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The purpose of this communication is to present an idea, and its technical implementation, on how to estimate experimentally in vivo joint contact pressure distributions. The idea is illustrated for the cat patellofemoral joint. For this particular joint, the approach requires muscle force and hindlimb movement measurements during unrestrained locomotion, and the quantification of the joint contact pressures in situ for conditions approximating the in vivo conditions as closely as possible. Although the approach is time-consuming and has its limitations, it is, as far as we know, the first purely experimental approach to determine the in vivo joint contact pressures during normal movement. "Purely experimental" refers to the idea that the required movements, muscle forces and contact pressures are all measured rather than estimated theoretically.

Mechanical loading of articular cartilage affects the synthesis and degradation of matrix macromolecules. Much of the work in this area has involved mechanical loading of articular cartilage explants or cells in vitro and assessing biological responses at the mRNA and protein levels. In this study, we developed a new experimental technique to load an intact patellofemoral joint in vivo using muscle stimulation. The articular cartilages were cyclically loaded for one hour in a repeatable and measurable manner. Cartilage was harvested from central and peripheral regions of the femoral groove and patella, either immediately after loading or after a three hour recovery period. Total RNA was isolated from the articular cartilage and biological responses were assessed on the mRNA level using the reverse transcriptase-polymerase chain reaction. Articular cartilage from intact patellofemoral joints demonstrated heterogeneity at the mRNA level for six of the genes assessed independent of the loading protocol. Cyclical loading of cartilage in its native environment led to alterations in mRNA levels for a subset of molecules when assessed immediately after the loading period. However, the increases in TIMP-1 and decreases in bFGF mRNA levels were transient; being present immediately after load application but not after a three hour recovery period.

The purpose of this study was to determine the effectiveness of a novel Laser Scanning Confocal Arthroscope (LSCA) for the morphological quantification of articular cartilage chondrocytes.

Healthy and debrided regions of the knee articular cartilage of six (6) New Zealand White rabbits were imaged during open follow-up surgery. Quantitative morphological analysis of chondrocyte cell populations was performed and compared to known parameters. Optical histology images were compared to conventional histology of similar sites.

Optical histology revealed viable cells in normal hyaline cartilage tissue and enabled the visualization of fibro-cartilage in defect tissue. Morphological analysis was able to characterize the in vivo two-dimensional equivalent-area-diameter of chondrocytes. Significant differences (P<0.05) were seen between the morphology of chondrocytes observed in optical and conventional histology.

This study concludes that the LSCA is capable of illustrating the surface and sub-surface appearance of healthy and defect articular cartilage, thereby providing a non-destructive method for assessing cartilage condition in vivo. In this role the LSCA may find application in the investigation of cartilage pathologies or repair techniques.

Objective: The goal of this study was to demonstrate a methodology to observe the relationship between joint contact pressure and cartilage T2 relaxation times in three-dimensional space. Methods: One subject diagnosed with unilateral scapholunate dissociation had both injured and uninjured wrists scanned using a Siemens 3T Skyra magnetic resonance imaging (MRI) scanner. Four time echo scans were performed with TE ranging 15–61ms with the hand relaxed. T2 maps were constructed using a custom Matlab code, and these maps were registered to anatomical images for the same subject. The anatomical images were used to construct surface contact models and calculate contact pressures for a simple grasp activity in a prior study. Contact pressures and T2 relaxation times were analyzed using regression analysis. Results and Conclusion: This study demonstrates the feasibility of comparing T2 relaxation times and contact pressure data. For this single demonstration subject, it is not surprising that no relationship was found between T2 relaxation times for the articular cartilage and contact pressures in the normal wrist, contact pressures in the wrist with injury, nor contact pressure changes due to injury. However, the method has been demonstrated and may be useful to evaluate the influence of joint injuries or other pathologies on T2 relaxation times in the context of changes in joint contact pressures with larger cohorts of subjects.

Purpose: Our objective was to investigate the effects of injury and surgical repair on T2 relaxation time, as a non-invasive biomarker of changes in the biophysical and biochemical status of the cartilage in the wrist. Methods: Magnetic resonance imaging (MRI) was performed using 3T scanners. Nine subjects attended scan sessions for both their injured wrist and contralateral (normal) wrist pre-operation and post-operation. T2 relaxation times were individually calculated by each cartilage surface of the radioscaphoid, radiolunate, capitoscaphoid and capitolunate articulations. Results: T2 relaxation times were not found to vary significantly according to injury state. Overall, findings were not dependent on which cartilage surface was analyzed. Pre-operative and post-operative normal wrist T2 values were not significantly different. Conclusions: Articular cartilage changes due to scapholunate dissociation do not appear to result in changes of T2 relaxation times measured by MRI. The lack of significant findings was possibly a result of thin wrist cartilage and limited image resolution. Further development of MRI capabilities may allow for more accurate determination of the time progression of T2 changes with injury (if any) and/or establish the change in T2 values after surgery.

