Periodic Reporting for period 1 - MetaBioMec (Biomechanics of menisci: a multiscale experimental, theoretical and modelling approach for biomimetic meniscal replacements)
Reporting period: 2018-06-11 to 2020-06-10
The project focuses on the biomechanics of the knee meniscus and understanding the structure-function relationship of this tissue. This research has touched upon a number of length scales from nano-macro and on a number of research fields from advanced microscopy and imaging to experimental mechanical testing, involving new mathematical models used for large scale simulations of the knee joint. The scientific objectives: • Meniscal internal architecture: Advanced microscopy and imaging techniques elucidate the internal structure of the meniscus and how it change in the different regions of the meniscus. Micro-structural quantities such as: porosity, fractal dimension, pore size, tortuosity of channels were successfully quantied. This is the only to-date detailed study on the knee meniscus. • Mechanical characterization: Mechanical and poromechanics testings allow to study the behavior of the tissue under a set of quasi static and dynamic loading. Spatial dependent Hyperelastic, viscoleastic and poroelastic parameters are identified. More than 150 tests are planned/conducted. • Material models: A number of material models are explored, including an innovative fractional poroelastic combined with large deformations. The focus is s to try to understand which is the simplest material model that can be used to reproduce the behavior of the tissue. The output is to extrapolate material parameters needed to run a patient specific Finite Element Model (FEM) of the human knee. • Patient Specific model of the knee: A detailed FEM of a human knee is been built and used as a benchmark problem in order to understand what is the role of the meniscus on the contact pressure and contact areas in the femoral and tibial cartilage. It has been noted that contact is a great issue when changing material model of the meniscus. Also it was highlighted that anisotropy makes the difference when comparing linear/non linear (time dependent and non time dependent).
Below a summary of the activities in the four Work packages.
• WP 1 focused on the microstructure characterization of the meniscal tissue. Advanced microscopy techniques such as ESEM and MultiPhoton microscopy have been used in order to make a major breakthrough regarding the understanding of the meniscal architecture.
Highlights: (a) Two families of collagen fibers with an average wavelength of 15 micron are observed. Elastin fibers appear to follow the orientation of the collagen bundles. (b) At the macroscale (between 10-15 mm) large tie fiber bundle sheets (80 m width) in radial direction and tie fiber bundle sheets of decreasing size in vertical and oblique directions create an intricate network which ”tie” and also divide the meniscal tissue into a series of macro compartments of honeycomb shapes. It has been observed that the this is a recurrent self-similar structure from the macroscale to the microscale.
• WP 2 focused on the experimental characterization of the meniscal tissue. This tissue is a functionally graded fiber (collagen)-reinforced composite material, with properties and architecture changing from the central body to the posterior/anterior horns. Highlights: (c) The meniscus exhibits a remarkable properties at high frequency which were not revealed before. The damping capability, enhanced at high frequency, is due through fluid mobility which is strongly related to the breakthrough in the meniscal architecture. (d) Hyperelastic properties are clearly different in the superficial layers of the menisci (being stiffer to sustain the loading and the more internal layers which act as a damper. The central body of the meniscus in the vascular region exhibit higher stiffness with respect of the anterior and posterior horns. The opposite (i.e. anterior and posterior horns stiffer than the central body) happens in the white region (avascular). (e) Poromechanics properties (permeability) of the meniscus are functionally graded throughout the meniscus.
• WP 3 focused on the understanding of the fluid ow inside the meniscal tissue by using computational fluid dynamics (CFD) numerical testing performed on real microstructure reconstructed by high resolution medical images such as microCT scans. Preliminary results of quantification of microstructural features (porosity, frequency of pores, tortuosity) related to one segment of the meniscal tissue are also presented. Highlights: (f) High resolution MicroCT scan (600nm resolution). (g) Advanced imaging based modelling of the fluid flow within the meniscal channels.
• WP 4 This report deals with the patient specific Finite Element models of the knee joint under walking/power walking and running loading. The focus is in the effect of the meniscus material models based on experimental data collected. It has been noted that: (h) The FE model is complex and convergence is slow especially when hyperelasticity is considered. One of the issues is the fact that the surface in contact (femoral condile/meniscus/tibia) are not closely conforming. This is due to the mesh generation from MRI imaging. A manual smoothing process is needed during preprocessing. (i) The focus of the simulations is the quantification of (a) contact pressure, (b) contact area both in the meniscus and in the femoral/tibial cartilage and (c) the meniscus strain maps and how this varies when the meniscus is modelled as: linear elastic (isotropic/anisotropic), hyperelastic (isotropic/anisotropic) and with spatial dependent properties experimentally evaluated, viscoelastic (isotropic/anisotropic) – The results show that the most relevant difference is considering anisotropy and spatial dependency. There is not a huge difference in (a),(b), (c) between linear and non linear models. The computational time however increase by a factor 10.
• WP 1 focused on the microstructure characterization of the meniscal tissue. Advanced microscopy techniques such as ESEM and MultiPhoton microscopy have been used in order to make a major breakthrough regarding the understanding of the meniscal architecture.
