Macro-, micro- and ultrastructural investigation of how degeneration influences the response of cartilage to loading

Subchondral bone plate thickness is associated with micromechanical and microstructural changes in the bovine patella osteochondral junction with different levels of cartilage degeneration

The influence of joint degeneration on the biomechanical properties of calcified cartilage and subchondral bone plate at the osteochondral junction is relatively unknown. Common experimental difficulties include accessibility to and visualization of the osteochondral junction, application of mechanical testing at the appropriate length scale, and availability of tissue that provides a consistent range of degenerative changes. This study addresses these challenges. A well-established bovine patella model of early joint degeneration was employed, in which micromechanical testing of fully hydrated osteochondral sections was carried out in conjunction with high-resolution imaging using differential interference contrast (DIC) optical light microscopy. A total of forty-two bovine patellae with different grades of tissue health ranging from healthy to mild, moderate, and severe cartilage degeneration, were selected. From the distal-lateral region of each patella, two adjacent osteochondral sections were obtained for the mechanical testing and the DIC imaging, respectively. Mechanical testing was carried out using a robotic micro-force acquisition system, applying compression tests over an array (area: 200 μm × 1000 μm, step size: 50 μm) across the osteochondral junction to obtain a stiffness map. Morphometric analysis was performed for the DIC images of fully hydrated cryo-sections. The levels of cartilage degeneration, DIC images, and the stiffness maps were used to associate the mechanical properties onto the specific tissue regions of cartilage, calcified cartilage, and subchondral bone plate. The results showed that there were up to 20% and 24% decreases (p < 0.05) in the stiffness of calcified cartilage and subchondral bone plate, respectively, in the severely degenerated group compared to the healthy group. Furthermore, there were increases (p < 0.05) in the number of tidemarks, bone spicules at the cement line, and the mean thickness of the subchondral bone plate with increasing levels of degeneration. The decreasing stiffness in the subchondral bone plate coupled with the presence of bone spicules may be indicative of a subchondral remodeling process involving new bone formation. Moreover, the mean thickness of the subchondral bone plate was found to be the strongest indicator of mechanical and associated structural changes in the osteochondral joint tissues.

The ultrastructure of cartilage tissue and its swelling response in relation to matrix health

This multi-length scale anatomical study explores the influence of mild cartilage structural degeneration on the tissue swelling response. While the swelling response of cartilage has been studied extensively, this is the first study to reveal and correlate tissue microstructure and ultrastructure, with the swelling induced cartilage tissue strains. Cartilage sample strips (n = 30) were obtained from the distal-lateral quadrant of thirty mildly degenerate bovine patellae and, following excision from the bone, the cartilage strips were allowed to swell freely for 2 h in solutions of physiological saline and distilled water successively. The swelling response of this group of samples were compared with that of healthy cartilage, with (n = 20) and without the surface layer (n = 20). The subsequent curling response of cartilage showed that in healthy tissue it was highly variable, and with the surface removed some samples curved in the opposite direction, while in the mildly degenerate tissue group, virtually all tissue strips curved in a consistent upward manner. A significant difference in strain was observed between healthy samples with surface layer removed and mildly degenerate samples, illustrating how excision of the surface zone from pristine cartilage is insufficient to model the swelling response of tissue which has undergone natural degenerative changes. On average, total tissue thickness increased from 940 µm (healthy) to 1079 µm (mildly degenerate), however, looking at the zonal strata, surface and transition zone thicknesses both decreased while deep zone thickness increased from healthy to mildly degenerate tissue. Morphologically, changes to the surface zone integrity were correlated with a diminished surface layer which, at the ultrastructural scale, correlated with a decreased fibrillar density. Similarly, fibrosity of the general matrix visible at the microscale was associated with a loss of later interconnectivity resulting in large, aggregated fibril bundles. The microstructural and ultrastructural investigation revealed that the key differences influencing the tissue swelling strain response was (1) the thickness and extent of disruption to the surface layer and (2) the amount of fibrillar network destructuring, highlighting the importance of the collagen and tissue matrix structure in restraining cartilage swelling.

