Machine learning augmented near-infrared spectroscopy: In vivo follow-up of cartilage defects.

OBJECTIVE: To assess the potential of near-infrared spectroscopy (NIRS) for in vivo arthroscopic monitoring of cartilage defects. METHOD: Sharp and blunt cartilage grooves were induced in the radiocarpal and intercarpal joints of Shetland ponies and monitored at baseline (0 weeks) and at three follow-up timepoints (11, 23, and 39 weeks) by measuring near-infrared spectra in vivo at and around the grooves. The animals were sacrificed after 39 weeks and the joints were harvested. Spectra were reacquired ex vivo to ensure reliability of in vivo measurements and for reference analyses. Additionally, cartilage thickness and instantaneous modulus were determined via computed tomography and mechanical testing, respectively. The relationship between the ex vivo spectra and cartilage reference properties was determined using convolutional neural network. RESULTS: In an independent test set, the trained networks yielded significant correlations for cartilage thickness (ρ = 0.473) and instantaneous modulus (ρ = 0.498). These networks were used to predict the reference properties at baseline and at follow-up time points. In the radiocarpal joint, cartilage thickness increased significantly with both groove types after baseline and remained swollen. Additionally, at 39 weeks, a significant difference was observed in cartilage thickness between controls and sharp grooves. For the instantaneous modulus, a significant decrease was observed with both groove types in the radiocarpal joint from baseline to 23 and 39 weeks. CONCLUSION: NIRS combined with machine learning enabled determination of cartilage properties in vivo, thereby providing longitudinal evaluation of post-intervention injury development. Additionally, radiocarpal joints were found more vulnerable to cartilage degeneration after damage than intercarpal joints.

Evaluation of equine articular cartilage degeneration after mechanical impact injury using cationic contrast-enhanced computed tomography.

OBJECTIVE: Cationic agent contrast-enhanced computed tomography (cationic CECT) characterizes articular cartilage ex vivo, however, its capacity to detect post-traumatic injury is unknown. The study objectives were to correlate cationic CECT attenuation with biochemical, mechanical and histological properties of cartilage and morphologic computed tomography (CT) measures of bone, and to determine the ability of cationic CECT to distinguish subtly damaged from normal cartilage in an in vivo equine model. DESIGN: Mechanical impact injury was initiated in equine femoropatellar joints in vivo to establish subtle cartilage degeneration with site-matched controls. Cationic CECT was performed in vivo (clinical) and postmortem (microCT). Articular cartilage was characterized by glycosaminoglycan (GAG) content, biochemical moduli and histological scores. Bone was characterized by volume density (BV/TV) and trabecular number (Tb.N.), thickness (Tb.Th.) and spacing (Tb.Sp.). RESULTS: Cationic CECT attenuation (microCT) of cartilage correlated with GAG (r = 0.74, P < 0.0001), compressive modulus (Eeq) (r = 0.79, P < 0.0001) and safranin-O histological score (r = -0.66, P < 0.0001) of cartilage, and correlated with BV/TV (r = 0.37, P = 0.0005), Tb.N. (r = 0.39, P = 0.0003), Tb.Th. (r = 0.28, P = 0.0095) and Tb.Sp. (r = -0.44, P < 0.0001) of bone. Mean [95% CI] cationic CECT attenuation at the impact site (2215 [1987, 2443] Hounsfield Units [HUs]) was lower than site-matched controls (2836 [2490, 3182] HUs, P = 0.036). Clinical cationic CECT attenuation correlated with GAG (r = 0.23, P = 0.049), Eeq (r = 0.26, P = 0.025) and safranin-O histology score (r = -0.32, P = 0.0046). CONCLUSIONS: Cationic CECT (microCT) reflects articular cartilage properties enabling segregation of subtly degenerated from healthy tissue and also reflects bone morphometric properties on CT. Cationic CECT is capable of characterizing articular cartilage in clinical scanners.

Functional effects of an interpenetrating polymer network on articular cartilage mechanical properties.

