Patient-specific modeling of biomechanical interaction in transcatheter aortic valve deployment
Introduction
Aortic stenosis (AS) is the most common valvular disease in developed countries [Auricchio et al., 2011] and its prevalence is growing with an aging population [Carabello and Paulus, 2009, Conti et al., 2010]. While surgical aortic valve replacement is still the preferred choice for patients with symptomatic AS, minimally invasive transcatheter aortic valve (TAV) intervention has recently shown promise for elderly high-risk patients who have significant comorbidities [Dwyer et al., 2009, Ebenstein et al., 2009, Fung et al., 1993]. This revolutionary therapy also has a great potential to treat non high-risk patients, with the advantages of less trauma (without the rigors of open-chest surgery) and shorter recovery time, and thus may fundamentally change the current paradigm of surgical valve replacement.
During TAV intervention, the interventional cardiologist does not have direct access to the calcified aortic valve, and must rely on the interaction between the TAV stent and the host tissue to maintain proper device positioning and function. Many of the adverse effects [Fung et al., 1993, Gasser et al., 2006, Grube et al., 2007, Gurvitch et al., 2010] seen in clinical trials, such as impairment of coronary flow, cardiac tamponade, stroke, peripheral embolism, aortic injury, paravalvular leak and access site injury [Haj-Ali, 2008, Hauck et al., 2009], can be explained from the biomechanics perspective. For instance, excessive radial expansion force of the TAV stent may cause aortic injury, while insufficient force may lead to paravalvular leakage and device migration. Improper TAV positioning can also cause occlusion of the coronary ostia (CO). Thus, a quantitative understanding of the biomechanics involved in the TAV intervention is critical for the success of this procedure.
Due to the complex geometry, mechanical properties and contact between the TAV stent and the aortic root in TAV intervention, integrated experimental and computational methods are necessary to evaluate the biomechanical response. Finite element (FE) analysis has been utilized to study the biomechanics of the aortic root [Holzapfel et al., 2000, Holzapfel et al., 2004, Jeziorska et al., 1998, Kumar and Mathew, 2010, Labrosse et al., 2010] or TAV devices [Leon et al., 2010, Li and Sun, 2010, Lu et al., 2008, Mangini et al., 2011, Marrey et al., 2006] individually. However, to the best of our knowledge, the biomechanical interaction between the stenotic aortic valve and TAV stent has been largely unexplored, and therefore is the focus of the present work. Specifically, patient-specific FE models of aortic roots were reconstructed from multi-slice computed tomography (MSCT) scans, and stent expansion during TAV deployment was simulated. Contact force between the stent and aortic root, as well as stress and strain changes in aortic tissue due to the stent expansion were analyzed.
Section snippets
Methods
Patient-specific aortic root geometry. Full phase cardiac MSCT scans were collected from patients at Hartford Hospital (Hartford, CT). Institutional Review Broad approval to review de-identified images was obtained for this study. One stenotic patient with a tricuspid aortic valve and an aortic annulus size of 21 mm was identified from the database. Severe calcification was found in the leaflets of the patient. The MSCT examination was performed on a GE LightSpeed 64-channel volume computed
Results
Material models. The best-fitted material properties of aortic tissue as well as the corresponding biaxial test data are illustrated in Fig. 4. The obtained material parameters are listed in Table 1. It can be seen in Fig. 4 that there was very good correlation between simulation and biaxial results. From the biaxial results it can be seen that the sinus and aortic leaflet tissues had stiffer mechanical response in the circumferential direction, while the ascending aorta was stiffer in the
Discussion
Successful TAV deployment and function are heavily reliant on the aortic root-TAV stent interaction. Since the human aortic valve has a large variation of anatomic structures, e.g. different annulus size, sinus height, CO location [Martin et al., 2011, Wang et al., 2011, Webb and Cribier, 2010] and tissue stiffness [Stolzmann et al., 2009], determination of appropriate interaction forces between the TAV and the native tissue using either in vivo or ex vivo measurements is a challenging task. In
Conclusions
Patient-specific FE models of stenotic aortic roots were reconstructed from MSCT scans; and TAV stent deployment was simulated. The results showed that mechanical responses of the aortic root model directly reconstructed from MSCT scans were significantly lower than those of the model at the rapid ventricular pacing phase. In addition, inclusion of the myocardium slightly increased the mechanical responses. It was observed that maximum stresses and strains were in the region of leaflet
Conflict of interest statement
All authors disclose any financial and personal relationships with other people or organizations that could inappropriately influence (bias) their work.
Acknowledgments
This work was supported in part by the State of Connecticut Department of Public Health Biomedical Research Grant DPH2010-0085, a NSF GRFP fellowship, NIH 1R01HL104080 and 1R21HL108239 grants. We would like to thank Dr. Charles Primiano and Dr. Raymond McKay for providing CT scans. We would also like to thank Thuy Pham and Caitlin Martin for providing experimental data of the aortic tissues.
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