Cartilage-on-cartilage contact: effect of compressive loading on tissue deformations and structural integrity of bovine articular cartilage

A human meniscus explant model for studying early events in osteoarthritis development by proteomics

Degenerative meniscus lesions have been associated with both osteoarthritis etiology and its progression. We, therefore, sought to establish a human meniscus ex vivo model to study the meniscal response to cytokine treatment using a proteomics approach. Lateral menisci were obtained from five knee-healthy donors. The meniscal body was cut into vertical slices and further divided into an inner (avascular) and outer region. Explants were either left untreated (controls) or stimulated with cytokines. Medium changes were conducted every 3 days up to Day 21 and liquid chromatography-mass spectrometry was performed at all the time points for the identification and quantification of proteins. Mixed-effect linear regression models were used for statistical analysis to estimate the effect of treatments versus control on protein abundance. Treatment by IL1ß increased release of cytokines such as interleukins, chemokines, and matrix metalloproteinases but a limited catabolic effect in healthy human menisci explants. Further, we observed an increased release of matrix proteins (collagens, integrins, prolargin, tenascin) in response to oncostatin M (OSM) + tumor necrosis factor (TNF) and TNF+interleukin-6 (IL6) + sIL6R treatments, and analysis of semitryptic peptides provided additional evidence of increased catabolic effects in response to these treatments. The induced activation of catabolic processes may play a role in osteoarthritis development.

Second Harmonic Imaging of Nasal, Auricular, and Costal Cartilage

OBJECTIVE

There is little knowledge about the histological organization of facial and costal cartilages in terms of matrix structure and cell morphology. Second harmonic generation (SHG) imaging is a nonlinear imaging technique that capitalizes on signal generation from highly ordered macromolecules such as collagen fibers. The purpose of this study was to use SHG microscopy to image collagen extracellular matrix (ECM) structure, chondrocyte size, and density of these cartilages.

STUDY DESIGN

Experimental.

METHODS

Surgical remnants of septal, lower lateral, rib, and auricular cartilages were collected following surgery, sectioned into 0.5-1 mm thick samples and fixed to facilitate batch process imaging. A Leica TCS SP8 MP Microscope and multiphoton laser were used to image the specimens. Images were analyzed for cell size, cell density, and collagen fiber directionality patterns using ImageJ.

RESULTS

SHG images of septal specimens show mesh-like structure of the ECM. There appears to be a superficial layer, characterized by flattened lacunae and middle zone, marked by circular lacunae clusters, similar to what is observed in articular cartilage. The structure of the ECM depicts a visible orientation perpendicular to the surface of the perichondrium. Cell size and density analysis through ImageJ suggests variety across cartilage types. Directionality analysis indicates that the collagen in the ECM displays preferred direction.

CONCLUSION

This study establishes clear extracellular models of facial and costal cartilages. Limitations include heterogeneous cartilage thickness due to processing difficulties. Further studies include automating the cutting process to increase uniformity of tissue thickness and increasing sample size to further validate results.

LEVEL OF EVIDENCE

2 Laryngoscope, 133:3370-3377, 2023.

Imaging of joint response to exercise with MRI and PET

Imaging of the joint in response to loading stress may provide additional measures of joint structure and function beyond conventional, static imaging studies. Exercise such as running, stair climbing, and squatting allows evaluation of the joint response to larger loading forces than during weight bearing. Quantitative MRI (qMRI) may assess properties of cartilage and meniscus hydration and organization in vivo that have been investigated to assess the functional response of these tissues to physiological stress. [18F]sodium fluoride ([18F]NaF) interrogates areas of newly mineralizing bone and provides an opportunity to study bone physiology, including perfusion and mineralization rate, as a measure of joint loading stress. In this review article, methods utilizing quantitative MRI, PET, and hybrid PET-MRI systems for assessment of the joint response to loading from exercise in vivo are examined. Both methodology and results of various studies performed are outlined and discussed. Lastly, the technical considerations, challenges, and future opportunities for these approaches are addressed.

Multi-frame biomechanical and relaxometry analysis during in vivo loading of the human knee by spiral dualMRI and compressed sensing

PURPOSE

Knee cartilage experiences repetitive loading during physical activities, which is altered during the pathogenesis of diseases like osteoarthritis. Analyzing the biomechanics during motion provides a clear understanding of the dynamics of cartilage deformation and may establish essential imaging biomarkers of early-stage disease. However, in vivo biomechanical analysis of cartilage during rapid motion is not well established.

