The Björk-Shiley Heart Valve

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

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.

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

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

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)

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

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

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 Subjected to several design changes. Flat occluder disc, originally of Delrin , later changed to pyrolytic carbon. Stellite housing. Opens to 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.

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.

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)

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

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

“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

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.

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.

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

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

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.

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

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

Resources