The porcine (pig) valves in the early 1970’s provided a quiet alternative that also avoided long-term use of blood thinners. Pig valves could be produced in large numbers and various sizes with factory standards for sterilization and quality control. Other animal tissues have also been used to make valves, notably the bovine (cow) pericardium.
Tissue valves, xenografts, made from animals are structured more like the human aortic valve. They are usually mounted on support frames (stents), to maintain the valves’ functional shape and to facilitate implantation. These valves do not require long-term blood thinners.
Durability is the problem with the xenograft valves, especially in younger patients. About 20% calcify and/or deteriorate within the first eight years, requiring re-operation. After eight years the percentage of valves that must be replaced increases. The stents take up space in the channel from the left ventricle to the aorta, obstructing and creating turbulence in the blood flow. Newer generations of pericardial and porcine valves have shown enhanced durability, especially in older patients.
Stentless porcine valves are the latest types of animal tissue valve. They leave the pig leaflets in the pig’s aorta without a support frame. These are more natural but are more difficult to install. As of 2007 three stentless valves have been approved by the US FDA: the St. Jude Toronto SPV ™, the Medtronic Freestyle ™ valve, and the Edwards LifeSciences Prima Plus™. The latter two devices each have unique fixation and anti-mineralization treatments, which offer hope of enhanced durability.
… The Medtronic Freestyle ™ valve and Edwards Prima Plus™ can also be used as a complete aortic root replacement giving the patient with a small aortic root the benefits of a larger, more effective, valve. (The Toronto SPV™ cannot but a full root version of it with anti-mineralization treatment is currently in clinical trials.)
The stentless valves have a significant advantage over the stented valves in permitting more efficient and less turbulent blood flow. This places less stress on the valve leaflets and requires less work by the heart. The more demanding implantation techniques have made these devices slow to be adopted by many surgeons. The simpler stented valves are also quite adequate in the majority of older, sedentary patients. Ironically, some of the sickest patients may benefit most from the more extensive surgery required to give maximal opening capability to the aortic valve.