OBJECTIVE: Depletion of glycosaminoglycans (GAGs) and degradation of collagen network are early hallmarks of osteoarthritis (OA). Currently, there are no chondroprotective therapies that mitigate the loss of GAGs or effectively restore the collagen network. Recently, a novel polymeric cartilage supplement was described that forms a charged interpenetrating polymer network (IPN) reconstituting the hydrophilic properties of the extracellular matrix (ECM). To investigate the mechanism by which this hydrophilic IPN improves articular cartilage material properties, a finite element (FE) model is used to evaluate the IPN's effect on the fibrillar collagen network, nonfibrillar matrix, and interstitial fluid flow. METHODS: Bovine osteochondral plugs were degraded with chondroitinase ABC to selectively decrease GAG content. Samples were mechanically tested before and after IPN treatment using unconfined testing geometry and stress-relaxation protocol. Every measurement was modeled separately using a fibril-reinforced poroviscoelastic FE model. Measurement replication was achieved by optimizing the following model parameters: initial and strain-dependent fibril network modulus (Ef0, Efε, respectively), nonfibrillar matrix modulus (Enf), initial permeability (k0) and strain-dependent permeability factor (M). RESULTS: Based on the FE model results, treatment of native and GAG depleted cartilage with the hydrophilic IPN increases the ECM stiffness and impedes fluid flow. The IPN did not alter the stiffness of fibrillary network. Cartilage permeability and the strain-dependent permeability factor decreased with increasing IPN w/v%. CONCLUSIONS: The IPN reconstitutes cartilage material properties primarily by augmenting the hydrophilic ECM. This reinforcement of the solid phase also affects the fluid phase reestablishing low permeability.

Highly nonlinear stress-relaxation response of articular cartilage in indentation: Importance of collagen nonlinearity.

Modern fibril-reinforced computational models of articular cartilage can include inhomogeneous tissue composition and structure, and nonlinear mechanical behavior of collagen, proteoglycans and fluid. These models can capture well experimental single step creep and stress-relaxation tests or measurements under small strains in unconfined and confined compression. Yet, it is known that in indentation, especially at high strain velocities, cartilage can express highly nonlinear response. Different fibril reinforced poroelastic and poroviscoelastic models were used to assess measured highly nonlinear stress-relaxation response of rabbit articular cartilage in indentation. Experimentally measured depth-dependent volume fractions of different tissue constituents and their mechanical nonlinearities were taken into account in the models. In particular, the collagen fibril network was modeled using eight separate models that implemented five different constitutive equations to describe the nonlinearity. These consisted of linear elastic, nonlinear viscoelastic and multiple nonlinear elastic representations. The model incorporating the most nonlinearly increasing Young׳s modulus of collagen fibrils as a function of strain captured best the experimental data. Relative difference between the model and experiment was ~3%. Surprisingly, the difference in the peak forces between the experiment and the model with viscoelastic collagen fibrils was almost 20%. Implementation of the measured volume fractions did not improve the ability of the model to capture the measured mechanical data. These results suggest that a highly nonlinear formulation for collagen fibrils is needed to replicate multi-step stress-relaxation response of rabbit articular cartilage in indentation with high strain rates.

Very early osteoarthritis changes sensitively fluid flow properties of articular cartilage.

In this study, fibril-reinforced poroelastic (FRPE) modeling was used for rabbit knee after anterior cruciate ligament transection (ACLT) to assess how the mechanical properties of collagen, proteoglycans, and fluid in articular cartilage change in early osteoarthritis, and how site-specific these changes are. Unilateral ACLT was performed in eight skeletally mature, female New Zealand white rabbits. A separate control (CTRL) group consisted of knee joints of five non-operated rabbits. Animals were sacrificed at four weeks after ACLT and cartilage-on-bone samples from femoral groove, medial and lateral femoral condyles, and tibial plateaus were harvested. A stress-relaxation protocol in indentation geometry was applied and the FRPE model was fitted to the experimental force-time curve by minimizing the mean absolute error between experiment and simulation. The optimized parameters were: fibril network modulus (Ef), representing the collagen network; non-fibrillar matrix modulus (Enf), representing the PG matrix; and permeability (k), representing fluid flow. Permeability was increased significantly in the ACLT group compared to the CTRL group knees at all sites except for the medial tibial plateau. ACLT also caused a decrease in the Ef at all sites except for the medial and lateral tibial plateaus. The Enf of the ACLT group knees was altered only for the lateral femoral condyle. The results of this study suggest that early osteoarthritis primarily affects cartilage permeability and impairs the collagen network stiffness in a site-specific manner. These findings from early osteoarthritis indicate that fluid flow velocity in articular cartilage may change prior to quantifiable structural alterations in the tissue.

Estimation of articular cartilage properties using multivariate analysis of optical coherence tomography signal.

