Pericardium tissue layer and calcification characteristics - polarization-sensitive optical coherence tomography as material analysis tool for aortic valve bioprosthesis fabrication

Introduction Calcific aortic valve (AV) disease results in stenosis, the most treated heart valve disease. Surgical or transcatheter AV replacement or implantation are the only treatment options and bioprostheses fabricated from glutaraldehyde (GA)-fixed bovine pericardium are predominantly used. Remaining free aldehyde groups and xenogenous epitopes are the causes of bioprostheses deterioration within 10 to 15 years. Nevertheless, ECM structures and layer orientation but also pervading vessel structures impact bioprosthesis durability. Polarization-sensitive optical coherence tomography (PS-OCT) is applied to monitor ECM layer and fiber characteristics of native and GA-fixed pericardium, pericardial patches and biological AV prosthesis prior implantation or after explantation due to deterioration. Purpose Aim is the investigation of pericardium properties and of calcified explanted bioprostheses via PS-OCT. PS-OCT applicability as material analysis tool in bioprosthesis fabrication is focused. Methods Porcine and bovine pericardium (native, GA-fixed; n≥3) and a explanted bioprosthesis was investigated using PS-OCT. The commercial PS-OCT system (TEL221PS) delivered ~5 µm axial and 20 µm lateral resolution. Custom MATLAB processing extracted complementary polarization contrasts, i.e. intensity, birefringence and depolarization from 3D averaged Stokes parameters to visualize collagen and elastin layers, quantify retardation and map depolarizing calcifications. Subsequently, histology using van Kossa, elastica and Movat Pentachrom stain was performed. Infrared spectroscopic imaging and SEM was used to correlate calcification and to characterize tissue surfaces, respectively. Results PS-OCT reveals a layered pericardium structure in native and GA-fixed tissues mostly reflecting a cross-section dichotomy. Layer structure was proven by subsequent histology e.g. revealing a contrary elastin fiber orientation in elastica stain. OCT intensity imaging allows discrimination of single elastin fibers. Beside ECM layers, partial mesothelial covering of the smooth pericardium surface is depicted via PS-OCT allowing the estimation of preparation impairment on the mesothelial lining. SEM-analyses of experimentally prepared bovine pericardia support inherent smooth side mesothelial lining but also visualizes remaining connective tissue at the rough side. These remaining connective tissues lead to shadowing in OCT-analysis. Explanted pericardial bioprostheses sections exhibit widespread calcification in the tissue matrix reflected in PS-OCT signals validated using infrared spectroscopic imaging. Conclusion PS-OCT allows the label-free, non-destructive, 3D assessment of pericardium ECM structure and mesothelial or connective tissue covering but also calcification in explanted AV bioprostheses. Therefore PS-OCT is a powerful tool to retrieve material properties for fabrication optimization and to investigate the calcific degeneration.