore be used in combination with clinical evaluation to assess the integrity of breast implants.

The sensitivity of mammography to identify rupture has been reported to range between 16.2%,16 67%4 and 89%14. The accuracy of mammography in the prediction of implant rupture is known to increase where there is extracapsular rupture with silicone clearly present in breast tissue outside of the fibrous capsule.

Ultrasound is reported to be more sensitive to changes in the integrity of silicone gel breast implant shells with rates of detection of rupture reported to be up to 70%. Detection of uncollapsed rupture by ultrasound has been reported to be a weakness in the technique.7

CT has not been widely used to detect rupture of silicone breast implants owing to concern about the effect of increased radiation exposure on patients.

Magnetic resonance imaging also enables good images of implants to be obtained with reported accuracy of predicting rupture up to 98%.7 Distinction between normal folding and rupture and intra- and extracapsular rupture has been reported to be consistently possible using MRI imaging.

The limitations of imaging techniques described above often mean that surgical exploration is required to confirm the presence of implant rupture.

Cause of rupture
Several causes of implant rupture are cited in the literature. These include:

Trauma
Although it is generally accepted that most incidences of implant rupture do not appear to have an obvious traumatic cause, rupture caused by trauma has been suggested to result from a number of sources including trauma experienced during traffic accidents, closed capsulotomy and over-compression of the breast during routine mammography.

Closed capsulotomy has been implicated as a significant cause of implant rupture , and little is known about the stress placed on implants during a closed capsulotomy procedure. One study reports that closed capsulotomy increases the incidence of rupture by up to 15%, but that it is not a significant factor in causing rupture, as approaching 60% of the patients involved in the study did not have closed capsulotomies but still had ruptured implants or implants showing severe gel bleed.16 The implant also may be exposed to external friction during a closed capsulotomy from rough deposits on the inner surface of the capsule.

Excessive compression of the breast during mammography has also been suggested as a trauma-based cause of rupture and mammography under such circumstances may convert an incidence of intra-capsular rupture to extra-capsular rupture.29 There is however little evidence to conclusively identify mammography as a significant cause of rupture.

Undetected Damage at Time of Implant/Explant
Inadvertent damage during implantation (for example needle damage during suturing) may cause tiny flaws in the implant shell. This type of damage may not be easily visible and damage caused before or during implantation may result in implantation of a damaged prosthesis. Similarly damage caused during explantation may result in the implant being classified as ruptured.

Shell Weakness due to a Manufacturing Defect
Any intrinsic weakness in the structure of the implant shell as a result of an undetected manufacturing defect, may lead to reduced strength of the implant shell and subsequently to premature failure resulting from rupture.

Deterioration of the Implant Shell
Degradation of the elastomeric silicone shell may occur when the implant is in situ. The mechanical strength of the shell may then be decreased, leading to premature failure.

Rupture rates
An intrinsic problem in the study of rupture rates is that most studies of implant rupture published to date are influenced by inherent selection bias, with most prostheses being explanted from patients requesting surgery because they are experiencing problems with their implants. No population studies and few surgical studies of asymptomatic women have been published to date.5 Another problem is that many authors do not quote the rupture rate but only the incidence of failure in a particular sample.

Manufacturers have reported rates of rupture as low as 0.2 to 1.1% (time period not specified).1 The American Medical Association Council on Scientific Affairs and the US Food and Drugs Administration (FDA) Advisory Panel (1992) suggested an incidence of 4 to 6% was more likely in asymptomatic women. , At the time it noted that reports of even higher rates were being published and the FDA decided that a rate of 5% was not acceptable. Many studies of the rupture rate of silicone gel breast implants were published before and have been published since the FDA's 1992 report.6-8,14-16-30,35,36,43,56 Review of the literature shows that the rates of rupture quoted vary widely from study to study.

Percentage Failure	Average time (years)	Range (years)	Definition	Reference
0.2 - 1.1	-	-	rupture	1
0.8	-	-	rupture	33
4.6	-	-	rupture	17, 18
5	-	-	rupture	21
5.25	-	-	rupture	26
5.33	-	-	rupture	34
33	9.7	0.06 - 24	rupture	28
49	10	-	rupture/leaking	20
57	-	-	rupture/leaking	29
62.5	10	-	rupture	30
63	-	12+	rupture	8
65	-	1 - 25	rupture/leaking	16
69 - 70	6	-	rupture	56
71	14	-	rupture	16
95.4	20	-	rupture	16
96	-	10+	rupture	39
8	-	<5	pin holes and tears	31, 32

