Valve Durability Considerations: Fluid Dynamic Principles, Assessment Techniques, Predictors, and Minimum Standards Jorge H. Jimenez, Ph.D. Georgia Institute of Technology
Jorge H. Jimenez DISCLOSURES Apica Cardiovascular Technologies - Founder
RISK ANALYSIS OF MI THERAPIES Decades of experience from heart valve and stent industry International standards panel in place Standards are flexible as to conform to device specificity Framework is based on Risk & Hazard Analysis Risk evaluation ISO 14971
Risk Analysis Of MI Therapies Node/Component Description Severity of risk Procedure Delivery Anchoring Verification Correction Structure Loads Dimensions materials Calcification fatigue Fluid Functional characteristics Abnormal patterns Thrombosis/Thromboembolism Blood damage Tissue In-growth
Failure due to Fluid Mechanics Stenosis Leakage Undesirable Flow field Characteristics Loading of fluid on valve’s structure Virchow’s Triad Thrombosis and Thromboembolism
Stenosis & Leakage Stenosis Leakage Leaflet properties and suturing Leaflet stiffening and calcification Pannus overgrowth Leakage Frame over-expansion Leaflet failure Deployed geometry
High velocity forward jet Flow Downstream of First Generations of Valves Flow Visualization and Laser Doppler Velocimetry Ball and Cage Valve Forward flow 2 1 0 m/s EOA= 1.75 cm2 Vreg= 5.5 cm3/beat for a 27 mm valve Recirculation region Forward flow jet Tilting Disc Valve Forward flow 2 1 0 m/s EOA= 3.49 cm2 Vreg= 9.4 cm3/beat for a 27 mm valve Slow forward jet High velocity forward jet recirculation
Structural Leaflet Failure Wear in coaptation areas Frictional Delamination Calcification Leaflet dislodgement at suture interface Micro or macro orifices in tissue
Durability Testing To demonstrate reasonable assurance that minimally invasive tissue valve substitutes will remain functional for 200-300 million cycles Target peak differential pressure 95 % or more of all the test cycles At least 5% of each cycle Some concerns: Effects of excess leaflet tissue; folding leaflets Interface between frame and leaflets Definition of failure: Structural Regurgitation; Stenosis ????? (risk analysis)
Structural Failure Complete structural failure Collapsed frame Partial failure of frame Leaflet dislodgment or malcoaptation Migration Crack propagation Long term failure www.endolab.de/computer/computersimulation_e.htm
Testing for Structural Failure Understanding the interaction between loads, materials and geometry Design mechanical testing protocols to ensure structural integrity
LOADS Do we really know the loads our devices will be subject to (animals, humans, bench)? At level of annulus On prosthetic valve leaflets On stent superstructure by calcified native aortic valve and aorta On calcified native leaflets and annulus XXX
MATERIALS Raw material Post manufacturing Preconditioned Standards certification Post manufacturing Thermo-structural changes Corrosion Finish & micro-cracks Preconditioned Crimping Cold working Implanted material Interaction with human body & environment (ASTM F2516, Tolomeo, Duerig, and Tolomeo; SMST ’00)
Computational Models - FEA www.endolab.de/computer/computersimulation_e.htm Validated computational tools & models used to complement fatigue tests Complex loading Geometrical variations Worst case scenario http://exploration.grc.nasa.gov
Structural Testing To demonstrate structural integrity 400-600 million cycles – test to success Dynamic failure mode tests (extended time or load): Weak link Failure Model validation Definition of failure Complete Crack formation & propagation
Summary New minimally invasive technologies pose new challenges 70+ years of combined experience from heart valve and stent industry Collaboration between Academia, Industry and FDA Appropriate risk-based standards will result in safer technologies, improving patient welfare by getting to market needed technologies.