Surface ultrastructure of silicone rubber aortic valve poppetts after long-term implantation. A scanning electron microscope study of four poppets

Prospective Study on Trilucent Soybean Oil-Filled Breast Prosthesis

BACKGROUND

The use of Trilucent breast implants dates back to 1995. These implants were produced in an attempt to overcome the perceived shortcomings of other implants filled with saline and silicone gel. The filler material, which was derived from soybean oil, was said to be biodegradable and biocompatible. When this prosthesis was subsequently found to be prone to rapid aging and rupture of the silicone shell, the company that manufactured this implant decided to withdraw it from the market and offered to remove or replace all Trilucent prostheses.

METHODS

The authors present a clinical study of the Trilucent implant in which 36 patients received a total of 63 implants and 33 patients had 58 implants removed. The authors also examined the implant shell of the removed prostheses to investigate the possible cause of silicone implant aging and rupture.

RESULTS

Each of the implants removed was carefully inspected visually, and samples of the implant shell were analyzed by scanning electron microscopy. All the implants removed had a very bizarre appearance; they were variegated in color, with shades of yellow and brown, and were studded with whitish yellow nodules whose number grew in proportion to the time the prosthesis was implanted. Scanning electron microscopic analysis of the implant shell revealed a heterogeneous inner surface of the implant together with fissures and globoid deposits on the cut surface.

CONCLUSIONS

The authors' findings on Trilucent implants are similar to those reported in previously published studies for other types of breast implants for the first 2 years after implantation, after which a progressive, rapid deterioration was observed.

What Exactly Was Wrong with the Trilucent Breast Implants? A Unifying Hypothesis

Trilucent soybean oil-filled breast implants were initially announced as the ultimate prostheses for breast augmentation. However, after an increasing number of reports of local complications and hazardous metabolites attributable to lipid oxidation, first the United Kingdom Medical Devices Agency and later the Belgian National Ministry of Health urged all plastic surgeons to contact their patients and advise them to have the implants removed and, if desired, replaced with another type of prosthesis. In our plastic surgery department, 13 patients received bilateral implants with triglyceride-filled prostheses between February and July of 1996, for primary breast augmentation or replacement of previously implanted prostheses. For 12 of those 13 patients, the prostheses have been explanted, because of unilateral breast enlargement attributable to a ruptured prosthesis for five patients and following the recommendation of the Belgian National Ministry of Health for the other seven patients. Before explantation, all patients underwent standard clinical examinations, with assessments of breast shape, volume, and firmness. Blood analyses were performed, with a special focus on liver enzymes, as were urinalyses. Magnetic resonance imaging scans were obtained before explantation; for two patients, the scans revealed a fluid level separating two liquid layers in an intact prosthesis. This is the first report of such a finding. The removed implants were examined for any damage or shell deterioration and for changes in color and viscosity, the weights and volumes were measured and compared with the initial values for the implanted prostheses, and complete biochemical analyses of the accumulated fluid in cases of ruptured prostheses and of the filler material in cases of intact prostheses were performed. This small but well-documented series illustrates the multitude of problems associated with triglyceride-filled implants, including bleeding of the triglyceride filler; shell deterioration, as indicated by a loss of texture and extreme fragility of the implant (with rupture or delamination with a simple finger touch); an increase in osmotic pressure exerted by the degraded filler material; progressive weakening of the outer silicone shell, with influx of plasma proteins of up to 750 kDa, eventually resulting in rupture of the prosthesis; a lack of oxidative stability and the formation of toxic oxidation products; a lack of biocompatibility, with the formation of insoluble organic soap-like material; and a pronounced inflammatory reaction. It is concluded that the sequential and/or simultaneous occurrence of (1) implant bleeding, (2) lipid infiltration of the silicone elastomer, and (3) inflammation attributable to oxidation products provides an overall explanation or unifying hypothesis for the wide variety of adverse events related to soybean oil-filled implants.

Study of aging of silicone rubber biomaterials with NMR

Multinuclear nuclear magnetic resonance (NMR) spectroscopy (29Si, 13C, 1H) is used to characterize the aging process of silicone rubber-based biomaterials in a rat model. 1H NMR relaxation measurements (spin-lattice, T1, and spin-spin, T2, relaxation times) were performed to better understand the molecular dynamics of polysiloxane chains in implants. After 1 year of implantation in animals, changes in the 1H T2 relaxation times and the NMR spectra were observed in polydimethylsiloxane, Silastic sheets and chin implants, while these measurements remain unchanged in finger joints. Very small amounts of fat were detected in all types of silicone rubber implants at the end of the implantation period. This work shows that free silicone migrates from the implants to adjacent tissues and distant sites, such as spleen or liver, and is chemically modified.

Silicone breast implants: immunotoxic and epidemiologic issues

Silicone gel implants for breast augmentation and reconstruction have been in use since 1962. Significant local complications include capsular contracture, rupture, gel "bleed", and spread of the implant material to regional lymph nodes (1-7) as well as histologic findings of foreign body granulomas in the capsular tissue and in lymph nodes (7-9). Through magnetic resonance spectroscopy and atomic emission spectroscopy, silicon compounds were found in the blood of some women with silicone breast implants; silicone and silica have also been found in liver (10). Well-publicized case reports have raised significant concerns regarding an association between implants and systemic disease. However, despite the availability of silicone implants for over 30 years, controlled epidemiological studies were not carried out until 1992. Currently available epidemiologic data are extremely limited. In part, because the majority of implants were used after 1981, the incidence of long-term problems is not yet known. In 1992, due to the unavailability of studies demonstrating the safety of implants, the U.S. Food and Drug Administration advised that silicone breast implants should be used only in reconstructive surgery and as part of clinical trials (11). This decision spurred a wave of research on the bioreactivity of silicone and clinical observations of patients with implants. Herein, we review the adverse immune effects following contact with silicone as well as the epidemiologic data available.

