Depth-dependent shear behavior of bovine articular cartilage: relationship to structure

Effect of Articular Surface Compression on Cartilage Extracellular Matrix Deformation

Early stage osteoarthritis is characterized by disruption of the superficial zone (SZ) of articular cartilage, including collagen damage and proteoglycan loss, resulting in "mechanical softening" of the extracellular matrix (ECM). The role of the SZ in controlling fluid exudation and imbibition during loading and unloading, respectively, was studied using confined creep compression tests. Bovine osteochondral (OC) plugs were subjected to either a static (88 kPa) or cyclic (0-125 kPa at 1 Hz) compressive stress for five minutes, and the cartilage deformation and recovery were measured during tissue loading and unloading, respectively. During unloading, the articular surface of the cartilage was either loaded with a small 1% tare load (∼1 kPa) applied through a porous load platen (covered), or completely unloaded (uncovered). Then the SZ (∼10%) of the cartilage was removed and the creep tests were repeated. Randomized tests were performed on each OC specimen to assess variability within and between plugs. Static creep strain was always greater than cyclic creep strain except at the beginning of loading (10-20 cycles). Uncovering the articular surface after creep deformation resulted in faster thickness recovery compared to the covered recovery. Removal of the SZ resulted in increased static and cyclic creep strains, as well as an increase in the cyclic peak-to-peak strain envelope. Our results indicate that an intact SZ is essential for normal cartilage mechanical function during joint motion by controlling fluid exudation and imbibition, and concomitantly ECM deformation and recovery, when loaded and unloaded, respectively.

High resolution three-dimensional strain measurements in human articular cartilage

An unresolved challenge in osteoarthritis research is characterising the localised intra-tissue mechanical response of articular cartilage. The aim of this study was to explore whether laboratory micro-computed tomography (micro-CT) and digital volume correlation (DVC) permit non-destructive quantification of three-dimensional (3D) strain fields in human articular cartilage. Human articular cartilage specimens were harvested from the knee, mounted into a loading device and imaged in the unloaded and loaded states using a micro-CT scanner. Strain was measured throughout the cartilage volume using the micro-CT image data and DVC analysis. The volumetric DVC-measured strain was within 5% of the known applied strain. Variation in strain distribution between the superficial, middle and deep zones was observed, consistent with the different architecture of the material in these locations. These results indicate DVC method may be suitable for calculating strain in human articular cartilage.

The nonlinear relationship between speed of sound and compression in articular cartilage: Measurements and modeling

We measured speed of sound in bovine articular cartilage as a function of compressive strain. Using techniques we developed, it was possible to apply strain starting from the unstrained, full height of a sample. Our measurements showed that speed of sound was not a monotonic function of strain as reported in earlier investigations. Speed increased with increasing strain over a range of lower strains. It reached a maximum, and then decreased as the strain increased further. These results were corroborated using a model of wave propagation in deformable porous materials. Using this model, we also established conditions under which a maximum in the speed would exist for samples in compression. Our measurements and analysis resolve the conflicting results reported in previous studies.

Apparatus and Method for Rapid Detection of Acoustic Anisotropy in Cartilage

Purpose

Articular cartilage is known to be mechanically anisotropic. In this paper, the acoustic anisotropy of bovine articular cartilage and the effects of freeze-thaw cycling on acoustic anisotropy were investigated.

Methods

We developed apparatus and methods that use a magnetic L-shaped sample holder, which allowed minimal handling of a tissue, reduced the number of measurements compared to previous studies, and produced highly reproducible results.

Results

SOS was greater in the direction perpendicular to the articular surface compared to the direction parallel to the articular surface (N=17, P = 0.00001). Average SOS was 1,758 ± 107 m/s perpendicular to the surface, and 1,617 ± 55 m/s parallel to it. The average percentage difference in SOS between the perpendicular and parallel directions was 8.2% (95% CI: 5.4% to 11%). Freeze-thaw cycling did not have a significant effect on SOS (P>0.4).

Conclusion

Acoustic measurement of tissue properties is particularly attractive for work in our laboratory since it has the potential for nondestructive characterization of the properties of developing engineered cartilage. Our approach allowed us to observe acoustic anisotropy of articular cartilage rapidly and reproducibly. This property was not significantly affected by freeze-thawing of the tissue samples, making cryopreservation practical for these assays.

