A Comparative Study of Bovine Pericardium Mineralization: A Basic and Practical Approach

The biomineralization of bovine pericardium (BP) heart valve bioprostheses was investigated by simulation of the process under in vitro and in vivo conditions. The nature and composition of calcific deposits that formed in human heart valve bioprostheses were compared with the mineral formed on BP discs immersed in a calcifying medium or subcutaneously implanted into Sprague-Dawley rats. The early stage of experimental biomineralization in vitro took place on the surface only, while in vivo deposition appeared to be intrinsic, as documented by histological cross sections. The chemical composition of the initial mineral phase formed under in vitro conditions had a mean molar Ca/P ratio = 1.255, standard deviation (S.D.) = 0.057, (n = 6). The composition of mineral formed under in vivo conditions had a low initial Ca/P ratio. From 4 days to three weeks the average in vivo ratio was 1.239, S.D. = 0.203 (n = 38). This ratio increased to 1.707, S.D. = 0.038 (n = 6) following seven weeks of implantation. The latter ratio is similar to that of biomineral formed in human bioprostheses: Ca/P = 1.645, S.D. = 0.021 (n = 6). The deproteinated biomineral showed a population of microparticles in the range of 50-200 nm with a microcrystalline apatitic X-ray diffraction pattern. The biomineral contained a significant amount of carbonate, and the solubility was markedly higher than the solubility of hydroxyapatite. Combined information from in vitro, in vivo and bioprosthetic mineralization supports the concept that octacalcium phosphate is a precursor that transforms into bioapatite and is implicated in the calcification of bioprosthetic heart valves.