Table 1. Summary of reported failure rates and times

Several studies offer evidence that silicone-gel implants wear out or fail according to a predictable timescale (summarised in Table 1). 33% of the prostheses examined were observed to have ruptured in a study considering implants which had been implanted for durations between 22 days to 24 years, with the mean duration of implantation cited as 9.7 years.28 49% of implants were shown to be ruptured or leaking at 10 years in another study.20 57% of implants studied by one author were quoted as having failed by leaking/rupturing. Another author observed a rupture rate of 62.5% after 10 years in situ . A rate of 63% ruptured at 12 years or more has also been proposed,14 with another study stating that up to 65% of prostheses which had been in place between 1 and 25 years had ruptured or were leaking.16 Higher rates of 69 to 70% have been observed in implants in place for 6 years,55 with other workers reporting rupture rates of 71% at 14 years and 95.4% at 20 years using survival analysis.16 In another study 96% of silicone gel implants were ruptured at or beyond 10 years in vivo.35 One study reports that the frequency of pin-holes and frank tears was observed to be 8% after 0 to 5 years implanted, rising to 61% after 10 years implanted, increasing further to 73% at 20 years or more.

By contrast, the few studies published involving random selection of patients with silicone-gel implants indicate lower rates of rupture, for example 0.8%, 5%,21 5.25%26 and 5.33%. However it should be noted that although the patients were selected randomly and most of these studies used the more accurate imaging techniques, not all of the patients involved in the studies were followed-up surgically to confirm any false negatives or false positives.

The variation in the published rates of rupture suggests that as yet it is not possible to predict with accuracy the life span of silicone gel breast implants. Each study uses a combination of different implant types and models produced by several manufacturers which further complicates the comparison of rupture rates determined by different authors. In most of the studies control samples are not available for comparison of identical unused prostheses with corresponding samples implanted for a period of time. It is therefore difficult to definitively assess the effect of any in vivo degradation of the implant shell that may occur.

Survival Curves
Survival curves have been presented in many cases to give a guideline as to the probability that an implant will be ruptured after a certain implantation time. The validity of the statistical methods employed by individual authors are often the subject of debate. Problems associated with the extraction of relevant data from studies published in the literature are particularly difficult to account for when data published in several studies are grouped together to provide a larger number of implants for statistical analysis. The selection and exclusion of certain studies from the group has been open to discussion by other authors.

Kaplan-Meier survival curves have been published in the literature. One such curve predicts a 50% failure rate after 12 years16. In the same study 89% were predicted to still be intact at 8 years, prompting the author to suggest that 8 years should be taken as a threshold in estimating the acceptable lifespan of silicone gel breast implants. Kaplan-Meier survival curves published in another study indicate an increase in the progression of failure observed between 10 to 15 years of implant age.28 The study also suggests that 50% of implants could be expected to have failed at approximately 15 years. Further statistical analysis yielded statistically significant break-points at 10, 12 and 14 years. For all three break-points respectively the implants of an older age were noted to show evidence of failure statistically more frequently than those younger than that implant age. It is generally accepted that the incidence of rupture increases with implant age.16

Data listed in five separate studies14,16,35, ,55 representing a total number of 1099 implants, were summarised and considered as one statistical group in one study. Separate survival curves were estimated based on the Kaplan-Meier algorithm. For each of the 5 studies data points were weighted according to the type and size of study and comparisons were made. Estimates of survival at 5, 10 and 15 years were obtained along with estimates of the median time to failure. At 5 years the unweighted estimate of survival rate was 96.7%, at 10 years it was 79.1% and at 15 years it was estimated to be 48.7%. The unweighted mean, median lifespan was estimated to be 16.4 years.

Another author combined 12 literature studies of failure of silicone gel breast implants generating statistical data about a total of 1652 explanted prostheses. The survival curve produced predicts survival rates of 50% after 8 years, a value somewhat lower than the values published in the similar studies discussed above. The statistical methods employed by the author have been questioned by several of the authors whose results were cited in the study.32 Some of the authors involved in the study of 1652 implants have subsequently extended the data considered in their previous statistical analysis to encompass data from 33 peer reviewed studies listing data for some 8000 implants.41 Using prostheses with a mean implant age of 10 years, with actual duration of implantation ranging from 1 to 30 years, failure rates of 30% at 5 years, 50% at 10 years and 70% at 17 years were calculated, values more in line with those published previously. It was also suggested that from a master curve plotting all of the data collated from the other 33 studies estimation of average failure rates at any given interval of time was possible, for example 6% of implants per year would be expected would fail during the first 5 years of implant life, 4% per year during years 5 to 10 and 2.9% per year during years 10 to 17.

