High resolution three-dimensional strain measurements in human articular cartilage

Biomechanics of the Human Osteochondral Unit: A Systematic Review

The damping system ensured by the osteochondral (OC) unit is essential to deploy the forces generated within load-bearing joints during locomotion, allowing furthermore low-friction sliding motion between bone segments. The OC unit is a multi-layer structure including articular cartilage, as well as subchondral and trabecular bone. The interplay between the OC tissues is essential in maintaining the joint functionality; altered loading patterns can trigger biological processes that could lead to degenerative joint diseases like osteoarthritis. Currently, no effective treatments are available to avoid degeneration beyond tissues' recovery capabilities. A thorough comprehension on the mechanical behaviour of the OC unit is essential to (i) soundly elucidate its overall response to intra-articular loads for developing diagnostic tools capable of detecting non-physiological strain levels, (ii) properly evaluate the efficacy of innovative treatments in restoring physiological strain levels, and (iii) optimize regenerative medicine approaches as potential and less-invasive alternatives to arthroplasty when irreversible damage has occurred. Therefore, the leading aim of this review was to provide an overview of the state-of-the-art-up to 2022-about the mechanical behaviour of the OC unit. A systematic search is performed, according to PRISMA standards, by focusing on studies that experimentally assess the human lower-limb joints' OC tissues. A multi-criteria decision-making method is proposed to quantitatively evaluate eligible studies, in order to highlight only the insights retrieved through sound and robust approaches. This review revealed that studies on human lower limbs are focusing on the knee and articular cartilage, while hip and trabecular bone studies are declining, and the ankle and subchondral bone are poorly investigated. Compression and indentation are the most common experimental techniques studying the mechanical behaviour of the OC tissues, with indentation also being able to provide information at the micro- and nanoscales. While a certain comparability among studies was highlighted, none of the identified testing protocols are currently recognised as standard for any of the OC tissues. The fibril-network-reinforced poro-viscoelastic constitutive model has become common for describing the response of the articular cartilage, while the models describing the mechanical behaviour of mineralised tissues are usually simpler (i.e., linear elastic, elasto-plastic). Most advanced studies have tested and modelled multiple tissues of the same OC unit but have done so individually rather than through integrated approaches. Therefore, efforts should be made in simultaneously evaluating the comprehensive response of the OC unit to intra-articular loads and the interplay between the OC tissues. In this regard, a multidisciplinary approach combining complementary techniques, e.g., full-field imaging, mechanical testing, and computational approaches, should be implemented and validated. Furthermore, the next challenge entails transferring this assessment to a non-invasive approach, allowing its application in vivo, in order to increase its diagnostic and prognostic potential.

Comparison of two contrast-enhancing staining agents for use in X-ray imaging and digital volume correlation measurements across the cartilage-bone interface

OBJECTIVE

The pathogenesis of osteoarthritis (OA) is associated with subchondral bone changes, which is linked to abnormal strain distribution in the overlying articular cartilage. This highlights the importance of understanding mechanical interaction at the cartilage-bone interface. The aim of this study is to compare solutions of two contrast-enhancing staining agents (CESA) for combining high-resolution Contrast-Enhanced X-ray microfocus Computed Tomography (CECT) with Digital Volume Correlation (DVC) for full-field strain measurements at the cartilage-bone interface.

DESIGN

Bovine osteochondral plugs were stained with phosphotungstic acid (PTA) in 70% ethanol or 1:2 hafnium-substituted Wells-Dawson polyoxometalate (Hf-WD POM) in PBS. Mechanical properties were assessed using micromechanical probing and nanoindentation. Strain uncertainties (from CECT data) were evaluated following two consecutive unloaded scans. Residual strains were computed following unconfined compression (ex situ) testing.

RESULTS

PTA and Hf-WD POM enabled the visualisation of structural features in cartilage, allowing DVC computation on the CECT data. Residual strains up to ∼10,000 μɛ were detected up to the tidemark. Nanoindentation showed that PTA-staining caused an average ∼6-fold increase in articular cartilage stiffness, a ∼19-fold increase in reduced modulus and ∼7-fold increase in hardness, whereas Hf-WD POM-stained specimens had mechanical properties similar to pre-stain tissue. Micromechanical probing showed a 77% increase in cartilage surface stiffness after PTA-staining, in comparison to a 16% increase in stiffness after staining with Hf-WD POM.