Decellularized tracheal scaffolds in tracheal reconstruction: An evaluation of different techniques

In humans, the trachea is a conduit for ventilation connecting the throat and lungs. However, certain congenital or acquired diseases may cause long-term tracheal defects that require replacement. Tissue engineering is considered a promising method to reconstruct long-segment tracheal lesions and restore the structure and function of the trachea. Decellularization technology retains the natural structure of the trachea, has good biocompatibility and mechanical properties, and is currently a hotspot in tissue engineering studies. This article lists various recent representative protocols for the generation of decellularized tracheal scaffolds (DTSs), as well as their validity and limitations. Based on the advancements in decellularization methods, we discussed the impact and importance of mechanical properties, revascularization, recellularization, and biocompatibility in the production and implantation of DTS. This review provides a basis for future research on DTS and its application in clinical therapy.

The mechanical influence of bone spicules in the osteochondral junction: A finite element modelling study

While much has been done to study how cartilage responds to mechanical loading, as well as modelling such responses, arguably less has been accomplished around the mechanics of the cartilage-bone junction. Previously, it has been reported that the presence of bony spicules invading the zone of calcified cartilage, preceded the formation of new subchondral bone and the advancing of the cement line (Thambyah and Broom in Osteoarthr Cartil 17:456-463, 2009). In this study, the morphology and frequency of bone spicules in the cartilage-bone interface of osteochondral beams subjected to three-point bending were modelled, and the results are discussed within the context of biomechanical theories on bone formation. It was found that the stress and strain magnitudes, and their distribution were sensitive to the presence and number of spicules. Spicule numbers and shape were shown to affect the strain energy density (SED) distribution in the areas of the cement line adjacent to spicules. Stresses, strains and SED analyses thus provided evidence that the mechanical environment with the addition of spicules promotes bone formation in the cartilage-bone junction.

Impact-induced cartilage damage assessed using polarisation-sensitive optical coherence tomography

Non-invasive determination of structural changes in articular cartilage immediately after impact and rehydration provides insight into the response and recovery of the soft tissue, as well as provides a potential methodology for clinicians to quantify early degenerative changes. In this study, we use polarisation-sensitive optical coherence tomography (PS-OCT) to examine subtle alterations of the optical properties in healthy and early-stage degenerate articular cartilage immediately after impact loading to identify structurally relevant metrics required for understanding the mechanical factors of osteoarthritic initiation and progression. A custom-designed impact testing rig was used to deliver 0.9 J and 1.4 J impact energies to bovine articular cartilage. A total of 52 (n=26 healthy, n=26 mildly degenerate) cartilage-on-bone samples were imaged before, immediately after, and 3 h after impact. PS-OCT images were analyzed to assess changes relating to surface irregularity, optical attenuation, and birefringence. Mildly degenerate cartilage exhibits a significant change in birefringence following 1.4 J impact energies compared to healthy samples which is believed to be attributable to degenerate cartilage being unable to fully utilise the fluid phase to distribute and dampen the energy. After rehydration, the polarisation-sensitive images appear to 'optically-recover' reducing the reliability of birefringence as an absolute metric. Surface irregularity and optical attenuation encode diagnostically relevant information and may serve as markers to predict the mechanical response of articular cartilage. PS-OCT with its ability to non-invasively image the sub-surface microstructural abnormalities of cartilage presents as an ideal modality for cartilage degeneration assessment and identification of mechanically vulnerable tissue.

The mechanical significance of the zonally differentiated collagen network of articular cartilage in relation to tissue swelling

BACKGROUND

We hypothesise that the Benninghoff arcade fibril structure motif of cartilage is able to predict the swelling response of cartilage.

METHODS

A total of ten healthy adult bovine patellae were used for this study, yielding 20 paired full depth cartilage samples (half with surface layer intact and half with surface layer removed). Following excision from the bone, samples were allowed to equilibrate first in physiological saline for 2 h, and then in distilled water for another 2 h to maximise the swelling response. Images were captured using a stereomicroscope to measure strain and the fully-swollen samples were fixed in 10% formalin to retain shape for microscopic and ultrastructural imaging.