Migration and biodegradation of free silicone from silicone gel-filled implants after long-term implantation

In vivo 1H NMR chemical shift imaging (CSI), 1H NMR localized spectroscopy (STEAM) and multinuclear NMR spectroscopy (29Si, 13C, 1H) were used to characterize the aging process of silicone gel-filled implants in a rat model after long-term implantation. Although no significant changes could be observed in the implants or surrounding tissue by in vivo 1H chemical shift imaging, in vivo 1H localized spectroscopy of the livers from the longer term population revealed the presence of silicone. Ex vivo 29Si spectroscopy of the liver, spleen, and the capsule formed around the 9 and 12 month implants clearly demonstrated and confirmed for the first time that a significant amount of free silicone migrates from silicone gel-filled implants. Also, these results show that silicones are not metabolically inert, and their biodegradation in tissue and within the implant can be monitored after 9 and 12 months by changes in the 29Si chemical shifts seen in corresponding ex vivo spectra. The NMR findings are supported by those obtained by atomic absorption spectroscopy. Silicone aging changes not only the chemical composition of the gel, but also its proton T2 relaxation times, which increase with long implantation times. The three dimensional structure of the gel disintegrates (i.e., polymer chain rupture), increasing the molecular mobility of the polymer and, consequently, its protons T2 values. The relaxation data we obtained reflect this in vivo degradation, especially in the case of implant rupture. Additionally, small concentrations of fat in the silicone gel were found within the implants. The presence of these lipophilic substances also might increase the T2 values (plasticizer effect). These findings may assist in evaluating the implant integrity and disease symptoms related to their presence in humans.

In vivo localized proton NMR spectroscopy of silicone

1H NMR localized spectroscopy (STEAM) can assess unambiguously the presence of free chemically unchanged silicone in animal tissue after injection of silicone oil. Although the signal-to-noise ratio obtained in 1H imaging is sufficient to detect the distribution of relatively large amounts of silicone in vivo, the specificity of silicone detection can be improved by using 1H localized spectroscopy techniques. The sensitivity of the STEAM experiments is sufficient to detect silicone at a concentration of 0.5% in a voxel of 27 mm3. Preliminary results from rats with silicone gel-filled implants show no detectable amounts of silicone in sites such as lymph nodes, the liver or the spleen, 3 or 6 months after implantation.

In vivo degradation of silicones

29Si nuclear magnetic resonance (NMR) spectroscopy is applied to study the degradation of polysiloxanes (silicones) in vivo. Our results with animal models show that silicone migrates from the implant to the liver (29Si resonance at -20 ppm) and new silicon containing compounds form after the silicones are introduced into the rats. The new 29Si resonances in the chemical shift range of -40 to -85 ppm are related to hydrolyzed silicone, those at -90 to -115 ppm are indicative of the presence of silica (SiO2), and the peaks observed at -120 to -150 are related to high coordinated silicon complexes. These resonances are not present in the 29Si spectra of the silicones before implantation. Our findings demonstrate that silicones are not metabolically inert.

In vivo 1H chemical shift imaging of silicone implants

In order to study the aging process (i.e., silicone migration, fat infiltration) of silicone (polydimethylsiloxane, PDMS) based biomaterials in living subjects by NMR imaging, a hybrid 1H selective excitation and saturation chemical shift imaging technique (IR/CHESS-CSSE) has been developed. This sequence allows selective mapping of the distribution of silicone protons in vivo, while suppressing the contributions of fat and water. Our results indicate that a combined inversion recovery and CHESS pulse, followed by a spoiler gradient, must be applied to suppress all contributions of fat protons to the NMR signal. The sensitivity of our experiments allows the detection of a chemically unchanged silicone concentration of 5% in a voxel of 0.9 mm3 at a signal/noise ratio of 2.

Structural changes in glutaraldehyde-treated porcine heterografts used as substitute cardiac valves. Transmission and scanning electron microscopic observations in 12 patients

Scanning and transmission electron microscopic studies were made of (1) 12 glutaraldehyde-treated porcine valvular heterografts that had been implanted in patients for 2 days to 76 months; (2) 3 unimplanted commercially processed porcine aortic valves; and (3) 1 unprocessed porcine aortic valve. Comparison of unprocessed porcine valves and unimplanted commercially processed valves showed loss of endothelium and acid mucopolysaccharides during preimplantation processing. Short-term (less than 2 months) changes after implantation consisted of insudation of plasma proteins, penetration of erythrocytes into surface crevices, formation of a thin surface layer of fibrin, and deposition of macrophages, giant cells and a few platelets. Longer-term (more than 2 months) changes were proportional to the time interval after implantation and consisted of progressive disruption of collagen, erosion of the valve surfaces, formation of aggregates of platelets and accumulation of lipid. The surfaces of the leaflets did not become covered with endothelium or with a fibrous sheath. Calcific deposits were found in one valve and bacterial organisms in another. Thus, progressive breakdown of collagen appears to be a critical factor in determining the long-term durability of glutaraldehyde-treated porcine valvular heterografts.