Calcification of polyurethanes implanted subdermally in rats is enhanced by calciphylaxis

Abstract

Calcification complicates the use of the polymer polyurethane in cardiovascular implants. To date only costly experimental circulatory animal models have been useful for investigating this disease process. In this paper we report that polyurethane calcification in rat subdermal implants is enhanced by overdosing with a vitamin-D analog. The calcification-prone state, known as calciphylaxis, was induced in 4-week old rats by oral administration of a vitamin-D analog, dihydrotachysterol. We studied two commercially available polyurethanes (Biomer® and Mitrathane®) and two proprietary polyurethanes (PEU-2000 and PEU-100). PEU-100 is unique because it is derivatized with ethanehydroxy-bisphosphonate (EHBP) for calcification resistance. Polyurethane calcium and phosphate levels and morphological changes due to calciphylaxis were compared with those of control rat subdermal explants in 60-day studies. Increased polyurethane mineralization was observed due to calciphylaxis with 60-day rat subdermal explants of Biomer®, Mitrathane®, and PEU-2000 (calcium levels, respectively, 4.13 ± 0.56, 18.61 ± 2.73, and 3.37 ± 0.22 μg/mg, mean ± standard error) as compared to control explants (calcium levels, respectively, 1.22 ± 0.1, 12.57 ± 0.86, and 0.20 ± 0.86 μg/mg). The study also demonstrated that with 60-day implants calciphylaxis had no side effects on somatic growth and serum calcium levels. Explant surface morphology of these polyurethane explants examined by scanning electron microscopy, back scattering electron imaging coupled with energy dispersive X-ray spectroscopy, and light microscopy demonstrated the presence of predominantly surface-oriented calcification. PEU-100, derivatized with 100 n.moles/mg of EHBP, resisted calcification with explant calcium levels 0.51 ± 0.01 (calciphylaxis) and 0.38 ± 0.01 (control) μg/mg. It is concluded that calciphylaxis enhances superficial polyurethane calcification in rat subdermal implants and that an EHBP-modified polyurethane resists calcification despite calciphylaxis. Rat subdermal implants using calciphylaxis may be generally useful for evaluating the calcification potential of various biomedical polymers. © 1996 John Wiley & Sons, Inc.