Background

Equipoise of transcatheter aortic valve replacement with surgical aortic valve replacement in intermediate-risk patients has been demonstrated. As transcatheter aortic valve replacement usage expands, questions regarding long-term durability become paramount. Valve design impacts durability with regions of increased leaflet stress being vulnerable to early failure. However, transcatheter aortic valve (TAV) leaflet stresses are unknown. The objective of this study was to determine stent and leaflet stresses of second-generation balloon-expandable TAV.

Methods

Commercial 29-mm Edwards Sapien XT (Edwards Lifesciences, Irvine, CA) valves underwent high-resolution microcomputed tomography scanning to develop precise three-dimensional geometric mesh. Compressed and uncompressed TAVs were modeled under systemic pressure using finite element software. Material properties of stent were based on cobalt-chromium, whereas those for leaflets were obtained from surgical bioprostheses.

Results

Maximum and minimum principal stresses on uncompressed Sapien XT TAV were 1.63 MPa and -0.36 MPa on leaflets and 93.3 MPa and -105.6 MPa on stent at diastolic pressure. Peak leaflet stress was observed at commissural tips where leaflets connected to the stent. For compressed TAV to 26 mm, maximum and minimum principal stresses were 1.55 MPa and -0.63 MPa on leaflets and 526.1 MPa and -902.2 MPa on stent at diastolic pressure. Peak leaflet stress was located at similar position and also along the suture line with the Dacron (C. R. Bard, Haverhill, PA).

Conclusions

Stress analysis of two extreme deployed geometries of 29-mm Edwards Sapien XT using exact geometry from high-resolution scans demonstrated that peak stresses for TAV leaflets were present at commissural tips where leaflets were attached. These regions would be mostly likely to initiate degeneration.

Objective

Transcatheter aortic valve replacement is established therapy for high-risk and inoperable patients with severe aortic stenosis, but questions remain regarding long-term durability. Valve design influences durability. Increased leaflet stresses in surgical bioprostheses have been correlated with degeneration; however, transcatheter valve leaflet stresses are unknown. From 2007 to 2014, a majority of US patients received first-generation balloon-expandable transcatheter valves. Our goal was to determine stent and leaflet stresses in this valve design using finite element analyses.

Methods

A 26-mm Sapien Transcatheter Heart Valve (Edwards Lifesciences, Inc, Irvine, Calif) underwent high-resolution microcomputed tomography scanning to develop precise 3-dimensional geometry of the leaflets, the stent, and the polyethylene terephthalate elements. The stent was modeled using 3-dimensional elements and the leaflets were modeled using shell elements. Stent material properties were based on stainless steel, whereas those for leaflets were obtained from surgical bioprostheses. Noncylindrical Sapien valve geometry was also simulated. Pressure loading to 80 mm Hg and 120 mm Hg was performed using ABAQUS finite element software (Dassault Systèmes, Waltham, Mass).

Results

At 80 mm Hg, maximum principal stresses on Sapien leaflets were 1.31 megaspascals (MPa). Peak leaflet stress was observed at commissural tips where leaflets connected to the stent. Maximum principal stresses for the stent were 188.91 MPa and located at stent tips where leaflet commissures were attached. Noncylindrical geometry increased peak principal leaflet stresses by 16%.

Conclusions

Using exact geometry from high-resolution scans, the 26-mm Sapien Transcatheter Heart Valve showed that peak stresses for both stent and leaflets were present at commissural tips where leaflets were attached. These regions would be prone to leaflet degeneration. Understanding stresses in first-generation transcatheter valves allows comparison to future designs for relative durability.

Objectives

Rupture/dissection of ascending thoracic aortic aneurysms (aTAAs) carries high mortality and occurs in many patients who did not meet size criteria for elective surgery. Elevated wall stress may better predict adverse events, but cannot be directly measured in vivo, rather determined from finite element (FE) simulations. Current computational models make assumptions that limit accuracy, most commonly using in vivo imaging geometry to represent zero-pressure state. Accurate patient-specific wall stress requires models with zero-pressure three-dimensional geometry, material properties, wall thickness and residual stress. We hypothesized that wall stress calculated from in vivo imaging geometry at systemic pressure underestimates that using zero-pressure geometry. We developed a novel method to derive zero-pressure geometry from in vivo imaging at systemic pressure. The purpose of this study was to develop the first patient-specific aTAA models using magnetic resonance imaging (MRI) to assess material properties and zero-pressure geometry. Wall stress results from FE models using systemic pressure were compared with those from models using zero-pressure correction.

Methods

Patients with aTAAs <5 cm underwent ECG-gated computed tomography angiography (CTA) and displacement encoding with stimulated echo (DENSE)-MRI. CTA lumen geometry was used to create surface contour meshes of aTAA geometry. DENSE-MRI measured cyclic aortic wall strain from which wall material property was derived. Zero- and systemic pressure geometries were created. Simulations were loaded to systemic pressure using the ABAQUS FE software. Wall stress analyses were compared between zero-pressure-corrected and systemic pressure geometry FE models.

Results

Peak first principal wall stress (primarily aligned in the circumferential direction) at systolic pressure for the zero-pressure correction models was 430.62 ± 69.69 kPa, whereas that without zero-pressure correction was 312.55 ± 39.65 kPa (P = 0.004). Peak second principal wall stress (primarily aligned in the longitudinal direction) at systolic pressure for the zero-pressure correction models was 200.77 ± 43.13 kPa, whereas that without zero-stress correction was 156.25 ± 25.55 kPa (P = 0.02).

Conclusions

Previous FE aTAA models from in vivo CT and MRI have not accounted for zero-pressure geometry or patient-specific material property. We demonstrated that zero-pressure correction significantly impacts wall stress results. Future computational models that use wall stress to predict aTAA adverse events must take into account zero-pressure geometry and patient material property for accurate wall stress determination.