The knee meniscus, also known as ‘semi-lunar’ cartilage, is the crescent-shaped wedge of fibrocartilage present within the knee joint, situated between each femoral condyle and tibial plateau. The major components of the meniscus are water, collagen fibrils and the proteoglycan network^1–3. It is the interaction between these components that determines the major functions of the meniscus such as load bearing and transmission, shock absorption and joint stabilisation^4–7. Disruption of meniscal architecture as a result of degradation or trauma may affect the integrity of the extracellular matrix (ECM) organisation, shear stiffness, shear stress and resistance to tensile hoop stresses. This could result in loss of function and compromise the tribological performance. To better understand meniscal tears, it is essential to interrogate the internal structural components of the meniscus. Light microscopy and scanning electron microscopy are commonly used to investigate meniscal structure^8–11. However, these methods require destructive preparation of samples, and can be very time-consuming if a whole-tissue 3D investigation is required.

In the field of biological and medical engineering, an understanding of the biomechanical and tribological properties of the knee meniscus and cartilage (e.g., contact stress and friction of the articulating surfaces) and their correlation with tissue degeneration and degradation such as wear and tear are important in research and development of therapies for knee diseases. To detect and characterise lesions, and to investigate tissue structural changes as a consequence of mechanical or tribological experiments, a non-invasive and high-resolution imaging technique is needed. High field high-resolution magnetic resonance imaging (MRI) may provide this solution. MRI has been used to investigate the knee meniscus for decades^12,13, and high-field MRI scanners have been used for years to provide high-resolution images of small structure as was recently exemplified in an in vitro studies of cartilage wear and degradation¹⁴. The aims of this study were to investigate the ability of high-field (9.4 T) MRI to delineate porcine knee meniscal tissue structure and meniscal tears.

The 3D-FLASH 9 T MRI allowed delineation of intrameniscal structures (Fig. 2). A 16-h FLASH 3D scan achieved a fine in-plane spatial resolution (43 × 43 μm) with the mean SNR value of 16.4 with a standard deviation of 1.34. The images from a 33-h scan have a spatial resolution of 47 × 47 × 86 μm with the mean SNR of 40.58 with a standard deviation of 1.74. The intrameniscal structure could be clearly observed on MR images. A fine network of high signal lines within the meniscal tissue (Fig. 2) showed an appearance reminiscent of the radially oriented tie fibres and the circumferentially oriented fibres that have been observed using microscopy techniques^8,9,11. High-resolution MR images also allowed visualisation of the simulated meniscal tears, particularly the disruption to tissue network [Fig. 2(C–D)].

In the field of functional meniscus biomechanics and biotribology, it is of value to be able to investigate the natural tissue matrix of the specimen after tribological experiments, in order to assess the effects of loads and motions in fundamental experiments. This may involve laboratory experiments on human or animal knee joints mimicking walking or running to help understanding of the role of knee meniscus in the functioning of the knee joints and it requires a non-destructive method which enables visualisation of the specimen in its native form in three dimensions. The method should also be quick enough to prevent the specimen dehydrating. Histology requires dissection and sectioning of the meniscus, it is destructive and time-consuming. High-field 3D MRI allowed preservation of the integrity of tissue and provided a virtual cross-sectional view of the internal architecture of the entire meniscus at any desired angle. The native meniscus was subjected to an MRI scan immediately following dissection. A half-day scan provided spatial resolution and SNR ratio that were sufficiently high to demonstrate 3D meniscal architecture. Furthermore, the application of accelerated MRI acquisition techniques may reduce the required scanning time, whilst preserving image quality.

However, there are several limitations of this study. Firstly, the image finding could not be deciphered as clearly as histology, though the structures seen on the MR images showed a strong resemblance to the actual fibrous structures revealed by microscopy in other reports. The porcine knees used in this study were from skeletally immature 5- to 6-month-old animals. It is known that immature menisci were more vascularised than mature menisci. Therefore, it would be highly possible that part of the intrameniscal signals were from blood vessels, which were difficult to be differentiated from fibrous structures on the images. Secondly, dehydration and deformation of the tissue could occur during a long scan, which would result in blurring of the images. In addition, the meniscal damage created in this study had limitations to mimic the naturally occurred tears, hence the disruption of the meniscal architecture we observed might differ from the natural tear signals.

In conclusion, high-field MRI is a promising technique to non-destructively examine the internal structural components and damage/wear of meniscal tissue in 3D. It has tremendous potential in the field of functional cartilage/meniscus biomechanics and biotribology. Future study will involve characterisation of meniscal components by contrast enhanced imaging and MRI–histological correlation study, characterisation of wear volume, and minimisation of the effect of dehydration and deformation using more sophisticated sample preparation and faster imaging protocols.

All authors have made substantial contributions to the conception and design of the study, interpretation of the image finding, critical revising of the draft, and final approval of the version to be submitted. Manyi Wang (manyiwang@hotmail.com) takes responsibility for the integrity of the work as a whole, from inception to finished article.

None of the authors of the above manuscript has declared any conflict of interest within the last 3 years which may arise from being named as an author on the manuscript.

M. Wang was funded by the Overseas Research Students Awards Scheme and the International Research Scholarship of the University of Leeds. Research was supported by EPSRC, the Leeds Centre of Excellence in Medical Engineering, WELMEC, funded by the Wellcome Trust and EPSRC, WT 088908/Z/09/Z and the Leeds Musculoskeletal Biomedical Research Unit (LMBRU), funded by NIHR.