The effect of different treatment modalities on the calcification potential and cross-linking stability of bovine pericardium

English: In the quest for suitable substitution materials to be used in surgical procedures, a large variety of different kinds of materials have been investigated. In cardiothoracic surgery, biological tissues such as porcine heart valves and bovine pericardium exhibited the most suitable properties for use as substitute material, while a variety of synthetic materials are also being used. Biological materials must meet a lengthy list of requirements, before it can be successfully and safely employed as substitution material. Amongst others, it needs to be stable against biological breakdown, easily sterilizable, express minimal immunogenicity, maintain mechanical strength and tissue stability, resist calcification, be non-carcinogenic and non-toxic and permit easy handling. Numerous chemicals and methodologies have been investigated in order to produce the most suitable materials attaining these properties. Glutaraldehyde has emerged as the chemical agent rendering most of these requirements to tissues following fixation and cross-linking with it. Despite the continued use of GA-fixed bovine pericardium worldwide, calcification and tissue degradation after 10-12 years post-implant remains a big problem. The main objective of this study was to try and identify additional biochemical treatment/s which can be employed in the fixation and storage of bovine pericardium, that will minimize the calcification potential of the tissue significantly without compromising the physical properties or the quality of the tissue. GA-fixed pericardial tissue was used as the control, and the outcomes of all the parameters for the other tissue treatments were compared against it. Numerical and categorical data were collected. In the first phase of study, four different methods of tissue treatment were compared for extractable calcium and water contents following 8 weeks implantation of treated samples in the subcutaneous rat model. Aluminium as treatment model was discarded due to the severe calcification of the implants. Results of tissue treated with GAGs were promising and compared favorably with commercial Glycar-treated tissue, and this prompted more detailed investigation. In the next phase of the study, mechanical properties (tensile strength) and crosslinking stability (thermal denaturation temperatures) of tissues treated with different concentrations of GAGs were compared with GA and Glycar-treated tissue. Treatment with a GAG concentration of 0.01M yielded tissue with comparable tensile strength and thermal denaturation temperatures above the minimum benchmark. This concentration was identified as the optimal GAG concentration to be investigated in subcutaneous rat implant studies. In the final phase, treated pericardial samples were implanted into weanling rats for 8 weeks and evaluated on the calcification potential, water content, antigenicity and extent of cross-linking of the collagen in the tissues. Tissue treated with 0.01M GAG compared favorably with the commercial Glycar patches regarding all of these parameters, outperforming GA-fixed control tissue significantly. Significant evidence was however found that added GAGs were still not effectively stabilized despite adding metaperiodate as fixative. GAGs leached out of tissue following an extended storage period. Only a limited amount of GAGs was visible on the outer surface of the explants compared to the layer of GAGs superficially bound to the tissue before implantation. Despite decreasing the tissue calcification substantially while maintaining good mechanical strength and low antigenicity, stabilization of the GAGs in treated tissues will have to be adequately addressed before clinical application of such tissues can be approved.