FINDINGS

We expected all swelling samples with an intact 'strain-limiting' surface layer to curl upwards, instead only 70% of them did. For samples without a surface layer, we expected the swelling to be evenly distributed and to remain relatively uncurled; but in 40% of the samples there was a downward curvature (i.e. opposite to that of the previous group). Micro-to-ultrastructural imaging, to determine fibrillar structure and organisation, revealed the deep zone cartilage was an additional counter layer limiting swelling strain, and was the likely cause of the unexpected swelling responses.

INTERPRETATION

Our expectations that the surface layer alone will influence the swelling response, was based on the assumptions of the Benninghoff arcade model. This study highlights the additional importance of sub-micron scale fibrillar interconnectivity and the role of the deep zone.

Macroscopically healthy articular cartilage with fibrillar-scale early tissue degeneration subject to impact loading results in greater extent of cell-death

From previous investigations it has been shown that there exists healthy-appearing articular cartilage that contains collagen fibril network destructuring. It is hypothesised that such sub-micron scale destructuring not only presents an increased vulnerability to tissue scale damage following impact loading, but an increase in cell death as well. Cartilage-on-bone blocks from 12 patellae, six healthy (G0) and the other six with sub-micron fibrillar destructuring (G1), were obtained and subject to 2.3 J impact loading. Two sets of sub-samples were obtained for each block tested. One set was used to examine for the live/dead cell response using calcein-AM and propidium iodide staining, imaged with confocal microscopy. The tissue microstructural matrix was imaged from the other matched set, unstained and in its fully hydrated state, using differential interference contrast optical light microscopy. High speed imaging of the impact was used to calculate the velocity changes or coefficient of restitution (COR) and used as a proxy of energy that the tissue absorbed. A previously defined tissue matrix damage score was used to quantify the extent of fracturing and cracking in the matrix. The cell death (PCD) was counted and presented as a percentage against all cells live plus dead. The energy absorbed was 36.5% higher in G1 than in G0 (p = 0.034). However, the damage score and PCD of samples in the G1 group was much larger than the G0 group, ~300% and 161% respectively. Microscopy showed that cell death is associated to both matrix compaction and further fibrillar destructuring from the ECM to the territorial matrix regions of the chondron. Following impact loading, cartilage tissue that appears normal but contains sub-micron fibrillar matrix destructuring responds with significantly increased cell death.

Detecting human articular cartilage degeneration in its early stage with polarization-sensitive optical coherence tomography

Detecting articular cartilage (AC) degeneration in its early stage plays a critical role in the diagnosis and treatment of osteoarthritis (OA). Polarization-sensitive optical coherence tomography (PS-OCT) is sensitive to the alteration and disruption of collagen organization that happens during OA progression. This study proposes an effective OA evaluating method based on PS-OCT imaging. A slope-based analysis is applied on the phase retardation images to segment articular cartilage into three zones along the depth direction. The boundaries and birefringence coefficients (BRCs) of each zone are quantified. Two parameters, namely phase homogeneity index (PHI) and zonal distinguishability (Dz), are further developed to quantify the fluctuation within each zone and the zone-to-zone variation of the tissue birefringence properties. The PS-OCT based evaluating method then combines PHI and Dz to provide a G _PS score for the severity of OA. The proposed method is applied to human hip joint samples and the results are compared with the grading by histology images. The G _PS score shows very strong statistical significance in differentiating different stages of OA. Compared to using the BRC of each zone or a single BRC for the entire depth, the G _PS score shows great improvement in differentiating early-stage OA. The proposed method is shown to have great potential to be developed as a clinical tool for detecting OA.

