XRM2024 - Tue13K - "Investigating the relation between microstructure and mechanical response in the meniscus by in situ loading and synchrotron phase contrast tomography"

The aim of the study was to characterize the microstructural organization of collagen fibers in human medial menisci and the response to mechanical loading in relation to age and tissue degeration, we implemented the use of a rheometer setup for simultaneous 12 loading and synchrotron X-ray phase-contrast tomography (SR-PhC-μCT). Structural changes leading to altered biomechanical properties occur in themeniscus due to aging, injuries or disease processes, and these changes are often associated withdevelopment or progression of osteoarthritis. However, the evolution of degeneration at themicroscale and how changes in collagen fiber organization relate to the mechanical response remain unclear.This gap is due to the challenges of imaging relatively thick, low X-ray-absorbance soft tissues, especially during mechanical testing. This study aimed to characterize the microstructural organization of collagen fibersin human medial menisci and the response to mechanical loading in relation to age and OA. For this purpose, we used a rheometer setup for simultaneous compression and synchrotron X-ray phase-contrast tomography(SR-PhC-μCT) earlier tested on bovine tissue [1]. The fiber 3D orientations and structural changes during loading were determined using a structure tensor approach.

Healthy samples exhibited a highly organized, layered fiber structure with a 30°-50° angular shift in the azimuth plane, along the meniscus height. In contrast, degenerated menisci showed less distinct layering. Degeneration did not clearly correlate with donor age .
Tracking fiber orientation and density during loading shows that the loading pattern is reflected in the fiber behaviour. Compression is redistributed uniformly between all layers. Organization along themain fiber axes was reduced and failed to recover during relaxation. Perpendicular organization  increased upon loading but was restored during relaxation. Even if during stress-relaxation  all menisci displayed similar mechanical respose, samples from older donors were characterized by larger variability.

To conclude, concurrent in-situ loading and SR-PhC-μCT provide unprecedented microstructural and mechanical characterization of the human meniscus improving our understanding the structure-function relationship. The structure tensor approach successfully revealed alternating fiber layers and their behavior during loading. While no clear correlation was found across all samples, a distinction in mechanical and fiber response was seen when comparing clearly degenerated versus healthy tissue. By providing a detaled 3D map of the meniscal microstructure, this work help to improve our understanding of how the mechanical response of the microstructural feuteres may lead to macroscopic tissue damage.