Cartilage and collagen mechanics under large-strain shear within in vivo and at supraphysiogical temperatures

Human joints, particularly those of extremities, experience a significant range of temperatures in vivo. Joint temperature influences the mechanics of both joint and cartilage, and the mechanics of cartilage can affect the temperature of both joint and cartilage. Thermal treatments and tissue repairs, such as thermal chondroplasty, and ex vivo tissue engineering may also expose cartilage to supraphysiological temperatures. Furthermore, although cartilage undergoes principally compressive loads in vivo, shear strain plays a significant role at larger compressive strains. Thus, we aimed to determine whether and how the bulk mechanical responses of cartilage undergoing large-strain shear change (1) within the range of temperatures relevant in vivo, and (2) both during and after supraphysiological thermal treatments. We completed large-strain shear tests (10 and 15%) at four thermal conditions: 24^∘C and 40^∘C to span the in vivo range, and 70^∘C and 24^∘C repeated after 70^∘C to explore mechanics during and after potential treatments. We calculated the bulk mechanical responses (strain-energy dissipation densities, peak-to-peak shear stresses, and peak-effective shear moduli) as of function of temperature and used statistical methods to probe significant differences. To probe the mechanisms underlying differences we assessed specimens, principally the type II collagen, with imaging (second harmonic generation and transmission electron microscopies, and histology) and assessed the temperature-dependent mechanics of type II collagen molecules within cartilage using steered molecular dynamics simulations. Our results suggest that the bulk mechanical responses of cartilage depend significantly on temperature both within the in vivo range and at supraphysiological temperatures, showing significant reductions in all mechanical measures with increasing temperature. Using imaging and simulations we determined that one underlying mechanism explaining our results may be changes in the molecular deformation profiles of collagen molecules versus temperature, likely compounded at larger length scales. These new insights into the mechanics of cartilage and collagen may suggest new treatment targets for damaged or osteoarthritic cartilage.

Effects of Loading Conditions on Articular Cartilage in a Metal-on-Cartilage Pairing

The aim of this in vitro study was to investigate the response of articular cartilage to frictional load when sliding against a metal implant, and identify potential mechanisms of damage to articular cartilage in a metal-on-cartilage pairing. Bovine osteochondral cylinders were reciprocally slid against metal cylinders (cobalt-chromium-molybdenum alloy) with several variations of load and sliding velocity using a microtribometer. The effects of different loads and velocities, and the resulting friction coefficients on articular cartilage, were evaluated by measuring histological and metabolic outcomes. Moreover, the biotribocorrosion of the metal was determined. Chondrocytes stimulated with high load and velocity showed increased metabolic activity and cartilage-specific gene expression. In addition, higher load and velocity resulted in biotribocorrosion of the metal implant and damage to the surface of the articular cartilage, whereas low velocity and a high coefficient of friction increased the expression of catabolic genes. Articular cartilage showed particular responses to load and velocity when sliding against a metal implant. Moreover, metal implants showed tribocorrosion. Therefore, corrosion particles may play a role in the mechano-biochemical wear of articular cartilage after implantation of a metal implant. These findings may be useful to surgeons performing resurfacing procedures and total knee arthroplasty. © 2019 The Authors. Journal of Orthopaedic Research® published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society J Orthop Res 37:2531-2539, 2019.