METHODS

We used spiral displacement encoding with stimulated echoes (DENSE) MRI on in vivo human tibiofemoral cartilage during cyclic varus loading (0.5 Hz) and used compressed sensing on the k-space data. The applied compressive load was set for each participant at 0.5 times body weight on the medial condyle. Relaxometry methods were measured on the cartilage before (T1ρ , T2 ) and after (T1ρ ) varus load.

RESULTS

Displacement and strain maps showed a gradual shift of displacement and strain in time. Compressive strain was observed in the medial condyle cartilage and shear strain was roughly half of the compressive strain. Male participants had more displacement in the loading direction compared to females, and T1ρ values did not change after cyclic varus load. Compressed sensing reduced the scanning time up to 25% to 40% when comparing the displacement maps and substantially lowered the noise levels.

CONCLUSION

These results demonstrated the ease of which spiral DENSE MRI could be applied to clinical studies because of the shortened imaging time, while quantifying realistic cartilage deformations that occur through daily activities and that could serve as biomarkers of early osteoarthritis.

Inflammatory Process on Knee Osteoarthritis in Cyclists

Osteoarthritis is a disorder affecting the joints and is characterized by cellular stress and degradation of the extracellular matrix cartilage. It begins with the presence of micro- and macro-lesions that fail to repair properly, which can be initiated by multiple factors: genetic, developmental, metabolic, and traumatic. In the case of the knee, osteoarthritis affects the tissues of the diarthrodial joint, manifested by morphological, biochemical, and biomechanical modifications of the cells and the extracellular matrix. All this leads to remodeling, fissuring, ulceration, and loss of articular cartilage, as well as sclerosis of the subchondral bone with the production of osteophytes and subchondral cysts. The symptomatology appears at different time points and is accompanied by pain, deformation, disability, and varying degrees of local inflammation. Repetitive concentric movements, such as while cycling, can produce the microtrauma that leads to osteoarthritis. Aggravation of the gradual lesion in the cartilage matrix can evolve to an irreversible injury. The objective of the present review is to explain the evolution of knee osteoarthritis in cyclists, to show the scarce research performed in this particular field and extract recommendations to propose future therapeutic strategies.

High frame rate deformation analysis of knee cartilage by spiral dualMRI and relaxation mapping

PURPOSE

Daily activities including walking impose high-frequency cyclic forces on cartilage and repetitive compressive deformation. Analyzing cartilage deformation during walking would provide spatial maps of displacement and strain and enable viscoelastic characterization, which may serve as imaging biomarkers for early cartilage degeneration when the damage is still reversible. However, the time-dependent biomechanics of cartilage is not well described, and how defects in the joint impact the viscoelastic response is unclear.

METHODS

We used spiral acquisition with displacement-encoding MRI to quantify displacement and strain maps at a high frame rate (25 frames/s) in tibiofemoral joints. We also employed relaxometry methods (T1 , T1ρ , T2 , T2 *) on the cartilage.

RESULTS

Normal and shear strains were concentrated on the bovine tibiofemoral contact area during loading, and the defected joint exhibited larger compressive strains. We also determined a positive correlation between the change of T1ρ in cartilage after cyclic loading and increased compressive strain on the defected joint. Viscoelastic behavior was quantified by the time-dependent displacement, where the damaged joint showed increased creep behavior compared to the intact joint. This technique was also successfully demonstrated on an in vivo human knee showing the gradual change of displacement during varus load.

CONCLUSION

Our results indicate that spiral scanning with displacement encoding can quantitatively differentiate the damaged from intact joint using the strain and creep response. The viscoelastic response identified with this methodology could serve as biomarkers to detect defects in joints in vivo and facilitate the early diagnosis of joint diseases such as osteoarthritis.

Compressive Mechanical Behavior of Partially Oxidized Polyvinyl Alcohol Hydrogels for Cartilage Tissue Repair