Mechanism of rupture
Analysis of aged silicone implants supports the hypothesis that prolonged cyclic mechanical loading over time causes degeneration of the silicone envelope.

Most silicone gel breast implants consist of a two part system; a highly crosslinked elastomeric silicone shell usually containing amorphous silica to enhance mechanical properties, and a crosslinked gel filling. The silicone most commonly used for both components of the implant is poly(dimethylsiloxane) (PDMS). In more recent designs a barrier layer of fluoro- or phenyl-siloxane is sandwiched between PDMS layers. Long term environmental aging of silicone elastomers may lead to one of several effects:

1. degradation of the base polymer to a lower average molecular weight material
2. degradation leading to production of cyclic compounds
3. formation of additional crosslinks causing embrittlement
4. degradation or weakening of the overall bonding between the silicone elastomer and the silica reinforcing agent and/or
5. swelling of the silicone shell by low molecular weight components of the gel it encapsulates.

Research to date has concentrated on swelling of the silicone shell as a failure mechanism. The nature and chemistry of silicones, including the gel/fluid which is largely used to fill breast implants, means that the bulk material will contain molecules of a range of chain lengths or molecular weights. Silicone gel is produced by chemically crosslinking the material to increase the viscosity and provide a medium which can provide the desired cosmetic properties of a breast implant. There is some debate as to what degree the silicone materials used to fill implants are crosslinked. Some authors propose that the implant filler consists of only 5 to 15% chemically crosslinked silicone gel, leaving 85 to 95% of the low molecular mass filling material with a fairly low viscosity.

Uncrosslinked short (low molecular weight) silicone chains are more mobile than the longer chains as a result of reduced chain entanglement. Migration of the low viscosity, low molar mass material by diffusion through the silicone implant shell is reported as being well established.40 The mobile molecules have been reported to swell the implant shell, reducing its strength. The low molecular weight fluid acts as a plasticiser, essentially softening the material and decreasing mechanical properties such as tensile strength, tear strength and elasticity of the shell. Recent research has indicated that control and explanted silicone shells can be expected to be swollen with up to 15 to 25% silicone fluid.42 It has been reported that all silicone gel breast implants show some diffusion or bleed of relatively low molecular weight silicone through the silicone elastomer shell. Ingress of the silicone fluid/gel into the implant shell will be observed as soon as the implant shell is exposed to the silicone gel during manufacture and will continue over the lifespan of the implant.

The mechanical strength of the implant shell is a measure of the shell's ability to withstand forces in vivo. Mechanical testing of explanted silicone shells has shown that the strength of the shell declines significantly with increasing age , with the shells being reported as becoming weaker and more compliant over time. Similarly the pressure required to rupture implants has also been shown to decrease with increasing age. A 50 % reduction in the applied pressure required to rupture implant shell was observed from prostheses explanted after approximately 6 years.

The mechanical strength of implants implanted for between 4 months and 20 years, which were either intact, leaking or ruptured has also been assessed. A strong correlation was observed between the decrease in the maximum stress at break of the samples and increasing implantation time for implants implanted for 1 to 18 years.44 In the first 3 to 4 years of implantation one study noted a decrease in strength approaching 50%.43 After 7 years in situ, the tensile strength of implants was observed to have reduced further to approximately 70% of the initial value. A study reporting a large decrease in the mechanical properties of Silastic II implants has been presented recently. After 8 years implanted a decreases were observed of approximately 50% in tensile strength, 40% in tear strength and 30% in elongation. In another study it was shown that the average force to break for sections of explanted implant shells was less than that for intact shells.43

Any tear formed in the capsule or fixation patches of an implant will inevitably alter the stress distribution around the shell and repeated cyclic loading may lead to enlargement of the size of the defect in a mechanically weakened shell. Although many ruptured explanted devices have been observed to be torn along the circumference, the failure stress of the silicone shell has not been observed to vary with position for individual shells.43 Specimens taken from the circumference of implants were however shown to be generally thinner than those taken from anterior and posterior surfaces. The reason for this is unclear.

The introduction of barrier layers is reported to have improved the rupture rates of silicone gel implants. One study reports that the tensile and tear strengths of an unused, uncoated control implant were shown to have decreased by 43% and 46% respectively 3.5 years after manufacture. Similarly the tensile and tear strengths of a Silastic-I implant were observed to decrease by 32% and 79% after 1 year (un-implanted). For a fluorosiloxane bleed barrier coated Silastic-II implant, tensile strength was only observed to decrease by 23% and there was little change in the tear strength after one year.

It is also possible that the relative mechanical strengths of successive generations of silicone gel implants may yield different rupture rates as a result of the variation in both the thickness of the shell, and the composition of the gel contained within the silicone envelope.