OBJECTIVE: The aim was to investigate the applicability of multivariate analysis of optical coherence tomography (OCT) information for determining structural integrity, composition and mechanical properties of articular cartilage. DESIGN: Equine osteochondral samples (N = 65) were imaged with OCT, and their total attenuation and backscattering coefficients (µt and µb) were measured. Subsequently, the Mankin score, optical density (OD) describing the fixed charge density, light absorbance in amide I region (Aamide), collagen orientation, permeability, fibril network modulus (Ef) and non-fibrillar matrix modulus (Em) of the samples were determined. Partial least squares (PLS) regression model was calculated to predict tissue properties from the OCT signals of the samples. RESULTS: Significant correlations between the measured and predicted mean collagen orientation (R(2) = 0.75, P < 0.0001), permeability (R(2) = 0.74, P < 0.0001), mean OD (R(2) = 0.73, P < 0.0001), Mankin scores (R(2) = 0.70, P < 0.0001), Em (R(2) = 0.50, P < 0.0001), Ef (R(2) = 0.42, P < 0.0001), and Aamide (R(2) = 0.43, P < 0.0001) were obtained. Significant correlation was also found between µb and Ef (ρ = 0.280, P = 0.03), but not between µt and any of the determined properties of articular cartilage (P > 0.05). CONCLUSION: Multivariate analysis of OCT signal provided good estimates for tissue structure, composition and mechanical properties. This technique may significantly enhance OCT evaluation of articular cartilage integrity, and could be applied, for example, in delineation of degenerated areas around cartilage injuries during arthroscopic repair surgery.

Characterization of site-specific biomechanical properties of human meniscus-Importance of collagen and fluid on mechanical nonlinearities.

Meniscus adapts to joint loads by depth- and site-specific variations in its composition and structure. However, site-specific mechanical characteristics of intact meniscus under compression are poorly known. In particular, mechanical nonlinearities caused by different meniscal constituents (collagen and fluid) are not known. In the current study, in situ indentation testing was conducted to determine site-specific elastic, viscoelastic and poroelastic properties of intact human menisci. Lateral and medial menisci (n=26) were harvested from the left knee joint of 13 human cadavers. Indentation tests, using stress-relaxation and dynamic (sinusoidal) loading protocols, were conducted for menisci at different sites (anterior, middle, posterior, n=78). Sample- and site-specific axisymmetric finite element models with fibril-reinforced poroelastic properties were fitted to the corresponding stress-relaxation curves to determine the mechanical parameters. Elastic moduli, especially the instantaneous and dynamic moduli, showed site-specific variation only in the medial meniscus (p<0.05 between the sites). The instantaneous and dynamic elastic moduli of the anterior horn were significantly (p<0.05) greater in the medial than lateral meniscus. The phase angle showed no statistically significant variation between the sites (p>0.05). The values for the strain-dependent fibril network modulus (nonlinear behaviour of collagen) were significantly different (p<0.05) between all sites in the medial menisci. Additionally, there was a significant difference (p<0.01) in the strain-dependent fibril network modulus between the lateral and medial anterior horns. The initial permeability was significantly different (p<0.05) in the medial meniscus only between the middle and posterior sites. For the strain-dependent permeability coefficient, only anterior and middle sites showed a significant difference (p<0.05) in the medial meniscus. This parameter demonstrated a significant difference (p<0.05) between lateral and medial menisci at the anterior horns. Our results reveal that under in situ indentation loading, medial meniscus shows more site-dependent variation in the mechanical properties as compared to lateral meniscus. In particular, anterior horn of medial meniscus was the stiffest and showed the most nonlinear mechanical behaviour. The nonlinearity was related to both collagen fibrils and fluid.

Alterations in subchondral bone plate, trabecular bone and articular cartilage properties of rabbit femoral condyles at 4 weeks after anterior cruciate ligament transection.

