Experimental Validation of SAPIENTM Transcatheter Heart Valve Finite Element Analysis Models

Abstract

The Edwards SAPIENTM transcatheter heart valve is designed for heart-valve replacement in patients with severe aortic stenosis without open-heart surgery. Physiological finite element analysis (FEA) has been performed to provide an assessment of the fracture and fatigue resistance of the device during deployment and operation. Experimental validation is an essential step in establishing the credibility of computational modeling and simulation [1,2]. The present study validates FEA frame models by comparing the crimping behavior of the FEA models with the results of crimping experiments. FEA models for the Edwards SAPIENTM transcatheter-heart-valve frames were created to assess the structural integrity and durability of the frame, in conjunction with a number of accelerated reliability tests. When a SAPIENTM valve is prepared for delivery, the valve is reduced in diameter onto a delivery catheter using a radial force fixture called a “crimper”. During crimping, the frame deforms elastically at the beginning of crimping, and later, the struts experience large displacements and substantial plastic deformation to achieve the desired diameter reduction. This crimping step includes all the representative deformation modes encountered during the full physiological FEA simulation. Therefore, a rigorous physical crimping experiment was developed to validate the FEA models, encompassing the full range of elastic and plastic deformation experienced during the physiological simulations. Specifically, crimping experiments were conducted using a radial force test system to compare the measured structural response of the frame to an FEA simulation. Figure 1Fig. 1Configuration of the radial force test system shows the radial force test system (MSI Radial Force tester model RX650). The results of the experiment were then compared with the predictions of the FEA models.Two FEA models of SAPIENTM valve frames were constructed using abaqus/standard [3]. These two frame models included representative design conditions in the simulation of the crimping process. The dimensional range of these models encompassed the dimensions of the physical test specimens: one at the low end and the other at the high end. After simulation, the resultant force-diameter curves were extracted from the FEA output database. Crimping experiments were performed using actual SAPIENTM valve frames as the test specimens. The radial force tester progressively reduced the diameter of each frame specimen by applying a uniform radial displacement through an array of 12 radial displacement elements [4]. The reaction force at each element was measured by instrumentation integral to the radial force test device. The radial force was continuously measured along with the outside diameter of the frame during the crimping process. After the crimp test, the resultant force-diameter curves were extracted and compared with the resultant force-diameter curves from FEA. The test system compliance was measured to compensate for the elastic deformation of the loading mechanism.The force-diameter curve defines the structural response of the frame over the complete crimping cycle. Qualitatively, by plotting the curves on the same axes, the curves of the FEA can be seen to agree well with the curves from the physical experiments (Fig. 2). Quantitatively, the curves can be compared using parameters such as radial strength, radial stiffness, and average crimping force. These parameters represent important crimping attributes and span the complete range of elastic and plastic deformation of the frame during the crimping process. A linear regression was performed for each parameter using the simulation results and the experimental data. Figure 3 shows an example of radial strength comparison for the experimental and FEA data. The radial strength represents the force at which plastic-frame deformation occurs. All experimental data points fall within 5% of the linear relationship established from the FEA simulation predictions. Validating a simulation model also aids in assuring the expected in vivo behavior of the implant. The adequacy of the radial strength is evaluated in a verified model and is confirmed clinically. Wilson et al. [5] have demonstrated the SAPIENTM and SAPIEN XTTM design success and have reported 98% circularity upon implantation.Finite element analyses of SAPIENTM valve frames were conducted to simulate the crimping process. The analogous crimping test was performed on SAPIENTM frames using a radial force test system. The force-diameter curves from FEA agree well with those from the physical experiments, capturing the important transition points and following similar slopes. Furthermore, the FEA simulation results are within 5% of the physical experiments for all three important frame characteristic parameters (radial strength, radial stiffness, and average crimping force). This good agreement validates the FEA models for the Edwards SAPIENTM transcatheter heart valve frames and demonstrates high credibility for their use in stress analyses and fatigue life analysis.