The effect of Folding of the implant surface
Fold flaws have been observed in silicone gel breast implants, with gel being clearly visible between the folds. Wrinkles or folds have been reported in 15 to 67% of silicone/saline implants explanted and are thought to originate from contraction of the fibrous capsule surrounding the prostheses.36, , The stress concentrations arising in an implant which has been distorted by the formation of folds are thought to cause rupture of the shell by means of the formation and propagation of a tear. Implants in place for longer periods (generally greater than 10 years) and those surrounded by a tight capsule have been observed to be particularly prone to significant distortion with clear evidence of folding.35 Motion of the breast and implant on the chest wall or through muscular contraction over a submuscular implant, may impose mild repetitive forces on the implant shell. The subsequent abrasion of the shell material at the surface of a fold may cause a surface defect at the shell surface. A tear may initiate at a suitable surface flaw. The relatively low tear strength of silicone materials means that only a small amount of energy will be required to propagate a tear.

The effect of lipid infiltration
Silicones used in other biomedical applications have been shown to adsorb lipids.51 Lipids have been observed to adversely affect the mechanical properties of silicones. For example silicone heart valves were shown to exhibit cracking and discolouration after being implanted. Silicone coated pacemaker leads also showed decreased tensile strength of approaching 70% after 11 years implanted. In an implantable finger prosthesis, simple lipid adsorption in relatively static conditions was observed to reach an equilibrium lipid content of 1.5% by weight during a 10 month period of implantation. The same authors suggested that additional absorption/adsorption of complex lipid products could occur when the elastomer underwent long term continual flexing in the presence of circulating blood.

Although breast implants are exposed to different in vivo conditions to the type of prosthesis studied previously, it has been proposed that silicone breast implants are vulnerable to lipid infiltration and Nuclear Magnetic Resonance (NMR) studies have noted the presence of lipids in explanted shells. , Lipids and cholesterol are suspected to create an emulsion in silicone prostheses have also been shown to be adsorbed on implant shells and also in the silicone gel of breast prostheses using NMR spectroscopy. Another factor in the adsorption/absorbtion of lipids in silicone breast implants is the suggestion that the presence of silica in the filler of the gel may also influence the infiltration of lipids.50,51

Variation in rupture rate with type (generation) of silicone gel implant
Several studies reported in the literature include study of the rupture of different generations of silicone gel implants. Unfortunately, many of the studies consider the implants used as a whole and no distinction is made between the rupture rate of each of the types of implant used.

A comparison of data obtained in one study with manufacturer data confirms that the strength of silicone breast implants is dependent on the implant type, shell thickness and implantation time43 and other studies support this view.

Some authors report that first generation implants, manufactured with thicker implant shells are much stronger than the thinner shelled generation developed subsequently, and that the thicker shelled implants appear to be resistant to rupture and leaking43. The group of first generation implants in one study showed no evidence of rupture, while 95% of the second generation implants were shown to be ruptured after 12 years.56 In the same study only 3.5% of the third generation implants examined were found to have ruptured. Another study reports that no evidence of rupture was found when a series of third generation implants were examined. In one study, while the strength of second generation implants with a thin silicone shell was observed to decrease significantly with increasing implantation time and many of the devices were ruptured, 3 thick walled first generation silicone implants included in the study were observed to be intact after 18 years.43

One author noted in a study reflecting implants in place for up to 17 years that the implants found to have drier shells were more frequently found to be intact. This may be a reflection of the thicker, possibly stronger shells of the newer silicone implants.35

In consideration of the effect of multiple lumen implant shells, there is general agreement that rupture rates are broadly analogous, with two reports that the percentage of implants showing disruption, a term used to encompass rupture and severe gel bleed, on explant was 63.8 and 72.7 % and 45% and 48% 53 for bi-lumen and single-lumen implants respectively. A further study observed that all of the first generation implants explanted were intact while 78% of the second generation implants had failed.29

Conclusions
There is no doubt that some breast implants will rupture. It is not possible to obtain an accurate estimate of either the rate of rupture nor its incidence at any particular point in time for breast implants in general, different manufacturers or any particular model. There are differences in shell structure resulting from the barrier layer, which could affect the mechanical properties of the shell in addition to decreasing gel bleed. Caution must be exercised in the extrapolation of the performance of earlier designs of silicone gel breast implants to current designs. Rupture can only be definitively detected following surgical removal of breast implants, so estimates of rupture rate in the implanted population as a whole must rely on the less accurate non-invasive imaging methods. Patients and surgeons require more information on the performance of current implant designs in vivo to enable them to make informed decisions.

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