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.

In vitro glycation of articular cartilage alters the biomechanical response of chondrocytes in a depth-dependent manner.

OBJECTIVE: To determine if increasing cartilage cross-links through in vitro glycation of cartilage explants can alter the biomechanical response of chondrocytes to compressive deformation. METHOD: Bovine osteochondral explants were either incubated with cell culture solution supplemented with (n = 7) or without (n = 7) ribose for 42 h in order to induce glycation. Deformation-induced changes in cell volume, dimensions and local tissue strains were determined through confocal laser scanning microscopy (CLSM) and the use of a custom built micro-compression device. Osteochondral explants were also utilized to demonstrate changes in depth-wise tissue properties, biomechanical tissue properties and cross-links such as pentosidine (Pent), hydroxylysyl pyridinoline (HP) and lysyl pyridinoline (LP). RESULTS: The ribose treated osteochondral samples experienced reduced cell volume deformation in the upper tissue zone by ∼ 8% (P = 0.005), as compared the control samples, through restricting cell expansion. In the deeper tissue zone, cell volume deformation was increased by ∼ 12% (P < 0.001) via the transmission of mechanical signals further into the tissue depth. Biomechanical testing of the ribose treated osteochondral samples demonstrated an increase in the equilibrium and dynamic strain dependent moduli (P < 0.001 and P = 0.008, respectively). The biochemical analysis revealed an increase in Pent cross-links (P < 0.001). Depth-wise tissue property analyses revealed increased levels of carbohydrate content, greater levels of fixed charge density and an increased carbohydrate to protein ratio from 6 to 16%, 55-100% and 72-79% of the normalized tissue thickness (from the surface), respectively, in the ribose-treated group (P < 0.05). CONCLUSION: In vitro glycation alters the biomechanical response of chondrocytes in cartilage differently in upper and deeper zones, offering possible insights into how aging could alter cell deformation behavior in cartilage.

Interrelationship of cartilage composition and chondrocyte mechanics after a partial meniscectomy in the rabbit knee joint - Experimental and numerical analysis.

Site-specific and depth-dependent properties of cartilage were implemented within a finite element (FE) model to determine if compositional or structural changes in the tissue could explain site-specific alterations of chondrocyte deformations due to cartilage loading in rabbit knee joints 3 days after a partial meniscectomy (PM). Depth-dependent proteoglycan (PG) content, collagen content and collagen orientation in the cartilage extracellular matrix (ECM), and PG content in the pericellular matrix (PCM) were assessed with microscopic and spectroscopic methods. Patellar, femoral groove and samples from both the lateral and medial compartments of the femoral condyle and tibial plateau were extracted from healthy controls and from the partial meniscectomy group. For both groups and each knee joint site, axisymmetric FE models with measured properties were generated. Experimental cartilage loading was applied in the simulations and chondrocyte volumes were compared to the experimental values. ECM and PCM PG loss occurred within the superficial cartilage layer in the PM group at all locations, except in the lateral tibial plateau. Collagen content and orientation were not significantly altered due to the PM. The FE simulations predicted similar chondrocyte volume changes and group differences as obtained experimentally. Loss of PCM fixed charge density (FCD) decreased cell volume loss, as observed in the medial femur and medial tibia, whereas loss of ECM FCD increased cell volume loss, as seen in the patella, femoral groove and lateral femur. The model outcome, cell volume change, was also sensitive to applied tissue geometry, collagen fibril orientation and loading conditions.

Site-specific cell-tissue interactions in rabbit knee joint articular cartilage.

