Stress state during fixation determines susceptibility to fatigue-linked biodegradation in bioprothetic heart valve materials

Mechanical loading contributes to the structural deterioration of bioprosthetic heart valves. The influence of stress state during fixation may play a substantial role in their failure, linking fatigue damage caused by buckling and tension and the enzymatic degradation of glutaraldehyde-crosslinked collagen. Bovine pericardia were obtained immediately postmortem and 100 mm × 15 mm samples were cut in the base-to-apex direction. Half the samples were subjected to a uniaxial tensile stress of 250 kPa and half remained unloaded during a crosslinking treatment in 0.5% glutaraldehyde. Tissue samples were rinsed and cut into 16 mm × 4 mm test strips. Half of these strips were exposed to cyclic compressive buckling and alternating tension at 30Hz for 20 million cycles (appprox. 7.5 days) using a custom-built multi-sample fatigue system. Fatigue-damaged and non-damaged samples were subsequently incubated at 37 C for 48hrs in: (i) Type I bacterial collagenase (20U/ml) buffered in 0.05M Tris, 10mM CaCl₂ 2H₂O (pH 7.4) or (ii) 0.05M Tris buffer (pH 7.4) only. In both cases, the samples were loaded sinusoidally between 40 and 80g using a previously described microtensile culture system. Tissue removed from the bath was rinsed in 0.1M EDTA solution and mounted in a servo-hydraulic mechanical testing system (MTS). Ultimate tensile strength (UTS), maximum tissue modulus, and fracture strain were determined. The percent collagen solubilized was assessed by a colourmetric hydroxyproline assay of the enzyme bath and tissue sample. All data were analyzed by analysis of variance (ANOVA). The results confirmed the synergy between fatigue damage and collagenase proteolysis in these materials; however, there were no significant differences in this effect between simple fixation and stress-fixation up to 20 million cycles. There were significant decreases in the mechanical properties and an increase in the amount of collagen solubilized with increased exposure to fatigue cycling.