Effect of osteochondral graft orientation in a biotribological test system

Autologous osteochondral transplantation (AOT) utilizing autografts is a widely used technique for the treatment of small-to-medium cartilage defects occurring in knee and ankle joints. The application of viable cartilage and bone ensures proper integration, early weight bearing, as well as restoration of biomechanical and biotribological properties. However, alignment of the autografts onto the defect site remains a pivotal aspect of reinstating the properties of the joint toward successful autograft integration. This is the first study to perform tests with different orientations of osteochondral grafts in a cartilage-on-cartilage test system. The objective was to estimate if there are differences between aligned and 90°-rotated grafts concerning molecular biological and biomechanical parameters. Tissue viability, assessed by XTT assay indicated lower metabolic activity in tested osteochondral grafts (aligned, p = 0.0148 and 90°-rotated, p = 0.0760) in favor of a higher anabolic gene expression (aligned, p = 0.0030 and 90°-rotated, 0.0027). Tissue structure was evaluated by Safranin O histology and microscopic images of the surface. Aligned and 90°-rotated grafts revealed no apparent differences between proteoglycan content or cracks and fissures on the cartilage surface. Test medium analyzed after tribological tests for their sulfated glycosaminoglycan content revealed no differences (p = 0.3282). During the tests, both the friction coefficient and the relative displacement between the two cartilage surfaces were measured, with no significant difference in both parameters (COF, p = 0.2232 and relative displacement, p = 0.3185). From the methods we deployed, this study can infer that there are no differences between aligned and 90°-rotated osteochondral grafts after tribological tests in the used ex vivo tissue model. © 2019 The Authors. Journal of Orthopaedic Research® Published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society. J Orthop Res.

Maximum shear strain-based algorithm can predict proteoglycan loss in damaged articular cartilage

Post-traumatic osteoarthritis (PTOA) is a common disease, where the mechanical integrity of articular cartilage is compromised. PTOA can be a result of chondral defects formed due to injurious loading. One of the first changes around defects is proteoglycan depletion. Since there are no methods to restore injured cartilage fully back to its healthy state, preventing the onset and progression of the disease is advisable. However, this is problematic if the disease progression cannot be predicted. Thus, we developed an algorithm to predict proteoglycan loss of injured cartilage by decreasing the fixed charge density (FCD) concentration. We tested several mechanisms based on the local strains or stresses in the tissue for the FCD loss. By choosing the degeneration threshold suggested for inducing chondrocyte apoptosis and cartilage matrix damage, the algorithm driven by the maximum shear strain showed the most substantial FCD losses around the lesion. This is consistent with experimental findings in the literature. We also observed that by using coordinate system-independent strain measures and selecting the degeneration threshold in an ad hoc manner, all the resulting FCD distributions would appear qualitatively similar, i.e., the greatest FCD losses are found at the tissue adjacent to the lesion. The proposed strain-based FCD degeneration algorithm shows a great potential for predicting the progression of PTOA via biomechanical stimuli. This could allow identification of high-risk defects with an increased risk of PTOA progression.

ESTABLISHING A LIVE CARTILAGE-ON-CARTILAGE INTERFACE FOR TRIBOLOGICAL TESTING

Mechano-biochemical wear encompasses the tribological interplay between biological and mechanical mechanisms responsible for cartilage wear and degradation. The aim of this study was to develop and start validating a novel tribological testing system, which better resembles the natural joint environment through incorporating a live cartilage-on-cartilage articulating interface, joint specific kinematics, and the application of controlled mechanical stimuli for the measurement of biological responses in order to study the mechano-biochemical wear of cartilage. The study entailed two parts. In Part 1, the novel testing rig was used to compare two bearing systems: (a) cartilage articulating against cartilage (CoC) and (b) metal articulating against cartilage (MoC). The clinically relevant MoC, which is also a common tribological interface for evaluating cartilage wear, should produce more wear to agree with clinical observations. In Part II, the novel testing system was used to determine how wear is affected by tissue viability in live and dead CoC articulations. For both parts, bovine cartilage explants were harvested and tribologically tested for three consecutive days. Wear was defined as release of glycosaminoglycans into the media and as evaluation of the tissue structure. For Part I, we found that the live CoC articulation did not cause damage to the cartilage, to the extent of being comparable to the free swelling controls, whereas the MoC articulation caused decreased cell viability, extracellular matrix disruption, and increased wear when compared to CoC, and consistent with clinical data. These results provided confidence that this novel testing system will be adequate to screen new biomaterials for articulation against cartilage, such as in hemiarthroplasty. For Part II, the live and dead cartilage articulation yielded similar wear as determined by the release of proteoglycans and aggrecan fragments, suggesting that keeping the cartilage alive may not be essential for short term wear tests. However, the biosynthesis of glycosaminoglycans was significantly higher due to live CoC articulation than due to the corresponding live free swelling controls, indicating that articulation stimulated cell activity. Moving forward, the cell response to mechanical stimuli and the underlying mechano-biochemical wear mechanisms need to be further studied for a complete picture of tissue degradation.