Bi-layered micro-fibre reinforced hydrogels for articular cartilage regeneration

Articular cartilage has limited capacity for regeneration and when damaged cannot be repaired with currently available metallic or synthetic implants. We aim to bioengineer a microfibre-reinforced hydrogel that can capture the zonal depth-dependent mechanical properties of native cartilage, and simultaneously support neo-cartilage formation. With this goal, a sophisticated bi-layered microfibre architecture, combining a densely distributed crossed fibre mat (superficial tangential zone, STZ) and a uniform box structure (middle and deep zone, MDZ), was successfully manufactured via melt electrospinning and combined with a gelatin-methacrylamide hydrogel. The inclusion of a thin STZ layer greatly increased the composite construct's peak modulus under both incongruent (3.2-fold) and congruent (2.1-fold) loading, as compared to hydrogels reinforced with only a uniform MDZ structure. Notably, the stress relaxation response of the bi-layered composite construct was comparable to the tested native cartilage tissue. Furthermore, similar production of sulphated glycosaminoglycans and collagen II was observed for the novel composite constructs cultured under mechanical conditioning w/o TGF-ß1 supplementation and in static conditions w/TGF-ß1 supplementation, which confirmed the capability of the novel composite construct to support neo-cartilage formation upon mechanical stimulation. To conclude, these results are an important step towards the design and manufacture of biomechanically competent implants for cartilage regeneration. STATEMENT OF SIGNIFICANCE: Damage to articular cartilage results in severe pain and joint disfunction that cannot be treated with currently available implants. This study presents a sophisticated bioengineered bi-layered fibre reinforced cell-laden hydrogel that can approximate the functional mechanical properties of native cartilage. For the first time, the importance of incorporating a viable superficial tangential zone (STZ) - like structure to improve the load-bearing properties of bioengineered constructs, particularly when in-congruent surfaces are compressed, is demonstrated. The present work also provides new insights for the development of implants that are able to promote and guide new cartilaginous tissue formation upon physiologically relevant mechanical stimulation.

Effects of low-level laser therapy on the organization of articular cartilage in an experimental microcrystalline arthritis model

The aim of this study was to evaluate the effects of low-level laser therapy using the gallium arsenide laser (λ = 830 nm) on the articular cartilage (AC) organization from knee joint in an experimental model of microcrystalline arthritis in adult male Wistar rats. Seventy-two animals were divided into three groups: A (control), B (induced arthritis), and C (induced arthritis + laser therapy). The arthritis was induced in the right knee using 2 mg of Na₄P₂O₇ in 0.5 mL of saline solution. The treatments were daily applied in the patellar region of the right knee after 48 h of induction. On the 7th, 14th, and 21st days of treatment, the animals were euthanized and their right knees were removed and processed for structural and biochemical analysis of the AC. The chondrocytes positively labeled for the TUNEL reaction were lower in C than in B on the 14th and 21st days. The content of glycosaminoglycans and hydroxyproline in A and C was higher than B on the 21st day. The amount of tibial TNF-α in B and C was lower than in A. The amount of tibial BMP-7 in B and C was higher than in A. The femoral MMP-13 was lower in B and C than for A. The tibial TGF-β for C was higher than the others. The femoral ADAMT-S4 content of A and C presented similar and inferior data to B on the 21st day. The AsGa-830 nm therapy preserved the content of glycosaminoglycans, reduced the cellular changes and the inflammatory process compared to the untreated group.

Quantifying birefringence in the bovine model of early osteoarthritis using polarisation-sensitive optical coherence tomography and mechanical indentation

Recent studies have shown potential for using polarisation sensitive optical coherence tomography (PS-OCT) to study cartilage morphology, and to be potentially used as an in vivo, non-invasive tool for detecting osteoarthritic changes. However, there has been relatively limited ability of this method to quantify the subtle changes that occur in the early stages of cartilage degeneration. An established mechanical indenting technique that has previously been used to examine the microstructural response of articular cartilage was employed to fix the bovine samples in an indented state. The samples were subject to creep loading with a constant compressive stress of 4.5 MPa and, when imaged using PS-OCT, enabled birefringent banding patterns to be observed. The magnitude of the birefringence was quantified using the birefringence coefficient (BRC) and statistical analysis revealed that PS-OCT is able to detect and quantify significant changes between healthy and early osteoarthritic cartilage (p < 0.001). This presents a novel utilization of PS-OCT for future development as an in vivo assessment tool.

