Jim Curtis

Medical Applications of Silicones

A variety of silicone materials have been prepared, many possessing excellent properties including chemical and thermal stability, low surface tension, hydrophobicity, and gas permeability. These characteristics helped originate the use of silicones in the medical field, and are key to the materials’ reported biocompatibility and biodurability. Since the 1960s, silicones have enjoyed expanded medical application and today are one of the most thoroughly tested and important biomaterials.

Comments on a Case Report of a Tenckhoff Catheter Allergy

To the Editor: The recent case report by Patel et al. (1), entitled Pruritic Rash and Eosinophilia in a Patient Receiving Peritoneal Dialysis recounts the experience of a 28year-old woman on peritoneal dialysis with rash and eosinophilia. An allergy to the Tenckhoff dialysis catheter was suspected and a patch test gave positive results. Following a successful kidney transplant, dialysis was discontinued, the catheter removed, and the eosinophilia as well as the rash resolved. The positive patch test using ‘‘shaved fragments taken from a new Tenckhoff catheter’’ taken together with the rapid resolution of immune symptoms after catheter explantation suggest that the patient may indeed have had a rare allergic reaction to a material in the catheter. However, the supposition of an allergy to silicone is not supported by the literature. Silicone materials have been extensively studied and scrutinized as a result of the breast implant debate. Three independent review panels (the Institute of Medicine, the National Scientific Panel appointed by the judge for the U.S. Multidistrict Litigation, and the United Kingdom’s Independent Review Group) carefully assessed the specific issue of silicone immunogenicity. All three groups concluded that there is no convincing evidence to support, or lend biologic plausibility to an association of silicone breast implants with immune-related human health conditions. They further found only insufficient or flawed evidence that silicones can elicit an immunotoxic response, trigger a specific immune reaction, or amplify an autoimmune-like disease (2–4). As such, the authors should have considered other sources of antigenic stimulation before using the phrase ‘‘silicone allergy.’’ There is more in the catheter than just silicone. Dialysis catheters are generally sold sterile and ethylene oxide is commonly utilized to sterilize Tenckhoff catheters (5). Residual ethylene oxide can cause tissue reactions ranging from simple allergy (6) to analphylaxis (7) to burns (8). Ethylene oxide is a highly reactive alkylating agent that can react with endogenous proteins to create a neo-antigen and precipitate an allergic reaction (9). In addition, the presence of chemical additives or their by-products might explain the patient’s unusual reaction. Tenckhoff catheters are assembled from several components, including most prominently an extruded silicone elastomer tube with adhered Dacron cuff(s). Room-temperature vulcanizing (RTV) silicone adhesive may be used to bond the Dacron cuff(s) to the silicone tube. The majority of such materials are tin-catalyzed and the immunogenicity of tin moieties has been noted in the clinical literature (10–12). Alternatively, adhesive reaction by-products may have caused false-positive readings in the immune sensitization tests. The most common reaction mechanism for medical silicone RTV adhesives is acetoxy-cure, which liberates acetic acid as a by-product. If the catheter was made with peroxide-initiated silicone rubber, other sources of antigenic stimulation should also have been considered. Residual by-products are generated from the decomposition of the peroxide vulcanization agent and if not properly removed, can adversely affect biocompatibility of the elastomer. Depending on the initiator used peroxide by-products might include 2,4-dichlorobenzoic acid, tert-butyl alcohol, 2-methyl-3-butyn-2-ol, and methyl vinyl ketone (13). Material safety data sheets for 2-methyl-3-butyn2-ol (14–16) and Commission of the European Communities review of methyl vinyl ketone (17) indicate that allergic reactions are possible with these substances. In summary, while initial case reports do add value to the clinical literature, it is incumbent upon the authors to rule out other plausible explanations for their reported observations before using provocative phrases like ‘‘silicone allergy.’’ If the patient’s rash is linked to an allergic response, based on extensive study of silicone implants—critically reviewed by independent scientific panels—it most certainly is not an allergic response to silicone.

Letter to the editor: The effect of the environment on the mechanical properties of medical grade silicones.

Sir, We have read the recent paper from Leslie et al. with interest. The authors postulate that in vivo failure of silicone finger joint implants could be partly attributed to the effects of environmental ageing of the silicone material. They associate this supposition to results of their mechanical (tensile) testing of specimens stored in Ringer’s solution, distilled water, and air for up to 27 weeks at 378C and 4 weeks at 808C. We have reservations about the relevance of their data as there are a number of erroneous assumptions in analytical procedures and materials selection. The most evident error was the selection of a Dow Corning material, which is never used in such implants. The Dow Corning material examined by the researchers is not comparable with the specially-formulated implant material which comprised Silastic Finger Joint Implants, Swanson Design. The researchers tested two different silicone elastomers which they improperly associated with silicone finger joint implants: C6-180 from Dow Corning and Med82-5010-80 from NuSil. The C6-180 was obtained in an uncured sample kit, whereas the NuSil material was provided in cured sheet form. They contend that ‘‘C6-180 was chosen as it is the limited implantation version of Silastic which was used in previous years for the Swanson silicone spacer.’’ The authors cited Hutchinson et al. for that statement. However, C6-180 is not mentioned in that reference; rather, it describes the 55 durometer material designed specifically for flexural fatigue crack growth resistance, a material known as ‘‘HP100.’’ Moreover, C6-180 did not exist as a Dow Corning product until the year 2000, three years after the Hutchinson article published and 6 years after Dow Corning divested of its Medical Device Business including the Silastic Finger Joint implants. Incidentally, to obtain the C6-180 sample kit from Dow Corning, Ms. Leslie signed a statement indicating that it would not be used in load bearing applications. As shown in Table I, the C6-180 was never used for finger joint implants and this material is not similar to the Dow Corning material used in the finger joint implants. The most obvious difference is durometer, a viscoelastic material property. C6-180 has a nominal durometer of 80A which is significantly harder than Silastic finger joint implants. Silicone elastomers used in medical applications incorporate fumed amorphous silica filler for reinforcement and to increase durometer. As the amount of silica in the elastomer is raised, tensile strength increases until it reaches maximum and then begins to decline as excessive silica dilutes the polymer in the formulation. In medical grade high consistency silicone elastomers, the tensile strength maximum usually occurs around durometer 45 to 55 Shore A. For example, we compare the typical tensile strengths of the 50 durometer elastomer C6-150 and the 80 durometer C6-180 in Table II. Whether expressed as engineering stress or true stress, the strength of C6-180 is simply not comparable to the 50 durometer elastomer of the same type. The authors indicated that ‘‘the mechanical strength of Med82-5010-80 is greater than that of C6-180.’’ This is to be expected because the Med82-5010-80 is made of material with a durometer of 50A. In addition to the clear distinction in elastomer hardness, there are additional fundamental differences in the compositions of C6-180 and HP100. HP100 was specifically formulated to maximize the material’s resistance to flexionfatigue-induced crack growth (flaw propagation) in flexible hinge implants. During development of the optimized material, DeMattia (ASTM D813) testing was conducted to 400 Correspondence to: J. Curtis (e-mail: jim.curtis@dowcorning.com) This letter is in response to Leslie LJ, Jenkins MJ, Shepherd DET, Kukureka SN. The effect of the environment on the mechanical properties of medical grade silicones. J Biomed Mater Res B Appl Biomater, 86B: 460-465, 2008. Mr. Curtis and Ms. El Houari are employees of Dow Corning. Dow Corning was the manufacturer of the Silastic Finger Joint Implant, Swanson Design. Professor Haggard has no current financial interest in any finger joint manufacturer.