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The Bj ö rk-Shiley Heart Valve

The Bj ö rk-Shiley Heart Valve. Normal cardiac function. pulmonary valve (to lungs). aortic valve (to body). mitral valve. Natural aortic valve. tricuspid valve. Heart Valve Disease. Problems Stenosis: hardening of the tissue, preventing complete opening Incompetence: allowing backflow

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The Bj ö rk-Shiley Heart Valve

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  1. The Björk-Shiley Heart Valve

  2. Normal cardiac function pulmonary valve (to lungs) aortic valve (to body) mitral valve Natural aortic valve tricuspid valve

  3. Heart Valve Disease • Problems • Stenosis: hardening of the tissue, preventing complete opening • Incompetence: allowing backflow • Causes • Rheumatic fever (stenosis) • Congenital defects • Bacterial infection (inflammation, scarring) • 60,000 valve replacements each year in U.S.

  4. Design Requirements • Unidirectional flow • Durable: 40 million cycles/year • Blood compatible: no thrombus, embolus • Central flow: laminar, not turbulent • Closing not damaging to blood cells

  5. Starr-Edwards caged ball valves The original Starr-Edwards mitral valve, implanted 1960. Lucite cage, Silastic rubber ball occluder. Starr-Edwards Model 1260 introduced 1968. Teflon and polypropylene sewing ring, cast Stellite 21 cage. Still in use. Starr-Edwards 2400 introduced 1972. Hollow Stellite ball, cloth covered Stellite cage. Discontinued 1980.

  6. Materials for Artificial Heart Valves • Cage/flange • Lucite (Plexiglas, PMMA = polymethyl methacrylate) • Stainless steel • Stellite, Haynes (Co-Cr-Mo-Ni) • Titanium • Pyrolitic carbon (LTI carbon) • Occluder • Silicone • Delrin (POM = polyoxymethylene) • Pyrolitic carbon (LTI carbon) • Sewing ring • Dacron (PET = polyethylene terephthalate) • Teflon (PTFE = polytetrafluoroethylene)

  7. Other caged occluder valves Barnard-Goosens Smeloff-Cutter Harken Kay-Shiley

  8. Advantages of caged occluder valves • Indestructible—caged ball still used • Built-in redundancy of multiple struts • Low regurgitation (backflow) • Shortcomings of caged occluder valves • Turbulent flow • Large profile • Damage to cells • Requires permanent anticoagulant therapy • A new design was needed

  9. Björk-Shiley Tilting Disc Valves • Donald P. Shiley, an engineer in Santa Ana, CA with a garage • Late 1960’s began developing heart valves • Viking O. Björk, a Swedish heart surgeon came on board a few years later Radial-spherical (R/S) valve introduced in 1974. Subjected to several design changes. Flat occluder disc, originally of Delrin, later changed to pyrolytic carbon. Stellite housing. Opens to 50-60 degrees. Inlet and outlet struts are welded to the flange. Early failures of the inlet strut welds were eliminated with a change to the welding process.

  10. Björk-Shiley Convexo-Concave valve Section view illustration showing open (left) and closed (right) positions of the Björk-Shiley Convexo-Concave valve. Blood flows from left to right. The flange and inlet and outlet struts retain the occluder disc as it floats freely between the open and closed positions. Photo of the Björk-Shiley Convexo-Concave valve, a tilting-disc valve. Inlet strut is integral with the flange; outlet strut is welded to the flange.

  11. Björk-Shiley Convexo-Concave valve: The Good News • C/C disc improves hemodynamics—decreased complications due to thromboembolism (TE) • Received FDA approval in 6 months • Became the most popular valve in the world (ultimately represented half of all valves implanted)

  12. Björk-Shiley Convexo-Concave valve: The (Really) Bad News • First outlet strut fracture in 1978 during clinical trials

  13. Cause of failure? • Manufacturing or Design? • Failure always at outlet strut weld • Based on experience with R/S valve, changed welding process—focused on manufacturing • Instituted 5 other manufacturing changes • increased the outlet strut angle to reduce stresses induced during installation of the occluder disc • instituted a strut deflection test to identify valves which exhibit high deflections under a test load • implemented a test for residual stresses • developed an inspection procedure for weld structure and penetration • tightened tolerances for the location of the outlet strut on the flange

  14. “Earn as you learn” • Valve not withdrawn from the market during the time these changes were made • Real cause of failure not conclusively determined • Patients with BSCC valves not notified of potential fracture (failure of honor system) • FDA guidelines for defining acceptable failure

  15. The Real Culprit: Bimodal Closure • Faulty logic: the weld fails, so the weld is the problem • Bimodal Closure and Interference • contour mismatch between the disc occluder and the adjacent surfaces of the inlet strut • multiple equilibrium points for the occluder disc in its closed position • rocking of the occluder on the inlet strut during closure could result in the occluder contacting the outlet strut at its tip rather than at its base as designed • contact at the tip of the outlet strut would apply excessive bending stresses to the weld union and result in eventual fatigue failure.

  16. A Design Failure Section view illustration showing a Björk-Shiley Convexo-Concave valve in the closed position. The valve's design calls for the occluder disc to impact the outlet strut at its base, but in bimodal closure the disc impacts the outlet strut at its tip. This induces greater stress in the weld between the flange and outlet strut and hence reduces the fatigue life of the valve.

  17. Resolution • Bimodal closure phenomenon recognized and corrected in the spring of 1984 • None of 8000 valves produced after 1984 fractured • Valve finally pushed off the market in 1986, just as the fracture problem was solved

  18. Björk-Shiley Monostrut Valve • Introduced 1982 • Opening angle 80° • Flange and both struts machined from a single piece of titanium (no welds) • Alliance Medical Technologies bought rights to monostrut from Sorin Biomedical • 120,000 implanted since 1982 with no fractures

  19. Another alternative: Bileaflet valves Photo of the St. Jude valve, the first bileaflet valve. The occluder leaflets swing open and closed on hinges, eliminating the problems associated with strut impact. The leaflets do not close completely, however, allowing some backflow to occur.

  20. Other Bileaflet valves St. Jude Medical Sorin Carbomedics Edwards-Duromedics

  21. Why did I tell you this story? • The engineering design process • it involves iteration • Biomechanics • Fluid mechanics • Mechanics of materials • Materials • Blood compatibility • Fatigue resistance • Creativity • FDA considerations • Ethics

  22. Resources http://www.me.utexas.edu/~uer/heartvalves/index.html http://members.evansville.net/ict/prostheticvalveimagegallery.htm

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