Highlights: (a) Two families of collagen fibers with an average wavelength of 15 micron are observed. Elastin fibers appear to follow the orientation of the collagen bundles. (b) At the macroscale (between 10-15 mm) large tie fiber bundle sheets (80 m width) in radial direction and tie fiber bundle sheets of decreasing size in vertical and oblique directions create an intricate network which ”tie” and also divide the meniscal tissue into a series of macro compartments of honeycomb shapes. It has been observed that the this is a recurrent self-similar structure from the macroscale to the microscale.
• WP 2 focused on the experimental characterization of the meniscal tissue. This tissue is a functionally graded fiber (collagen)-reinforced composite material, with properties and architecture changing from the central body to the posterior/anterior horns. Highlights: (c) The meniscus exhibits a remarkable properties at high frequency which were not revealed before. The damping capability, enhanced at high frequency, is due through fluid mobility which is strongly related to the breakthrough in the meniscal architecture. (d) Hyperelastic properties are clearly different in the superficial layers of the menisci (being stiffer to sustain the loading and the more internal layers which act as a damper. The central body of the meniscus in the vascular region exhibit higher stiffness with respect of the anterior and posterior horns. The opposite (i.e. anterior and posterior horns stiffer than the central body) happens in the white region (avascular). (e) Poromechanics properties (permeability) of the meniscus are functionally graded throughout the meniscus.
• WP 3 focused on the understanding of the fluid ow inside the meniscal tissue by using computational fluid dynamics (CFD) numerical testing performed on real microstructure reconstructed by high resolution medical images such as microCT scans. Preliminary results of quantification of microstructural features (porosity, frequency of pores, tortuosity) related to one segment of the meniscal tissue are also presented. Highlights: (f) High resolution MicroCT scan (600nm resolution). (g) Advanced imaging based modelling of the fluid flow within the meniscal channels.
• WP 4 This report deals with the patient specific Finite Element models of the knee joint under walking/power walking and running loading. The focus is in the effect of the meniscus material models based on experimental data collected. It has been noted that: (h) The FE model is complex and convergence is slow especially when hyperelasticity is considered. One of the issues is the fact that the surface in contact (femoral condile/meniscus/tibia) are not closely conforming. This is due to the mesh generation from MRI imaging. A manual smoothing process is needed during preprocessing. (i) The focus of the simulations is the quantification of (a) contact pressure, (b) contact area both in the meniscus and in the femoral/tibial cartilage and (c) the meniscus strain maps and how this varies when the meniscus is modelled as: linear elastic (isotropic/anisotropic), hyperelastic (isotropic/anisotropic) and with spatial dependent properties experimentally evaluated, viscoelastic (isotropic/anisotropic) – The results show that the most relevant difference is considering anisotropy and spatial dependency. There is not a huge difference in (a),(b), (c) between linear and non linear models. The computational time however increase by a factor 10.
Remarkable results towards and beyond the state of the art are summarized below:
• Discovery 1 New architecture of the meniscus at the micro/nano scale . The meniscus is a highly porous “cushion” made of macro/micro/nano channels of collagen fascicles through which uid ows. The external layer is highly impermeable as large hydraulic pressures are required to force uid ow through meniscal tissue. • Discovery 2 Damping properties at high frequency. It has been observed through DMA tests that the meniscus exhibits a remarkable properties at high frequency which were not revealed before. The damping capability, enhanced at high frequency, is due through uid mobility which is strongly related to the breakthrough in the meniscal architecture. • Discovery 3 Material parameters are spatial dependent. Mechanical properties are clearly dierent in the supercial layers of the menisci (being stier to sustain the loading and the more internal layers which act as a damper. The central body of the meniscus in the vascular region exhibit higher stiness with respect of the anterior and posterior horns. • Discovery 4 High resolution MicroCT scans (600nm resolution) on the meniscal tissue- currently the only-to-date. Quantication of microstructural features (porosity, frequency of pores, tortuosity) and uid ow simulations by using computational uid dynamics (CFD) on real microstructure reconstructed by MicroCT scans.
• Discovery 1 New architecture of the meniscus at the micro/nano scale . The meniscus is a highly porous “cushion” made of macro/micro/nano channels of collagen fascicles through which uid ows. The external layer is highly impermeable as large hydraulic pressures are required to force uid ow through meniscal tissue. • Discovery 2 Damping properties at high frequency. It has been observed through DMA tests that the meniscus exhibits a remarkable properties at high frequency which were not revealed before. The damping capability, enhanced at high frequency, is due through uid mobility which is strongly related to the breakthrough in the meniscal architecture. • Discovery 3 Material parameters are spatial dependent. Mechanical properties are clearly dierent in the supercial layers of the menisci (being stier to sustain the loading and the more internal layers which act as a damper. The central body of the meniscus in the vascular region exhibit higher stiness with respect of the anterior and posterior horns. • Discovery 4 High resolution MicroCT scans (600nm resolution) on the meniscal tissue- currently the only-to-date. Quantication of microstructural features (porosity, frequency of pores, tortuosity) and uid ow simulations by using computational uid dynamics (CFD) on real microstructure reconstructed by MicroCT scans.