OBJECTIVE: To quantify early osteoarthritic-like changes in the structure and volume of subchondral bone plate and trabecular bone and properties of articular cartilage in a rabbit model of osteoarthritis (OA) induced by anterior cruciate ligament transection (ACLT). METHODS: Left knee joints from eight skeletally mature New Zealand white rabbits underwent ACLT surgery, while the contralateral (CTRL) right knee joints were left unoperated. Femoral condyles were harvested 4 weeks after ACLT. Micro-computed tomography imaging was applied to evaluate the structural properties of subchondral bone plate and trabecular bone. Additionally, biomechanical properties, structure and composition of articular cartilage were assessed. RESULTS: As a result of ACLT, significant thinning of the subchondral bone plate (P < 0.05) was accompanied by significantly reduced trabecular bone volume fraction and trabecular thickness in the medial femoral condyle compartment (P < 0.05), while no changes were observed in the lateral compartment. In both lateral and medial femoral condyles, the equilibrium modulus and superficial zone proteoglycan (PG) content were significantly lower in ACLT than CTRL joint cartilage (P < 0.05). Significant alterations in the collagen orientation angle extended substantially deeper into cartilage from the ACLT joints in the lateral femoral condyle relative to the medial condyle compartment (P < 0.05). CONCLUSIONS: In this model of early OA, significant changes in volume and microstructure of subchondral bone plate and trabecular bone were detected only in the femoral medial condyle, while alterations in articular cartilage properties were more severe in the lateral compartment. The former finding may be associated with reduced joint loading in the medial compartment due to ACLT, while the latter finding reflects early osteoarthritic changes in the lateral compartment.

Site-dependent changes in structure and function of lapine articular cartilage 4 weeks after anterior cruciate ligament transection.

OBJECTIVE: The aim of this study was to investigate the site-dependent changes in the structure and function of articular cartilage in the lapine knee joint at a very early stage of osteoarthritis (OA), created experimentally by anterior cruciate ligament transection (ACLT). METHODS: Unilateral ACLT was performed in eight mature New Zealand white rabbits. ACL transected and contralateral (C-L) joints were prepared for analysis at 4 weeks after ACLT. Three rabbits with intact joints were used as a control group (CNTRL). Femoral groove, medial and lateral femoral condyles, and tibial plateaus were harvested and used in the analysis. Biomechanical tests, microscopy and spectroscopy were used to determine the biomechanical properties, composition and structure of the samples. A linear mixed model was chosen for statistical comparisons between the groups. RESULTS: As a result of ACLT, the equilibrium and dynamic moduli were decreased primarily in the femoral condyle cartilage. Up to three times lower moduli (P < 0.05) were observed in the ACLT group compared to the control group. Significant (P < 0.05) proteoglycan (PG) loss in the ACLT joint cartilage was observed up to a depth of 20-30% from the cartilage surface in femoral condyles, while significant PG loss was confined to more superficial regions in tibial plateaus and femoral groove. The collagen orientation angle was increased (P < 0.05) up to a cartilage depth of 60% by ACLT in the lateral femoral condyle, while smaller effects, but still significant, were observed at other locations. The collagen content was increased (P < 0.05) in the middle and deep zones of the ACLT group compared to the control group samples, especially in the lateral femoral condyle. CONCLUSION: Femoral condyle cartilage experienced the greatest structural and mechanical alterations in very early OA, as produced by ACLT. Degenerative alterations were observed especially in the superficial collagen fiber organization and PG content, while the collagen content was increased in the deep tissue of femoral condyle cartilage. The current findings provide novel information of the early stages of OA in different locations of the knee joint.

Structure-function relationships in osteoarthritic human hip joint articular cartilage.

OBJECTIVES: It is currently poorly known how different structural and compositional components in human articular cartilage are related to their specific functional properties at different stages of osteoarthritis (OA). The objective of this study was to characterize the structure-function relationships of articular cartilage obtained from osteoarthritic human hip joints. METHODS: Articular cartilage samples with their subchondral bone (n = 15) were harvested during hip replacement surgeries from human femoral necks. Stress-relaxation tests, Mankin scoring, spectroscopic and microscopic methods were used to determine the biomechanical properties, OA grade, and the composition and structure of the samples. In order to obtain the mechanical material parameters for the samples, a fibril-reinforced poroviscoelastic model was fitted to the experimental data obtained from the stress-relaxation experiments. RESULTS: The strain-dependent collagen network modulus (E(f)(ε)) and the collagen orientation angle exhibited a negative linear correlation (r = -0.65, P < 0.01), while the permeability strain-dependency factor (M) and the collagen content exhibited a positive linear correlation (r = 0.56, P < 0.05). The nonfibrillar matrix modulus (E(nf)) also exhibited a positive linear correlation with the proteoglycan content (r = 0.54, P < 0.05). CONCLUSION: The study suggests that increased collagen orientation angle during OA primarily impairs the collagen network and the tensile stiffness of cartilage in a strain-dependent manner, while the decreased collagen content in OA facilitates fluid flow out of the tissue especially at high compressive strains. Thus, the results provide interesting and important information of the structure-function relationships of human hip joint cartilage and mechanisms during the progression of OA.