Research

Plan

The meniscus plays a critical role in load transmission, stability and energy dissipation in the knee joint. Loss of the meniscus leads to joint degeneration and osteoarthritis. In a number of cases replacement of the resected meniscal tissue by a synthetic implant might avoid the articular cartilage degeneration. None of the available implants presents optimal biomechanics characteristic due to the fact the biomechanics functionality of the meniscus is not yet fully understood.

Mimicking the native biomechanical characteristics of the menisci seems to be the key factor in meniscus replacement functioning. This is extremely challenging due to its complex inhomogeneous microstructure, the lack of a full experimental characterization of the material properties and the lack of 3D theoretical, numerical and computational models which can reproduce and validate the experimental results.

Therefore, the aim of this work is a thorough understanding of the menisci biomechanics with the view of translating the knowledge to the orthopaedic implants arena.

The objective of the proposal be achieved through

  1. designing and performing a range of innovative experimental tests to characterize the behaviour of the meniscus tissue at the micro and macroscale
  2. building an appropriate and novel multiscale anisotropic model at the tissue level which takes into account the fractal dimension of the porous menisci’s tissue
  3. implementing the material model in commercial finite element (FE) software
  4. build and validate an accurate FE biomechanical model of the knee joint (which includes the meniscus) in order to model the biomechanical behaviour of the menisci when subjected to a range of mechanical stress which is not reproducible in an experimental context

Results

The project focused 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.

Remarkable results towards and beyond the objectives of the project:

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 fluid flows. The external layer is highly impermeable as large hydraulic pressures are required to force fluid flow through meniscal tissue.

New architecture of the meniscus at the micro/nano scale (a) ESEM image of untreated specimens of the circumferential section (Lateral meniscus, posterior region) obtained at a pressure of 10Pa, scale bar 200µm (b) Higher magnification of an area of (a), scale bar 30 µm. (a) Three of these large tie fibers (60-80 μm width) bundle sheets (yellow lines) emanating/conveying into a common "node" (red circle). (b) higher magnification on the node in (a). with details of the roughness of the large tie fibers. (c) A high-resolution image of pore of about 30 μm. (d) Microscale view of the portion inside a macro honeycomb compartment. (e) Micro-honeycomb- like structure is delimited by micro tie fiber bundles with a diameter of about 5 μm of the medial meniscus highlighting tie fiber bundles and a "honeycomb-like" network. https://www.nature.com/articles/s41598-019-55243-2


Circumferential section of the central portion of the lateral meniscus Circumferential section of the central portion of the lateral meniscus. Representative 1024 x1024 section of Z-Stacks at one height. (a) the overlay of the two channels is reported, (b) green channel associated with collagen is reported, (c) red channel in which a number of 1 μm thick elastin fibers is evident (white arrows). https://www.nature.com/articles/s41598-019-55243-2


Tapping mode AFM images of meniscal tissue showing collagen fibres (left) & (right) channels for synovial fluid flow Tapping mode AFM images of meniscal tissue showing collagen fibres (left) & (right) channels for synovial fluid flow. [Jared Maritz, Francesca Murphy, K. Dragnevski, O. Barrera, ‘Development and optimisation of micromechanical testing techniques to study the properties of meniscal tissue, Materials Today proceeding 2020: in press’]



Discovery 2: Damping properties at high frequency. It has been observed through DMA tests that the meniscus exhibits 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 proposed in.

Typical stress-strain curve using strain calculated from the rig (blue) and DIC (black) (a) & (b) typical stress-time curve Typical stress-strain curve using strain calculated from the rig (blue) and DIC (black) (a) & (b) typical stress-time curve [Jared Maritz, Francesca Murphy, K. Dragnevski, O. Barrera, ‘Development and optimisation of micromechanical testing techniques to study the properties of meniscal tissue, Materials Today proceeding 2020: in press’]


Typical strain-time curves obtained from a walking (a) & (b) running DMA simulation Typical strain-time curves obtained from a walking (a) & (b) running DMA simulation. [Jared Maritz, Francesca Murphy, K. Dragnevski, O. Barrera, ‘Development and optimisation of micromechanical testing techniques to study the properties of meniscal tissue, Materials Today proceeding 2020: in press’]



Discovery 3: Material parameters are spatial dependent. Mechanical 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.

(a) 3D reconstruction from high resolution MicroCT scan, the volume analysed is a cylinder $3 mm$ in lengths and $2 mm$ diameter. (b) CFD mesh. (left) 3D reconstruction from high resolution MicroCT scan, the volume analysed is a cylinder $3 mm$ in lengths and $2 mm$ diameter. (right) CFD mesh.



Discovery 4: High resolution MicroCT scans (600nm resolution) on the meniscal tissue- currently the only-to-date. Quantification of microstructural features (porosity, frequency of pores, tortuosity) and fluid flow simulations by using computational fluid dynamics (CFD) on real microstructure reconstructed by MicroCT scans.

Data

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