Simulation of transcatheter aortic valve implantation through patient-specific finite element analysis: Two clinical cases
Introduction
The first percutaneous transcatheter implantation of an aortic valve prosthesis in humans was described more than 10 years ago, in 2002, by Cribier et al. (2002). Since then, such a minimally invasive procedure to restore valve functionality in case of calcific stenosis has become a routine approach for high-risk or even inoperable patients (Smith et al., 2011).
Many papers collecting the results of transcatheter aortic valve implantation (TAVI) demonstrate that, with respect to standard therapy, rates of death from any cause are reduced (Leon et al., 2010). Mid-term follow-up has shown no evidence of restenosis or prosthesis dysfunction (Zajarias and Cribier, 2013). Moreover, recent registers report satisfactory outcomes of TAVI in terms of feasibility, short-term hemodynamics, and functional improvement (Thomas et al., 2011, Eltchaninoff et al., 2011, Vahanian et al., 2008).
However, a high percentage of treated patients have shown moderate to severe perivalvular aortic regurgitation (Zajarias and Cribier, 2013), that is one of the most frequent complications associated with TAVI, which correlates with an increased rate of mortality (Generaux et al., 2013). Incomplete prosthesis apposition due to calcifications or annular eccentricity (Blanke et al., 2010), undersizing of the device, and malpositioning of the valve (Détaint et al., 2009) are the most common determinants of paravalvular leakage. As a direct consequence, an appropriate annular measurement, a correct evaluation of calcifications, and an appropriate sizing of the prosthetic valve are “of utmost importance” (Gurvitch et al., 2011, Delgado et al., 2010, Détaint et al., 2009).
Given such considerations, advanced computational tools integrating patient-specific information and accurate device modeling can be used to support pre-operative planning. In the literature, several studies addressing computer-based simulations of TAVI through finite element analysis (FEA) are already available. Specifically, Smuts et al. (2011) have developed new concepts for different percutaneous aortic leaflet geometries while Wang et al. (2012) and Sun et al. (2010) have investigated the post-operative TAVI mechanics and hemodynamics, respectively. Moreover, Capelli et al. (2012) have virtually evaluated the feasibility of TAVI in morphologies which are currently borderline cases for a percutaneous approach (e.g., failed bioprosthetic aortic valves). More recently, Auricchio et al. (2012a) have proposed a patient-specific FEA-based simulation accounting for all the procedural steps able to reproduce the prosthesis post-operative performance through the inclusion in the analysis of the biological valve sewn into the metallic frame. However, to the authors’ knowledge, a comparison with postoperative medical data has not been addressed yet.
In this context, the present work proposes a systematic approach to realistically simulate TAVI, tailored to the clinical practice; in particular, we propose a study, based on the analysis of pre-operative medical images of two real patients who underwent TAVI, with the final ambitious goal of predicting the post-operative performance of the prosthesis with respect to the specific anatomical features. The present work includes different issues which make it an original contribution, presenting the capabilities of an advanced tool for clinical support and, in particular: (i) the aortic valve model is complete of both the aortic sinuses and the native valve leaflets and the considered material model is calibrated on human data, (ii) the calcific plaque is included within the model on the basis of imaging records, (iii) the geometry of the prosthetic stent is very accurate, being obtained from micro-tomography (micro-CT) reconstruction.
Last but not least, post-operative data collected by physicians for patients’ follow-up are used for comparison with numerical results with the aim of assessing the capabilities of the proposed simulations to predict the procedural outcomes. Validation of TAVI simulation is a critical issue since it is usually difficult to obtain good quality post-operative data and images from standard post-operative procedures. Additionally, postoperative CT is not included in the routine protocol of transcatheter aortic valve implantation either to not overload renal activity of often already critical patients with the use of a contrast die, or to avoid high radiation doses for the patient. Instead, the operation outcome is generally evaluated by intraoperative angiography as well as by follow-up ultrasound. In the present paper, on the basis of such routinely obtained data, we try to address a comparison between the real procedure outcomes and the simulation results.
Section snippets
Materials and methods
Two patients that underwent TAVI have been included in the present study, both with severe symptomatic aortic stenosis: Patient-1 is a 83 year-old male subject, while Patient-2 is a 84 year-old male subject; both patients underwent TAVI through transapical access.1 In both cases, the preoperative planning started from a CT examination, which, at present, represents the standard methodology for device selection based on the measurement of
Results
The obtained results can be classified into two main groups: (i) from the simulation of stent expansion we can evaluate the impact of the metallic frame of the stent on the native calcified aortic root wall; (ii) from the simulation of valve closure, we can predict the post-operative device performance.
In Fig. 6a von Mises aortic wall stresses induced by stent expansion are shown from two different views for the two considered patients.
Such a distribution should be ideally uniform, resembling
Discussion
It is well-known and extensively reported in the literature that the selection of prosthetic device size and type is very important to avoid (or, at least, reduce) aortic regurgitation and/or other TAVI complications (Generaux et al., 2013, Delgado et al., 2010, Détaint et al., 2009). Such a critical choice not only depends on annular dimensions but also on the complex native aortic root morphology as well as on position and dimensions of calcifications (Feuchtner et al., 2013).
Computational
Limitations
Even though the present work represents a clear improvement with respect to the current state of the art due to the previously highlighted key aspects, it still presents some limitations. Material modeling has been simplified: homogeneous isotropic properties have been assigned to all the involved materials, even though calibrated on human experimental data. Our findings indicate a different level of post-implant solicitation of the root tissue. Such results call for further developments in two
Conclusions
Two clinical cases of transcatheter aortic valve implantation have been investigated through structural finite element analysis. In particular, the impact of patient-specific anatomical features of the native aortic valve on the postoperative performance of the balloon-expandable Edwards Sapien XT device has been analyzed. Stress distributions, geometrical changes, coaptation values, and risk of paravalvular leakage have been computed and evaluated for both patients. Comparison between the
Conflict of interest
The authors report no conflicts of interest.
Acknowledgments
This work is partially funded by the European Research Council through the Project no. 259229 entitled ‘ISOBIO: Isogeometric Methods for Biomechanics’, partially by MIUR through the PRIN Project no. 2010BFXRHS., and partially by Cariplo Foundation through the Project iCardioCloud no. 2013-1179 and by Regione Lombardia through the Project no. E18F13000030007. We would also like to thank Dr Anna Ferrara, Dept. of Civil Engineering and Architecture, University of Pavia, Italy, for providing the
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