Polyvinyl alcohol (PVA) hydrogels are extensively used as scaffolds for tissue engineering, although their biodegradation properties have not been optimized yet. To overcome this limitation, partially oxidized PVA has been developed by means of different oxidizing agents, obtaining scaffolds with improved biodegradability. The oxidation reaction also allows tuning the mechanical properties, which are essential for effective use in vivo. In this work, the compressive mechanical behavior of native and partially oxidized PVA hydrogels is investigated, to evaluate the effect of different oxidizing agents, i.e., potassium permanganate, bromine, and iodine. For this purpose, PVA hydrogels are tested by means of indentation tests, also considering the time-dependent mechanical response. Indentation results show that the oxidation reduces the compressive stiffness from about 2.3 N/mm for native PVA to 1.1 ÷ 1.4 N/mm for oxidized PVA. During the consolidation, PVA hydrogels exhibit a force reduction of about 40% and this behavior is unaffected by the oxidizing treatment. A poroviscoelastic constitutive model is developed to describe the time-dependent mechanical response, accounting for the viscoelastic polymer matrix properties and the flow of water molecules within the matrix during long-term compression. This model allows to estimate the long-term Young's modulus of PVA hydrogels in drained conditions (66 kPa for native PVA and 34-42 kPa for oxidized PVA) and can be exploited to evaluate their performances under compressive stress in vivo, as in the case of cartilage tissue engineering.

Characterization of intra-tissue strain fields in articular cartilage explants during post-loading recovery using high frequency ultrasound

This study aims to demonstrate the potential of ultrasound elastography as a research tool for non-destructive imaging of intra-tissue strain fields and tissue quality assessment in cartilage explants. Osteochondral plugs from bovine patellae were loaded up to 10, 40, or 70 N using a hemi-spherical indenter. The load was kept constant for 15 min, after which samples were unloaded and ultrasound imaging of strain recovery over time was performed in the indented area for 1 h. Tissue strains were determined using speckle tracking and accumulated to LaGrangian strains in the indentation direction. For all samples, strain maps showed a heterogeneous strain field, with the highest values in the superficial cartilage under the indenter tip at the bottom of the indent and decreasing values in the deeper cartilage. Strains were higher at higher load levels and tissue recovery over time was faster after indentation at 10 N than at 40 N and 70 N. At lower compression levels most displacement occurred near the surface with little deformation in the deep layers, while at higher levels strains increased more evenly in all cartilage zones. Ultrasound elastography is a promising method for high resolution imaging of intra-tissue strain fields and evaluation of cartilage quality in tissue explants in a laboratory setting. In the future, it may become a clinical diagnostic tool used to identify the extent of cartilage damage around visible defects.

Emerging polymeric material strategies for cartilage repair

Cartilage is found throughout the body, serving an array of essential functions. Owing to the limited healing capacity of cartilage, damage or degeneration is often permanent and so requires clinical intervention. Established surgical techniques generally rely on biological grafting. However, recent advances in polymeric materials provide an encouraging alternative to overcome limits of auto- and allografts. For regenerative engineering of cartilage, a polymeric scaffold ideally supports and instructs tissue regeneration while also providing mechanical integrity. Scaffolds direct regeneration via chemical and mechanical cues, as well as delivery and support of exogenous cells and bioactive factors. Advanced polymeric scaffolds aim to direct regeneration locally, replicating the heterogeneities of native tissues. Alternatively, new cartilage-mimetic hydrogels have potential to serve as synthetic cartilage replacements. Prepared as multi-network or composite hydrogels, the most promising candidates have simultaneously realized the hydration, mechanical, and tribological properties of native cartilage. Collectively, the recent rise in polymers for cartilage regeneration and replacement proposes a changing paradigm, with a new generation of materials paving the way for improved clinical outcomes.

Ultra-High Modulus Hydrogels Mimicking Cartilage of the Human Body

The human body is comprised of numerous types of cartilage with a range of high moduli, despite their high hydration. Owing to the limitations of cartilage tissue healing and biological grafting procedures, synthetic replacements have emerged but are limited by poorly matched moduli. While conventional hydrogels can achieve similar hydration to cartilage tissues, their moduli are substantially inferior. Herein, triple network (TN) hydrogels are prepared to synergistically leverage intra-network electrostatic repulsive and hydrophobic interactions, as well as inter-network electrostatic attractive interactions. They are comprised of an anionic 1^st network, a neutral 2^nd network (capable of hydrophobic associations), and a cationic 3^rd network. Collectively, these interactions act synergistically as effective, yet dynamic crosslinks. By tuning the concentration of the cationic 3^rd network, these TN hydrogels achieve high moduli of ≈1.5 to ≈3.5 MPa without diminishing cartilage-like water contents (≈80%), strengths, or toughness values. This unprecedented combination of properties poises these TN hydrogels as cartilage substitutes in applications spanning articulating joints, intervertebral discs (IVDs), trachea, and temporomandibular joint disc (TMJ).

A Comprehensive Coronal and Axial Bone Dimension and Cartilage Thickness Evaluation of the Distal Humerus: Age and Sex Differences

OBJECTIVE

There are limited data on bone dimension and cartilage thickness of the distal humeral articular surface. This study aimed to evaluate sex- and age-related bone dimension and cartilage thickness differences and assess the effect of cartilage thickness on distal humeral shape.