Bioluminescence and second harmonic generation imaging reveal dynamic changes in the inflammatory and collagen landscape in early osteoarthritis

Osteoarthritis (OA) is a leading cause of chronic disability whose mechanism of pathogenesis is largely elusive. Local inflammation is thought to play a key role in OA progression, especially in injury-associated OA. While multiple inflammatory cytokines are detected, the timing and extent of overall inflammatory activities in early OA and the manner by which joint inflammation correlates with cartilage structural damage are still unclear. We induced OA via destabilization of the medial meniscus (DMM) in NFκB luciferase reporter mice, whose bioluminescent signal reflects the activity of NFκB, a central mediator of inflammation. Bioluminescence imaging data showed that DMM and sham control joints had a similar surge of inflammation at 1-week post-surgery, but the DMM joint exhibited a delay in resolution of inflammation in subsequent weeks. A similar trend was observed with synovitis, which we found to be mainly driven by synovial cell density and inflammatory infiltration rather than synovial lining thickness. Interestingly, an association between synovitis and collagen structural damage was observed in early OA. Using Second Harmonic Generation (SHG) imaging, we analyzed collagen fiber organization in articular cartilage. Zonal differences in collagen fiber thickness and organization were observed as soon as OA initiated after DMM surgery, and persisted over time. Even at 1-week post-surgery, the DMM joint showed a decrease in collagen fiber thickness in the deep zone and an increase in collagen fiber disorganization in the superficial zone. Since we were able detect and quantify collagen structural changes very early in OA development by SHG imaging, we concluded that SHG imaging is a highly sensitive tool to evaluate pathological changes in OA. In summary, this study uncovered a dynamic profile of inflammation and joint cartilage damage during OA initiation and development, providing novel insights into OA pathology.

Collagen Damage Location in Articular Cartilage Differs if Damage is Caused by Excessive Loading Magnitude or Rate

Collagen damage in articular cartilage is considered nearly irreversible and may be an early indication of cartilage degeneration. Surface fibrillation and internal collagen damage may both develop after overloading. This study hypothesizes that damage develops at these different locations, because the distribution of excessive strains varies with loading rate as a consequence of time-dependent cartilage properties. The objective is to explore whether collagen damage could preferentially occur superficially or internally, depending on the magnitude and rate of overloading. Bovine osteochondral plugs were compressed with a 2 mm diameter indenter to 15, 25, 35 and 45 N, and at 5, 60 and 120 mm/min. Surface fibrillation and internal collagen damage were graded by four observers, based on histology and staining of collagen damage. Results show that loading magnitude affects the degree of collagen damage, while loading rate dominates the location of network damage: low rates predominantly damage superficial collagen, while at high rates, internal collagen damage occurs. The proposed explanation for the rate-dependent location is that internal fluid flows govern the time-dependent internal tissue deformation and therewith the location of overstained and damaged areas. This supports the hypothesis that collagen damage development is influenced by the time-dependent material behaviour of cartilage.

How a decreased fibrillar interconnectivity influences stiffness and swelling properties during early cartilage degeneration

OBJECTIVE

The functional coupling between the fibrillar network and the high-swelling proteoglycans largely determines the mechanical properties of the articular cartilage matrix. The objective of this new study was to show specifically how changes in fibrillar interconnectivity arising from early cartilage degeneration influence transverse stiffness and swelling properties at the tissue level.

DESIGN

Radial zone transverse layers of cartilage matrix were obtained from intact and mildly degenerate bovine patellae. Each layer was then subdivided to assess tensile stiffness, free-swelling response, glycosaminoglycan (GAG) content, and micro- and ultra-structural features.

RESULTS

The tensile modulus was significantly lower and the degree of swelling significantly higher for the degenerate matrix compared to the intact. Scanning electron microscopy revealed a homogeneous response to transverse strain in the intact cartilage, whereas large non-fibrillar spaces between fibril aggregates were visible in the degenerate matrix. Although there were no significant differences in GAG content it did correlate significantly with stiffness and swelling in the intact samples but not in the degenerate.