Relationships between cartilage structure and superficial in situ chondrocyte deformations were investigated from 6 different knee joint locations (n=10 knees). Depth dependent cartilage structure and composition were quantified with microscopic/microspectroscopic methods. Medial tibial cartilages had the lowest superficial collagen content, highest collagen orientation angle, and highest proteoglycan content in the pericellular matrix relative to that in the extracellular matrix, coupled with the largest chondrocyte deformations. In contrast, femoral groove and lateral tibial cartilages had the highest superficial collagen contents, lowest collagen orientation angles, and low normalized proteoglycan contents in the pericellular matrix, coupled with the smallest chondrocyte deformations. To study cell-tissue interactions further, observations (n=57) from all locations were pooled and a multivariable linear regression was performed. Cell width deformations (R2=0.57) correlated with collagen orientation angle (standardized regression coefficient ß=0.398) and collagen content (ß=-0.402). Cell height deformations (R2=0.52) also correlated with collagen orientation (ß=-0.248) and collagen content (ß=0.455). Cell volume change upon cartilage compression (R2=0.41) correlated with collagen content (ß=0.435) and proteoglycan content (ß=0.279). In conclusion, higher collagen and proteoglycan contents combined with lower collagen orientation angle in the extracellular matrix were related to reductions in superficial chondrocyte deformations. Also, a steep gradient of proteoglycan content from the extracellular to the pericellular matrix was associated with increased cell deformation, particularly in the medial tibial plateau cartilage.

Early in situ changes in chondrocyte biomechanical responses due to a partial meniscectomy in the lateral compartment of the mature rabbit knee joint.

We determined the biomechanical responses of chondrocytes to indentation at specific locations within the superficial zone of cartilage (i.e. patellar, femoral groove, femoral condylar and tibial plateau sites) taken from female New Zealand white rabbits three days after a partial meniscectomy in the lateral compartment of a knee joint. Confocal laser scanning microscopy combined with a custom indentation system was utilized to image chondrocyte responses at sites taken from ten contralateral and experimental knee joints. Cell volume, height, width and depth changes, global, local axial and transverse strains and Young׳s moduli were determined. Histological assessment was performed and proteoglycan content from the superficial zone of each site was determined. Relative to contralateral group cells, patellar, femoral groove and lateral femoral condyle cells in the experimental group underwent greater volume decreases (p < 0.05), due to smaller lateral expansions (with greater decreases in cell height only for the lateral femoral condyle cells; p < 0.05) whereas medial femoral and medial tibial plateau cells underwent smaller volume decreases (p < 0.05), due to less deformation in cell height (p < 0.05). Proteoglycan content was reduced in the patellar (p > 0.05), femoral groove, medial femoral condyle and medial tibial plateau experimental sites (p < 0.05). The findings suggest: (i) cell biomechanical responses to cartilage loading in the rabbit knee joint can become altered as early as 3 days after a partial meniscectomy, (ii) are site-specific, and (iii) occur before alterations in tissue mechanics or changes detectable with histology.

Site-dependent biomechanical responses of chondrocytes in the rabbit knee joint.

Biomechanical responses of chondrocytes were determined in specific locations within the superficial zone of patellar, femoral groove, femoral condyle and tibial plateau cartilages obtained from female New Zealand White rabbits. A confocal laser scanning microscope combined with a custom indentation system was utilized for experimentation. Changes in cell volumes and dimensions (i.e. cell height, width and depth) due to loading, global, local axial and transverse strains were determined for each site. Tissue composition and structure was analysed at each indentation site with digital densitometry, polarized light microscopy and Fourier transform infrared imaging spectroscopy. Patellar cells underwent greater volume decreases (compared to femoral groove cells; p<0.05) primarily due to greater decreases in cell height (p<0.05), consistent with greater levels of both global and local axial strains (p<0.05). Lateral condyle cells underwent greater volume decreases (compared to lateral plateau cells; p<0.05) primarily due to greater decreases in cell height, consistent with greater levels of tissue strains (p<0.05). Medial condyle cells underwent smaller volume decreases (compared to medial plateau cells; p<0.05) primarily due to elevated cell expansions in the depth direction, which was consistent with greater levels of minor transverse strains (p<0.05). Site-dependent differences in collagen orientation angles agreed conceptually with the observed cell dimensional changes. Chondrocyte biomechanical responses were highly site-dependent and corresponded primarily with the orientation of the collagen fibrils. The observed differences were thought to be due to the different biomechanical loading conditions at each site.