CONCLUSION

Hf-WD POM is a more suitable CESA solution compared to PTA for CECT imaging combined with DVC as it allowed visualisation of structural features in the cartilage tissue whilst more closely maintaining tissue mechanical properties.

Bridging bench to body: ex vivo models to understand articular cartilage repair

With little to no ability to self-regenerate, human cartilage defects of the knee remain a major clinical challenge. Tissue engineering strategies include delivering specific types of cells and biomaterials to the injured cartilage for restoration of architecture and function. Pre-clinical models to test the efficacy of the therapies come with high costs and ethical issues, and imperfect prediction of performance in humans. Ex vivo models represent an alternative avenue to trial cartilage tissue engineering. Defined as viable explanted cartilage samples, ex vivo models can be cultured with a cell-laden biomaterial or tissue-engineered construct to evaluate cartilage repair. Though human and animal ex vivo models are currently used in the field, there is a need for alternative methods to assess the strength of integration, to increase throughput and manage variability and to optimise and standardise culture conditions, enhancing the utility of these models overall.

Development of a method to investigate strain distribution across the cartilage-bone interface in guinea pig model of spontaneous osteoarthritis using lab-based contrast enhanced X-ray-computed tomography and digital volume correlation

OBJECTIVE

Strain changes at the cartilage-bone interface play a crucial role in osteoarthritis (OA) development. Contrast-Enhanced X-ray Computed Tomography (CECT) and Digital Volume Correlation (DVC) can measure 3D strain changes at the osteochondral interface. Using lab-based CT systems it is often difficult to visualise soft tissues such as articular cartilage without staining to enhance contrast. Contrast-Enhancing Staining Agents (CESAs), such as Phosphotungstic Acid (PTA) in 70% ethanol, can cause tissue shrinkage and alter tissue mechanics. The aims of this study were, firstly, to assess changes to the mechanical properties of osteochondral tissue after staining with a PTA/PBS solution, and secondly, to visualise articular cartilage during loading and with CECT imaging in order to compare strain across the interface in both healthy and OA joints using DVC.

DESIGN

Nanoindentation was used to assess changes to mechanical properties in articular cartilage and subchondral bone before and after staining. Hindlimbs from Dunkin-Hartley guinea pigs were stained with 1% PTA/PBS at room temperature for 6 days. Two consecutive CECT datasets were acquired for DVC error analysis. In-situ compression with a load corresponding to 2x body weight was applied, the specimen was re-imaged, and DVC was performed between the pre- and post-load tomograms.

RESULTS

Nanoindentation before and after PTA/PBS staining showed similar cartilage stiffness (p < 0.05), however, staining significantly decreased the stiffness of subchondral bone (∼9-fold; p = 0.0012). In severe OA specimens, third principal/compressive (εp3) strain was 141.7% higher and shear strain (γ) was 98.2% higher in tibial articular cartilage compared to non-OA (2 - month) specimens. A 23.1% increase in third principal stain strain and a 54.5% significant increase in the shear (γ) strain (p = 0.0027) was transferred into the mineralised regions of calcified cartilage and subchondral bone in severe OA specimens.

CONCLUSIONS

These results indicate the suitability of PTA in PBS as a contrast agent for the visualisation of cartilage during CECT imaging and allowed DVC computation of strain across the cartilage-bone interface. However, further research is needed to address the reduction in stiffness of subchondral bone after incubation in PBS.

A novel tool to quantify in vivo lumbar spine kinematics and 3D intervertebral disc strains using clinical MRI

Medical imaging modalities that calculate tissue morphology alone cannot provide direct information regarding the mechanical behaviour of load-bearing musculoskeletal organs. Accurate in vivo measurement of spine kinematics and intervertebral disc (IVD) strains can provide important information regarding the mechanical behaviour of the spine, help to investigate the effects of injuries on the mechanics of the spine, and assess the effectiveness of treatments. Additionally, strains can serve as a functional biomechanical marker for detecting normal and pathologic tissues. We hypothesised that combining digital volume correlation (DVC) with 3T clinical MRI can provide direct information regarding the mechanics of the spine. Here, we have developed a novel non-invasive tool for in vivo displacement and strain measurement within the human lumbar spine and we used this tool to calculate lumbar kinematics and IVD strains in six healthy subjects during lumbar extension. The proposed tool enabled spine kinematics and IVD strains to be measured with errors that did not exceed 0.17 mm and 0.5%, respectively. The findings of the kinematics study identified that during extension the lumbar spine of healthy subjects experiences total 3D translations ranging from 1 mm to 4.5 mm for different vertebral levels. The findings of strain analysis identified that the average of the maximum tensile, compressive, and shear strains for different lumbar levels during extension ranged from 3.5% to 7.2%. This tool can provide base-line data that can be used to describe the mechanical environment of healthy lumbar spine, which can help clinicians manage preventative treatments, define patient-specific treatments, and to monitor the effectiveness of surgical and non-surgical interventions.