CONCLUSIONS

The lower degree of fibril network interconnectivity in the degenerate matrix led to both a decreased transverse stiffness and reduced resistance to osmotic swelling. This network 'de-structuring' also resulted in a reduced functional interaction between the fibrillar network and the proteoglycans. The study provides new insights into the role of the fibrillar network and how changes in the network arising from the degenerative cascade will influence tissue level behaviour.

Experimental Study on the Mechanical Properties of Porcine Cartilage with Microdefect under Rolling Load

Objectives

To investigate the mechanical responses of microdefect articular cartilage under rolling load and find out the failure rule.

Methods

Rolling load was applied to the porcine articular cartilage samples with rectangular notches of different depths. The displacement and strain near the notches were obtained by the noncontact digital image correlation technique.

Results

The strain value and peak frequency around the notch increased; the maximum equivalent strain value could be observed at both bottom corners of the notch; the equivalent strain value first increased and then decreased at the points in the superficial and middle layers with the increase of rolling velocity; the points in the deep layer were less affected by rolling velocity; the equivalent strain value of the points in the superficial layer declined after rising with the increase of defect depth, while a decreased trend could be found for the points in the middle and deep layers.

Conclusions

The shear strain, which rose with the increase in defect depth, was the main factor in cartilage destruction. The cartilage tended to be destructed firstly at the bottom corner of the defect. Rolling velocity showed significant effects on superficial and middle layers. Cartilage had the ability to resist destruction.

The contribution of collagen fibers to the mechanical compressive properties of the temporomandibular joint disc

OBJECTIVE

The Temporomandibular Joint (TMJ) disc is a fibrocartilaginous structure located between the mandibular condyle and the temporal bone, facilitating smooth movements of the jaw. The load-bearing properties of its anisotropic collagenous network have been well characterized under tensile loading conditions. However, recently it has also been speculated that the collagen fibers may contribute dominantly in reinforcing the disc under compression. Therefore, in this study, the structural-functional role of collagen fibers in mechanical compressive properties of TMJ disc was investigated.

DESIGN

Intact porcine TMJ discs were enzymatically digested with collagenase to disrupt the collagenous network of the cartilage. The digested and non-digested articular discs were analyzed mechanically, biochemically and histologically in five various regions. These tests included: (1) cyclic compression tests, (2) biochemical quantification of collagen and glycosaminoglycan (GAG) content and (3) visualization of collagen fibers' alignment by polarized light microscopy (PLM).

RESULTS

The instantaneous compressive moduli of the articular discs were reduced by as much as 50-90% depending on the region after the collagenase treatment. The energy dissipation properties of the digested discs showed a similar tendency. Biochemical analysis of the digested samples demonstrated an average of 14% and 35% loss in collagen and GAG, respectively. Despite the low reduction of collagen content the PLM images showed considerable perturbation of the collagenous network of the TMJ disc.

CONCLUSIONS

The results indicated that even mild disruption of collagen fibers can lead to substantial mechanical softening of TMJ disc undermining its reinforcement and mechanical stability under compression.

Multi-scalar mechanical testing of the calcified cartilage and subchondral bone comparing healthy vs early degenerative states

OBJECTIVE

The calcified cartilage layer is thought to be integral to force transmission between the compliant articular cartilage (AC) above and underlying stiff bone. This study aims to determine how such a stiffness gradient across the calcified cartilage and bone changes with joint degeneration and how different scalar levels of testing are correlated.

METHOD

Using a bovine model of early osteoarthritis (OA), multiple samples of calcified cartilage on subchondral bone (SB) from sixteen bovine patellae, displaying a range of cartilage states from intact (healthy) to moderately degenerate, were tested using macroscopic three-point bending, microhardness indentation, and nanoindentation. Mechanical properties were correlated to cartilage health and microstructural morphometric measurements obtained from high resolution imaging using Differential Interference Contrast (DIC) Microscopy.

