Design and biomechanical analysis of patientspecific porous tantalum prostheses for knee joint revision surgery.

Artificial joint revision surgery, as an increasingly common surgery in orthopedics, often requires patient-specific prostheses to repair the bone defect. Porous tantalum is a good candidate due to its excellent abrasion and corrosion resistance and good osteointegration. Combination of 3D printing technology and numerical simulation is a promising strategy to design and prepare patient-specific porous prostheses. However, clinical design cases have rarely been reported, especially from the viewpoint of biomechanical matching with the patient's weight and motion and specific bone tissue. This work reports a clinical case on the design and mechanical analysis of 3D-printed porous tantalum prostheses for the knee revision of an 84-year-old male patient. Particularly, standard cylinders of 3D-printed porous tantalum with different pore size and wire diameters were first fabricated and their compressive mechanical properties were measured for following numerical simulation. Subsequently, patientspecific finite element models for the knee prosthesis and the tibia were constructed from the patient's computed tomography data. The maximum von Mises stress and displacement of the prostheses and tibia and the maximum compressive strain of the tibia were numerically simulated under two loading conditions by using finite element analysis software ABAQUS. Finally, by comparing the simulated data to the biomechanical requirements for the prosthesis and the tibia, a patient-specific porous tantalum knee joint prosthesis with a pore diameter of 600 µm and a wire diameter of 900 µm was determined. The Young's modulus (5719.32 ± 100.61 MPa) and yield strength (172.71 ± 1.67 MPa) of the prosthesis can produce both sufficient mechanical support and biomechanical stimulation to the tibia. This work provides a useful guidance for designing and evaluating a patient-specific porous tantalum prosthesis.

The Comparison of Different Constitutive Laws and Fiber Architectures for the Aortic Valve on Fluid-Structure Interaction Simulation.

Built on the hybrid immersed boundary/finite element (IB/FE) method, fluid-structure interaction (FSI) simulations of aortic valve (AV) dynamics are performed with three different constitutive laws and two different fiber architectures for the AV leaflets. An idealized AV model is used and mounted in a straight tube, and a three-element Windkessel model is further attached to the aorta. After obtaining ex vivo biaxial tensile testing of porcine AV leaflets, we first determine the constitutive parameters of the selected three constitutive laws by matching the analytical stretch-stress relations derived from constitutive laws to the experimentally measured data. Both the average error and relevant R-squared value reveal that the anisotropic non-linear constitutive law with exponential terms for both the fiber and cross-fiber directions could be more suitable for characterizing the mechanical behaviors of the AV leaflets. We then thoroughly compare the simulation results from both structural mechanics and hemodynamics. Compared to the other two constitutive laws, the anisotropic non-linear constitutive law with exponential terms for both the fiber and cross-fiber directions shows the larger leaflet displacements at the opened state, the largest forward jet flow, the smaller regurgitant flow. We further analyze hemodynamic parameters of the six different cases, including the regurgitant fraction, the mean transvalvular pressure gradient, the effective orifice area, and the energy loss of the left ventricle. We find that the fiber architecture with body-fitted orientation shows better dynamic behaviors in the leaflets, especially with the constitutive law using exponential terms for both the fiber and cross-fiber directions. In conclusion, both constitutive laws and fiber architectures can affect AV dynamics. Our results further suggest that the strain energy function with exponential terms for both the fiber and cross-fiber directions could be more suitable for describing the AV leaflet mechanical behaviors. Future experimental studies are needed to identify competent constitutive laws for the AV leaflets and their associated fiber orientations with controlled experiments. Although limitations exist in the present AV model, our results provide important information for selecting appropriate constitutive laws and fiber architectures when modeling AV dynamics.

Photooxidation crosslinking to recover residual stress in decellularized blood vessel.

Decellularization method based on trypsin-digestion is widely used to construct small diameter vascular grafts. However, this method will reduce the opening angle of the blood vessel and result in the reduction of residual stress. Residual stress reduced has an adverse effect on the compliance and permeability of small diameter vascular grafts. To improve the situation, acellular blood vessels were treated with glutaraldehyde and photooxidation crosslinking respectively, and the changes of opening angle, circumferential residual strain of native blood vessels, decellularized arteries and crosslinked blood vessels were measured by means of histological examination, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in this study. The opening angle of decellularized arteries significantly restored after photooxidation crosslinking (P = 0.0216), while that of glutaraldehyde crosslinking blood vessels reduced. The elastic fibers inside the blood vessels became densely rearranged after photooxidation crosslinking. The results of finite element simulation showed that the residual stress increased with the increase of opening angle. In this study, we found at the first time that photooxidation crosslinking method could significantly increase the residual stress of decellularized vessels, which provides biomechanical support for the development of new biomaterials of vascular grafts.