A practical guide for in situ mechanical testing of musculoskeletal tissues using synchrotron tomography

Musculoskeletal tissues are complex hierarchical materials where mechanical response is linked to structural and material properties at different dimensional levels. Therefore, high-resolution three-dimensional tomography is very useful for assessing tissue properties at different scales. In particular, Synchrotron Radiation micro-Computed Tomography (SR-microCT) has been used in several applications to analyze the structure of bone and biomaterials. In the past decade the development of digital volume correlation (DVC) algorithms applied to SR-microCT images and its combination with in situ mechanical testing (four-dimensional imaging) have allowed researchers to visualise, for the first time, the deformation of musculoskeletal tissues and their interaction with biomaterials under different loading scenarios. However, there are several experimental challenges that make these measurements difficult and at high risk of failure. Challenges relate to sample preparation, imaging parameters, loading setup, accumulated tissue damage for multiple tomographic acquisitions, reconstruction methods and data processing. Considering that access to SR-microCT facilities is usually associated with bidding processes and long waiting times, the failure of these experiments could notably slow down the advancement of this research area and reduce its impact. Many of the experimental failures can be avoided with increased experience in performing the tests and better guidelines for preparation and execution of these complex experiments; publication of negative results could help interested researchers to avoid recurring mistakes. Therefore, the goal of this article is to highlight the potential and pitfalls in the design and execution of in situ SR-microCT experiments, involving multiple scans, of musculoskeletal tissues for the assessment of their structural and/or mechanical properties. The advice and guidelines that follow should improve the success rate of this type of experiment, allowing the community to reach higher impact more efficiently.

Preparation and characterization of a novel drug-loaded Bi-layer scaffold for cartilage regeneration

The incidence of articular cartilage defects is increasing year by year. In order to repair the cartilage tissue at the defect, scaffolds with nanofiber structure and biocompatibility have become a research hotspot. In this study, we designed and fabricated a bi-layer scaffold prepared from an upper layer of drug-dispersed gelatin methacrylate (GELMA) hydrogel and a lower layer of a drug-encapsulated coaxial fiber scaffold prepared from silk fiber (SF) and polylactic acid (PLA). These bi-layer scaffolds have porosity (91.26 ± 3.94%) sufficient to support material exchange and pore size suitable for cell culture and infiltration, as well as mechanical properties (2.65 ± 0.31 MPa) that meet the requirements of cartilage tissue engineering. The coaxial fiber structure exhibited excellent drug release properties, maintaining drug release for 14 days in PBS. In vitro experiments indicated that the scaffolds were not toxic to cells and were amenable to chondrocyte migration. Notably, the growth of cells in a bi-layer scaffold presented two states. In the hydrogel layer, cells grow through interconnected pores and take on a connective tissue-like shape. In the coaxial fiber layer, cells grow on the surface of the coaxial fiber mats and appeared tablet-like. This is similar to the structure of the functional partitions of natural cartilage tissue. Together, the bi-layer scaffold can play a positive role in cartilage regeneration, which could be a potential therapeutic choice to solve the current problems of clinical cartilage repair.

Digital volume correlation for the characterization of musculoskeletal tissues: Current challenges and future developments

Biological tissues are complex hierarchical materials, difficult to characterise due to the challenges associated to the separation of scale and heterogeneity of the mechanical properties at different dimensional levels. The Digital Volume Correlation approach is the only image-based experimental approach that can accurately measure internal strain field within biological tissues under complex loading scenarios. In this minireview examples of DVC applications to study the deformation of musculoskeletal tissues at different dimensional scales are reported, highlighting the potential and challenges of this relatively new technique. The manuscript aims at reporting the wide breath of DVC applications in the past 2 decades and discuss future perspective for this unique technique, including fast analysis, applications on soft tissues, high precision approaches, and clinical applications.