RESULTS

There was a significant decrease in the moduli obtained from tests done at increasing scalar levels. The macroscale average modulus obtained from three-point bending showed that the SB was 10 times stiffer than the calcified cartilage in healthy tissue, 5 times in tissue displaying mildly degenerate AC and 8 times with moderate degeneration. Microhardness testing of multiple points from the calcified cartilage to the SB revealed that there was a monotonic gradual increase in the mean modulus. The moduli obtained from nanoindentation testing indicated that the SB was about twice the stiffness of the calcified cartilage.

CONCLUSION

The mechanical transition from calcified cartilage to SB involves a graded continuum of increasing material stiffness. This stiffness gradient is altered in association with early degenerative change in the overlying AC, detectable only at the macro level.

How a radial focal incision influences the internal shear distribution in articular cartilage with respect to its zonally differentiated microanatomy

Articular surface fibrillation and the loss of both transverse interconnectivity and zonal differentiation are indicators of articular cartilage (AC) degeneration. However, exactly how these structural features affect the load-redistributing properties of cartilage is still poorly understood. This study investigated how a single radial incision made to varying depths with respect to the primary zones of AC influenced its deformation response to compression. Three depths of incision were applied to cartilage-on-bone tissue blocks: one not exceeding the transition zone; one into the mid-radial zone; and one down to the calcified cartilage. Also included were non-incised controls. All samples were compressed to a near-equilibrium strain using a flat-faced indenter that incorporated a central relief channel within which the incision could be positioned lengthwise along the channel axis. Employing fixation under load followed by decalcification, the structural responses of the cartilage-on-bone samples were investigated. The study provides an analysis of the micro-morphological response that is characteristic of a completely normal cartilage-on-bone system but which contains a defined degree of disruption induced by the focal radial incision. The resulting loss of transverse continuity of the cartilage with respect to its zonally differentiated structure is shown to lead to an altered pattern of internal matrix shear whose intensity varies with incision depth.

A numerical model to study mechanically induced initiation and progression of damage in articular cartilage

OBJECTIVE

Proteoglycan (PG) loss and surface roughening, early signs of osteoarthritis (OA), are likely preceded by softening of the ground substance and the collagen network. Insight in their relative importance to progression of OA may assist the development of treatment strategies for early OA. To support interpretation of experimental data, a numerical model is proposed that can predict damage progression in cartilage over time, as a consequence of excessive mechanical loading. The objective is to assess the interaction between ground substance softening and collagen fiber damage using this model.

DESIGN

An established cartilage mechanics model is extended with the assumption that excessive strains may damage the ground substance or the collagen network, resulting in softening of the overstrained constituent. During subsequent loading cycles the strain may or may not cross a threshold, resulting in damage to stabilize or to progress. To evaluate how softening of the ground substance and collagen may interact, damage progression is computed when either one of them, or both together are allowed to occur during stepwise increased loading.

RESULTS

Softening in the ground substance was predicted to localize in the superficial and transitional zone and resulted in cartilage softening. Collagen damage was most prominent in the superficial zone, with more diffuse damage penetrating deeper into the tissue, resulting in adverse strain gradients. Effects were more pronounced if both constituents developed damage in parallel.

CONCLUSION

Ground substance softening and collagen damage have distinct effects on cartilage mechanopathology, and damage in either one of them may promote each other.

The bovine patella as a model of early osteoarthritis

The bovine patella model has been used extensively for studying important structure-function aspects of articular cartilage, including its degeneration. However, the degeneration seen in this model has, to our knowledge, never been adequately compared with human osteoarthritis (OA). In this study, bovine patellae displaying normal to severely degenerate states were compared with human tissue displaying intact cartilage to severe OA. Comparisons of normal and OA features were made with histological scoring, morphometric measurements, and qualitative observations. Differential interference contrast microscopy was used to image early OA changes in the articular cartilage matrix and to investigate whether this method provided comparable quality of visualisation of key structural features with standard histology. The intact bovine cartilage was found to be similar to healthy human cartilage and the degenerate bovine cartilage resembled the human OA tissues with regard to structural disruption, cellularity changes, and staining loss. The extent of degeneration in the bovine tissues matched the mild to moderate range of human OA tissues; however, no bovine samples exhibited late-stage OA. Additionally, in both bovine and human tissues, cartilage degeneration was accompanied by calcified cartilage thickening, tidemark duplication, and the advancement of the cement line by protrusions of bony spicules into the calcified cartilage. This comparison of degeneration in the bovine and human tissues suggests a common pathway for the progression of OA and thus the bovine patella is proposed to be an appropriate model for investigating the structural changes associated with early OA.