DESIGN

Elbow magnetic resonance images of 180 healthy participants were evaluated. Cartilage thicknesses of the trochlea and capitellum were measured at 19 points using coronal and axial images. In addition, bone diameters were measured from the flexion-extension axis to the 19 points on the coronal and axial magnetic resonance images. Sex differences were evaluated, and the correlation between age and measurement parameters was assessed.

RESULTS

Significant sex differences regarding the diameters of the axial trochlear bone, coronal lateral trochlear bone, and medial capitellar bone, cartilage thickness at the apex of the lateral trochlear ridge in the axial and coronal plane and at the most lateral point of the capitellar articular surface in the axial plane were observed. A negative correlation was observed between age and axial plane trochlear bone dimensions and between age and coronal plane lateral trochlear and medial capitellar bone dimensions. No significant correlation was found between cartilage thickness and bone dimensions.

CONCLUSIONS

Bone dimension and cartilage thickness at the distal humerus vary according to sex and age. The data could be used in the donor site selection and graft preparation while osteochondral autograft transfer and allograft transplantation, and in the development of gender-compatible hemiarthroplasty implants.

Articular and Artificial Cartilage, Characteristics, Properties and Testing Approaches-A Review

The design and manufacture of artificial tissue for knee joints have been highlighted recently among researchers which necessitates an apt approach for its assessment. Even though most re-searches have focused on specific mechanical or tribological tests, other aspects have remained underexplored. In this review, elemental keys for design and testing artificial cartilage are dis-cussed and advanced methods addressed. Articular cartilage structure, its compositions in load-bearing and tribological properties of hydrogels, mechanical properties, test approaches and wear mechanisms are discussed. Bilayer hydrogels as a niche in tissue artificialization are presented, and recent gaps are assessed.

Age-related alterations of articular cartilage in pituitary adenylate cyclase-activating polypeptide (PACAP) gene-deficient mice

Pituitary adenylate cyclase activating polypeptide (PACAP) is an evolutionarly conserved neuropeptide which is produced by various neuronal and non-neuronal cells, including cartilage and bone cells. PACAP has trophic functions in tissue development, and it also plays a role in cellular and tissue aging. PACAP takes part in the regulation of chondrogenesis, which prevents insufficient cartilage formation caused by oxidative and mechanical stress. PACAP knockout (KO) mice have been shown to display early aging signs affecting several organs. In the present work, we investigated articular cartilage of knee joints in young and aged wild-type (WT) and PACAP KO mice. A significant increase in the thickness of articular cartilage was detected in aged PACAP gene-deficient mice. Amongst PACAP receptors, dominantly PAC1 receptor was expressed in WT knee joints and a remarkable decrease was found in aged PACAP KO mice. Expression of PKA-regulated transcription factors, Sox5, Sox9 and CREB, decreased both in young and aged gene deficient mice, while Sox6, collagen type II and aggrecan expressions were elevated in young but were reduced in aged PACAP KO animals. Increased expression of hyaluronan (HA) synthases and HA-binding proteins was detected parallel with an elevated presence of HA in aged PACAP KO mice. Expression of bone related collagens (I and X) was augmented in young and aged animals. These results suggest that loss of PACAP signaling results in dysregulation of cartilage matrix composition and may transform articular cartilage in a way that it becomes more prone to degenerate.

Simultaneous tractography and elastography imaging of the zone-specific structural and mechanical responses in articular cartilage under compressive loading

We quantified the precise zonal cartilage structural and mechanical responses to unconfined compressive loading by using simultaneous PSOCT based optical tractography and elastography imaging. Twelve bovine knee articular cartilage samples from six animals were imaged under bulk compression from 4% to 20%. The results revealed strong evidence that the conventional radial zone could be divided into two sub-zones with distinct mechanical properties. The "upper" part of the radial zone played a critical role in "absorbing" the mechanical compression. The study also showed that the zonal fiber organization greatly affected the cartilage structural and mechanical responses. A strong correlation was observed between the optical birefringence and logarithm of the Young's modulus. These new results provide useful information for improving mechanical modeling of articular cartilage and developing better cartilage-mimetic biomaterials.