Mechanical and morphometric study of mitral valve chordae tendineae and related papillary muscle.

The mitral valve (MV) apparatus is a complex mechanical structure including annulus, valve leaflets, papillary muscles (PMs) and connected chordae tendineae. Chordae anchor to the papillary muscles to help the valve open and close properly during one cardiac cycle. It is of paramount importance to understand the functional, mechanical, and microstructural properties of mitral valve chordae and connecting PMs. In particular, little is known about the biomechanical properties of the anterior and posterior papillary muscle and corresponding chords. In this work, we performed uniaxial and biaxial tensile tests on the anterolateral (APM) and posteromedial papillary muscle (PPM), and their respective corresponding chordae tendineae, chordaeAPM and chordaePPM, in porcine hearts. Histology was carried out to link the microstructure and macro-mechanical behavior of the chordae and PMs. Our results demonstrate that chordaePPM are less in number, but significantly longer and stiffer than chordaeAPM. These different biomechanical properties may be partially explained by the higher collagen core ratio and larger collagen fibril density of chordaePPM. No significant mechanical or microstructural differences were observed along the circumferential and longitudinal directions of APM and PPM samples. Data measured on chordae and PMs were further fitted with the Ogden and reduced Holzapfel - Ogden strain energy functions, respectively. This study presents the first comparative anatomical, mechanical, and structural dataset of porcine mitral valve chordae and related PMs. Results indicate that a PM based classification of chordae will need to be considered in the analysis of the MV function or planning a surgical treatment, which will also help developing more precise computational models of MV.

Some Effects of Different Constitutive Laws on FSI Simulation for the Mitral Valve.

In this paper, three different constitutive laws for mitral leaflets and two laws for chordae tendineae are selected to study their effects on mitral valve dynamics with fluid-structure interaction. We first fit these three mitral leaflet constitutive laws and two chordae tendineae laws with experimental data. The fluid-structure interaction is implemented in an immersed boundary framework with finite element extension for solid, that is the hybrid immersed boundary/finite element(IB/FE) method. We specifically compare the fluid-structure results of different constitutive laws since fluid-structure interaction is the physiological loading environment. This allows us to look at the peak jet velocity, the closure regurgitation volume, and the orifice area. Our numerical results show that different constitutive laws can affect mitral valve dynamics, such as the transvalvular flow rate, closure regurgitation and the orifice area, while the differences in fiber strain and stress are insignificant because all leaflet constitutive laws are fitted to the same set of experimental data. In addition, when an exponential constitutive law of chordae tendineae is used, a lower closure regurgitation flow is observed compared to that of a linear material model. In conclusion, combining numerical dynamic simulations and static experimental tests, we are able to identify suitable constitutive laws for dynamic behaviour of mitral leaflets and chordae under physiological conditions.

Modelling mitral valvular dynamics-current trend and future directions.

Dysfunction of mitral valve causes morbidity and premature mortality and remains a leading medical problem worldwide. Computational modelling aims to understand the biomechanics of human mitral valve and could lead to the development of new treatment, prevention and diagnosis of mitral valve diseases. Compared with the aortic valve, the mitral valve has been much less studied owing to its highly complex structure and strong interaction with the blood flow and the ventricles. However, the interest in mitral valve modelling is growing, and the sophistication level is increasing with the advanced development of computational technology and imaging tools. This review summarises the state-of-the-art modelling of the mitral valve, including static and dynamics models, models with fluid-structure interaction, and models with the left ventricle interaction. Challenges and future directions are also discussed.

A finite strain nonlinear human mitral valve model with fluid-structure interaction.

A computational human mitral valve (MV) model under physiological pressure loading is developed using a hybrid finite element immersed boundary method, which incorporates experimentally-based constitutive laws in a three-dimensional fluid-structure interaction framework. A transversely isotropic material constitutive model is used to characterize the mechanical behaviour of the MV tissue based on recent mechanical tests of healthy human mitral leaflets. Our results show good agreement, in terms of the flow rate and the closing and opening configurations, with measurements from in vivo magnetic resonance images. The stresses in the anterior leaflet are found to be higher than those in the posterior leaflet and are concentrated around the annulus trigons and the belly of the leaflet. The results also show that the chordae play an important role in providing a secondary orifice for the flow when the valve opens. Although there are some discrepancies to be overcome in future work, our simulations show that the developed computational model is promising in mimicking the in vivo MV dynamics and providing important information that are not obtainable by in vivo measurements.