Prevention of cartilage dehydration in imaging studies with a customized humidity chamber

Quantitative three-dimensional imaging methods such as micro-computed tomography (μCT) allow for the rapid and comprehensive evaluation of cartilage and bone in animal models, which can be used for drug development and related research in arthritis. However, when imaging fresh cartilage tissue in air, a common problem is tissue dehydration which causes movement artifact in the resulting images. These artifacts distort scans and can render them unusable, leading to a considerable loss of time and effort with sample preparation and measurement. The sample itself is also irretrievably damaged by the dehydration, often unable to return to its full tissue thickness upon rehydration. Additionally, imaging with ionic contrast agents such as Hexabrix(TM) must be performed in air, otherwise the agent will be washed out if immersed in a liquid. The first goal of this study was to design a customized humidity chamber to maintain cartilage hydration without the need for immersion. Following this, the use of the humidity chamber during a synchrotron radiation-μCT scan was validated and its performance evaluated. Results showed that the loss of fluid film volume is associated with scanning at low humidity (87%), and can be avoided using the humidity chamber. Coupling this technology with advances in synchrotron imaging (e.g., phase contrast imaging) or contrast agents is promising.

Further insight into the depth-dependent microstructural response of cartilage to compression using a channel indentation technique

Stress relaxation and structural analysis were used to investigate the zonally differentiated microstructural response to compression of the integrated cartilage-on-bone tissue system. Fifteen cartilage-on-bone samples were divided into three equal groups and their stress relaxation responses obtained at three different levels of axial compressive strain defined as low (~20%), medium (~40%) and high (~60%). All tests were performed using a channel indenter which included a central relief space designed to capture the response of the matrix adjacent to the directly loaded regions. On completion of each stress relaxation test and while maintaining the imposed axial strain, the samples were formalin fixed, decalcified, and then sectioned for microstructural analysis. Chondron aspect ratios were used to determine the extent of relative strain at different zonal depths. The stress relaxation response of cartilage to all three defined levels of axial strain displayed an initial highly viscous response followed by a significant elastic response. Chondron aspect ratio measurements showed that at the lowest level of compression, axial deformation was confined to the superficial cartilage layer, while in the medium and high axial strain samples the deformation extended into the midzone. The cells in the deep zone remained undeformed for all compression levels.

Biochemical changes caused by decellularization may compromise mechanical integrity of tracheal scaffolds

Tissue-engineered airways have achieved clinical success, but concerns remain about short-term loss of biomechanical properties, necessitating a stent. This study investigated the effect of chemical-enzymatic decellularization on biochemical properties of trachea important for cell attachment and vascularization (fibronectin and laminin) and cartilage matrix homeostasis (type II collagen and glycosaminoglycans (GAG)), as well as biomechanical status. Native trachea was used as a control, and NDC trachea stored in phosphate buffered saline (PBS) in parallel to decellularization was used as a time-matched control. Decellularization removed most cells, but chondrocytes and DNA remained after 25 cycles. Fibronectin was retained throughout the lamina propria and laminin at basement membranes. DNA accumulation along ECM fibres was seen. A decline in soluble collagen was observed in decellularized tissue. GAG content of cartilage rings was reduced, even in PBS control tissue from 20 cycles onwards (p<0.05), but decellularization caused the greatest loss (p<0.01). Tensile strength declined throughout the process, but was significant only at later time points. The data demonstrate that the substantial reduction in GAG might contribute to loss of mechanical integrity of biotracheas. Overcoming structural changes that cause an imbalance in cartilage matrix equilibrium will be necessary to optimize clinical benefit, enabling widespread use of biotracheas.