Design development and evaluation of an improved pericardial bioprosthetic heart valve
Pericardial bioprosthetic valves have shown good long-term clinical follow-up results over a period of 13 years with a low incidence of thrombo-embolism and calcification, and good haemodynamic function. However, doubts remain about the long-term durability of these valves and a significant incidence of primary tissue failure has been reported and this has been observed in our clinical series in Glasgow. I have identified tears in leaflets of explanted valves close to the edge of the cloth-covered stent which have caused prolapsed leaflets and large regurgitation. The performances of four pericardial valves, the Ionescu-Shiley Standard, Ionescu-Shiley Low Profile, Hancock and Mitral Medical pericardial valves have been evaluated in my test apparatus in an attempt to gain understanding of the mechanisms of these primary tissue failures. A hydrodynamic function test apparatus has been developed which allowed pressure difference, regurgitation and energy loss across the valve to be measured, and leaflet dynamics to be studied. Durability tests were carried out with Rowan Ash accelerated fatigue testers. The valve function and leaflet dynamics were dependent on the method of leaflet fixation, and the leaflet geometries and the coaption sutures used to close the leaflets together at the top of the posts. These could also affect the durability of the valves. However, accelerated fatigue test results showed premature failure for all four types of valves with tears in the leaflets caused by abrasion at the edge of the cloth-covered frames. In the Ionescu-Shiley Standard valve, tears were also seen at the commissure stitches. Although in these laboratory tests the mechanism of failure was abrasion and thinning of the leaflets as they were pulled over the edge of the cloth-covered frame, care has to be taken when extrapolating these results to clinical practice as biological effects, such as blood deposits on the cloth and tissue ingrowth, can reduce the abrasion to the leaflets at the edge of the frame. These processes are variable and leaflet abrasion on the cloth-covered frames remains the major cause of primary tissue failure in clinically explanted valves. The new three leaflet pericardial valve which I have developed has improved durability and comparable function in vitro to existing valves. The valve is based on a unique twin frame design. The inner support frame is covered with a single piece of bovine pericardium to reduce abrasion to the leaflets at the edge of the frame and has an array of radially-projecting pins and studs onto which the leaflets are mounted. The outer frame is covered with polyester cloth and this is located over the same radial pins to retain the leaflets in position. The assembled valve is secured with a fine locking ring in the base of the valve. The sewing ring is constructed from polyester cloth and can be positioned towards the inflow aspect of the valve in the aortic position to give a supra-annular configuration and away from the inflow aspect in the mitral position to reduce the length of post projecting into the ventricle. The posts on the outer frame are rounded to reduce the risk of damage of the ventricular wall, and a protective suture can be placed across the top of the posts to reduce the risk of suture-snaring during implantation in the mitral position. The mechanical properties of the pericardial tissue used for the leaflets have been investigated and fixation conditions analysed to produce uniform cross-linking throughout the tissue. Prototype valves have been tested with different leaflet geometries and differing methods of leaflet fixation, and an optimal leaflet geometry has been developed. The flexing position of the leaflet was moulded during fixation in a shape defined by two cylindrical surfaces which intersect in a spherical surface in the centre of the leaflet. This gave a stable closed position with deep coaption between the leaflets and synchronous movement of the leaflets to a uniform open position. Prototype valves have been manufactured in sizes 19 to 31 mm. Hydrodynamic tests on the prototype valves have shown comparable pressure difference, regurgitation and energy loss to other pericardial valves. Accelerated fatigue testers have shown greatly improved durability with eight out of nine valves cycled to over 400 million cycles, the equivalent of 10 years without failure. Implantation of size 25 mm valves in the mitral position in seven sheep for over three months has shown good short-term in vivo function. A randomised clinical trial is planned comparing a porcine valve with this improved pericardial valve.