Cartilage-on-cartilage cyclic loading induces mechanical and structural damage

Cartilage breaks down during mechanically-mediated osteoarthritis (OA). While previous research has begun to elucidate mechanical, structural and cellular damage in response to cyclic loading, gaps remain in our understanding of the link between cyclic cartilage loading and OA-like mechanical damage. Thus, the aim of this study was to quantify irreversible cartilage damage in response to cyclic loading. A novel in vitro model of damage through cartilage-on-cartilage cyclic loading was established. Cartilage was loaded at 1 Hz to two different doses (10,000 or 50,000 cycles) between -6.0 ± 0.2 MPa and -10.3 ± 0.2 MPa 1st Piola-Kirchhoff stress. After loading, mechanical damage (altered mechanical properties: elastic moduli and dissipated energy) and structural damage (surface damage and specimen thickness) were quantified. Linear and tangential moduli were determined by fitting the loading portion of the stress-strain curves. Dissipated energy was calculated from the area between loading and unloading stress-strain curves. Specimen thickness was measured both before and after loading. Surface damage was assessed by staining samples with India ink, then imaging the articular surface. Cyclic loading resulted in dose-dependent decreases in linear and tangential moduli, energy dissipation, thickness, and intact area. Collectively, these results show that cartilage damage can be initiated by mechanical loading alone in vitro, suggesting that cyclic loading can cause in vivo damage. This study demonstrated that with increased number of cycles, cartilage undergoes both tissue softening and structural damage. These findings are a first step towards characterizing the cartilage response to cyclic loading, which can ultimately provide important insight for delaying the initiation and slowing the progression of OA.

Combined enzymatic degradation of proteoglycans and collagen significantly alters intratissue strains in articular cartilage during cyclic compression

As degenerative joint diseases such as osteoarthritis (OA) progress, the matrix constituents, particularly collagen fibrils and proteoglycans, become damaged, therefore deteriorating the tissue's mechanical properties. This study aims to further the understanding of the effect of degradation of the different cartilage constituents on the mechanical loading environment in early stage OA. To this end, intact, collagen- and proteoglycan-depleted cartilage plugs were cyclically loaded in axial compression using an experimental model simulating in vivo cartilage-on-cartilage contact conditions in a micro-MRI scanner. Depletion of collagen and proteoglycans was achieved through enzymatic degradation with collagenase and chondroitinase ABC, respectively. Using a displacement-encoded imaging sequence (DENSE), strains were computed and compared in intact and degraded samples. The results revealed that, while degradation with one or the other enzyme had little effect on the contact strains, degradation with a combination of both enzymes caused an increase in the means and variance of the transverse, axial and shear strains, particularly in the superficial zone of the cartilage. This effect indicates that the balance between cartilage matrix constituents plays an essential role in maintaining the mechanical properties of the tissue, and a disturbance in this balance leads to a decrease of the load bearing capacity associated with degenerative joint diseases such as OA.

Functional assessment of strains around a full-thickness and critical sized articular cartilage defect under compressive loading using MRI

OBJECTIVE

The objective of this study was to evaluate the effect of full-thickness chondral defects on intratissue deformation patterns and matrix constituents in an experimental model mimicking in vivo cartilage-on-cartilage contact conditions.

DESIGN

Pairs of bovine osteochondral explants, in a unique cartilage-on-cartilage model system, were compressed uniaxially by 350 N during 2 s loading and 1.4 s unloading cycles (≈1700 repetitions). Tissue deformations under quasi-steady state load deformation response were measured with displacement encoded imaging with stimulated echoes (DENSE) in a 9.4 T magnetic resonance imaging (MRI) scanner. Pre- and post-loading, T₁, T₂ and T_1ρ relaxation time maps were measured. We analyzed differences in strain patterns and relaxation times between intact cartilage (n = 8) and cartilage in which a full-thickness and critical sized defect was created (n = 8).

RESULTS

Under compressive loading, strain magnitudes were elevated at the defect rim, with elevated tensile and compressive principal strains (Δϵ_max = 4.2%, P = 0.02; Δϵ_min = -4.3%, P = 0.02) and maximum shear strain at the defect rim (Δγ_max = 4.4%, P = 0.007). The opposing cartilage showed minimal increase in strain patterns at contact with the defect rim but decreased strains opposing the defect. After defect creation, T₁, T₂ and T_1ρ relaxation times were elevated at the defect rim only. Following loading, the overall relaxations times of the defect tissue and especially at the rim, increased compared to intact cartilage.

CONCLUSIONS

This study demonstrates that the local biomechanical changes occurring after defect creation may induce tissue damage by increasing shear strains and depletion of cartilage constituents at the defect rim under compressive loading.