Fatigue-induced changes to the biaxial mechanical properties of glutaraldehyde-fixed porcine aortic valve leaflets.

Circumferential extension is a direct measure of the preservation of functional collagen crimp in the fibrosal layer of aortic valve leaflets. The aim of this study was to determine whether the elastic properties of zero-pressure, glutaraldehyde-fixed leaflets are changed by mechanical fatigue. Nine Medtronic Freestyle bioprostheses were subjected to 200x10(6) cycles of accelerated fatigue and then biaxially tested to quantify the elastic properties of the leaflets. At physiological load (60 Nm(-1)) the radial extensibility was approximately halved relative to controls (P<10(-4)); there were also lesser reductions in the circumferential extensions (P<.01). The pulsatile regurgitant volume showed no change relative to the control leaflets. The natural corrugations of the fibrosal layer were flattened by the fatigue cycling, but this was not related to an increase in the radial size of the leaflets. Valve competency was maintained.

Hydrodynamic performance of the Medtronic Freestyle Aortic Root Bioprosthesis.

The Freestyle Aortic Root Bioprosthesis is comprised of a porcine aortic root preserved in buffered 0.2% glutaraldehyde solution. It is fixed at a pressure sufficient to distend the aortic wall, but the leaflets are fixed with no transvalvular gradient. The bioprosthesis is treated with alpha-amino-oleic acid (AOA) to reduce the potential for leaflet calcification. In order to conduct in-vitro performance studies, a simulated aorta test chamber was developed. The test chamber material formulation can be altered to produce a wide range of desired compliances, while maintaining repeatable dimensional specifications. The test chamber design also allows for performance testing simulating a variety of implant techniques. Both Freestyle and the stented control valves (Hancock Standard Model 242) were mounted within the simulated aortas for these studies. Steady state and pulsatile flow pressure drop studies demonstrated a significantly lower gradient across the Freestyle valve when compared to stented Hancock Standard controls. In addition, the smaller size (19,21,23 mm) Freestyle valves had superior EOA's compared to the Standard St. Jude bileaflet mechanical and Hancock modified orifice (MO) porcine valve designs. The regurgitant volumes were equivalent to the control (Hancock Standard model 242) valves when compared to the Freestyle valves with equivalent flow areas. Additional studies were conducted in order to determine the effects of sizing and configuration on hydrodynamic performance. These studies were performed in four configurations: total root, inclusion, partially scalloped (two sinuses) and fully scalloped valve. These studies demonstrated that both pressure drop and regurgitation are significantly affected by valve sizing and implant technique. Accelerated wear studies were performed on three of each size and six of the largest Freestyle valves. Stented Hancock Standard valves and non-AOA treated Freestyle valves were used as controls. Hydrodynamic performance tests were conducted at intervals throughout the study. No evidence was found to indicate a difference in wear or hydrodynamic performance between the AOA treated and untreated Freestyle valves.