Shear deformations of human articular cartilage: Certain mechanical anisotropies apparent at large but not small shear strains

Articular cartilage has pronounced through-the-thickness heterogeneity in both ultrastructure and mechanical function. The tissue undergoes a combination of large deformations in vivo, where shear is critical in both failure and chondrocyte death. Yet the microstructure mechanical response of cartilage to multi-axial large shear deformations is unknown. We harvested a total of 42 cartilage specimens from seven matched locations across the lateral femoral condyles and patellofemoral grooves of six human male donors (30.2±8.8yrs old, M±SD). With each specimen we applied a range of quasi-static, multi-axial large (simple) shear displacements both parallel and perpendicular to the local split-line direction (SLD). Shear stresses in cartilage specimens from the patellofemoral grooves were higher, and more energy was dissipated, at all applied strains under loading parallel to the local SLD versus perpendicular, while specimens from the lateral condyles were mechanically anisotropic only under larger strains of 20% and 25%. Cartilage also showed significant intra-donor variability at larger shear strains but no significant inter-donor variability. Overall, shear strain-energy dissipation was almost constant at 5% applied shear strain and increased nonlinearly with increasing shear magnitude. Our results suggest that full understanding of cartilage mechanics requires large-strain analyses to account for nonlinear, anisotropic and location-dependent effects not fully realized at small strains.

Cartilage Strain Distributions Are Different Under the Same Load in the Central and Peripheral Tibial Plateau Regions

There is increasing evidence that the regional spatial variations in the biological and mechanical properties of articular cartilage are an important consideration in the pathogenesis of knee osteoarthritis (OA) following kinematic changes at the knee due to joint destabilizing events (such as an anterior cruciate ligament (ACL) injury). Thus, given the sensitivity of chondrocytes to the mechanical environment, understanding the internal mechanical strains in knee articular cartilage under macroscopic loads is an important element in understanding knee OA. The purpose of this study was to test the hypothesis that cartilage from the central and peripheral regions of the tibial plateau has different internal strain distributions under the same applied load. The internal matrix strain distribution for each specimen was measured on osteochondral blocks from the tibial plateau of mature ovine stifle joints. Each specimen was loaded cyclically for 20 min, after which the specimen was cryofixed in its deformed position and freeze fractured. The internal matrix was viewed in a scanning electron microscope (SEM) and internal strains were measured by quantifying the deformation of the collagen fiber network. The peak surface tensile strain, maximum principal strain, and maximum shear strain were compared between the regions. The results demonstrated significantly different internal mechanical strain distributions between the central and peripheral regions of tibial plateau articular cartilage under both the same applied load and same applied nominal strain. These differences in the above strain measures were due to differences in the deformation patterns of the collagen network between the central and peripheral regions. Taken together with previous studies demonstrating differences in the biochemical response of chondrocytes from the central and peripheral regions of the tibial plateau to mechanical load, the differences in collagen network deformation observed in this study help to provide a fundamental basis for understanding the association between altered knee joint kinematics and premature knee OA.

Effect of actuating cell source on locomotion of organic living machines with electrocompacted collagen skeleton

In robotics, there is a need for small scale, compliant actuators for use in medical applications or minimally invasive environmental monitoring. Biohybrid devices offer one solution to this need by using muscle cells to actuate compliant scaffolds. Such devices typically use biocompatible synthetic polymers as compliant scaffolds, which require additional processing steps to promote cellular alignment and attachment. Instead, electrocompacted and aligned collagen (ELAC) can be used as a completely organic scaffold, requiring no additional processing steps, with alignment being innately promoted by the topography. Locomotive living machines have been fabricated in this study using ELAC scaffolds. Devices have been produced using either primary cardiomyocytes or primary skeletal muscle cells isolated from chick embryos as actuators. When tested under the same conditions, skeletal muscle cell powered devices were approximately an order of magnitude faster, having a mean velocity of 77.6 ± 86.4 μm min(-1), compared to 9.34 ± 6.69 μm min(-1) for cardiomyocyte powered devices. In conclusion, completely organic living machines have been fabricated using electrocompacted collagen skeletons, and it was found that skeletal muscle powered devices were significantly faster than cardiomyocyte powered devices.