Fresh patellar allograft without violating the continuum of the articular cartilage was evaluated in rabbits. Twenty-four skeletally immature New Zealand White rabbits underwent resurfacing of the patella with fresh allografts and 92% (22/24) of the allografts survived. These specimens were analyzed to assess the geometric parameters of the patellofemoral joint anatomy as well as the biomechanical and histological properties of the patellar articular cartilage at 12 (n=8), 26 (n=7) and 52 weeks (n=7) postoperatively. Despite incomplete restoration of the patellofemoral joint geometry, both the biomechanical and histologic results showed excellent preservation of the articular cartilage at 26 and 52 weeks. From the biomechanical testing, the aggregate modulus (Ha) and the permeability (k) of the transplanted cartilage for the 26- and 52-week groups showed no difference between the experimentals and the controls. For the 26-week group, the aggregate modulus was 0.70±0.07 MPa and 0.72±0.19 MPa for the experimental and control, respectively (p>0.5) and the permeability was (0.97±0.13)×10^-15m⁴/N-s and (1.17±0.33)×10^-15m⁴/N-s for the experimental and control, respectively (p>0.5). For the 52-week group, the aggregate modulus was 0.93±0.14 MPa and 1.03±0.06 MPa for the experimental and control, respectively (p>0.5) and the permeability was 2.32±0.57)×10^-15m⁴/N-s and 2.12±0.85)×10^-15m⁴/N-s for the experimental and control, respectively (p>0.5). This study clearly demonstrates the long-term viability of articular cartilage in entire osteochondral patellar allograft in rabbits.

Articular cartilage dissipates contact loads according to three dissipative mechanisms: frictional drag, intrinsic viscoelasticity, and surface friction. Estimation of dissipation due to these three mechanisms during gait is required to understand the dissipative properties of articular cartilage. Fourteen healthy subjects performed a gait analysis on treadmill. Tibiofemoral contact forces were estimated from inverse dynamic analysis and from a reductionist knee contact model. These contact forces and the results obtained from a preloading creep simulation were introduced into a biphasic poroviscoelastic articular cartilage model, and a one-dimensional confined compression was performed. Articular dissipation from each dissipative mechanism was estimated. Sensitivity analysis was performed to determine the effects of material parameters and length of the preloading simulation on the patterns of the dissipative mechanisms. Dissipative force patterns for all dissipative mechanisms were found to be similar to those of tibiofemoral contact forces. Frictional drag was found to be the dominant dissipative mechanism. The initial permeability and the viscoelastic spectrum parameters were found to have an important impact on the magnitude of the peaks of dissipative patterns. If appropriate material parameters are introduced, this model could be used to compare the difference between healthy and osteoarthritic human articular cartilage.

This study's purpose was to investigate the biomechanical effects of varus and valgus knee deformities in different degrees on the acetabulum's weight-bearing dome. We collected six lower extremity specimens from three fresh adult male cadavers, including the L5 vertebral body, pelvis, bilateral hips, bilateral knees and lower leg sections. We visually examined all specimens and then X-rayed them to exclude the presence of hip and knee pathology. We adjusted the tibiofemoral angle to 180° (knee varus at 10°, Group A), 190° (knee varus at 20°, Group B), 160° (knee valgus at 10°, Group C) and 150° (knee valgus at 20°, Group D) using the external fixator, respectively, fixed them, respectively, and then monitored them through the above 3D laser scanning imager. We found the acetabulum's weight-bearing dome area, stress and stress distribution by loading and coloring on the pressure-sensitive film. We measured them using a FPD-305 density meter and FPD-306 pressure converter. We compared the areas, average stresses and peak stresses of the acetabulum's weight-bearing dome between groups B and D and group A, respectively. The differences between groups B and D had no significance. Compared to group A's area, average stress and peak stress, the areas of groups C and E decreased, and their average stresses and peak stresses increased significantly. The differences between groups B and C and groups D and E were also significant. Due to different degrees of knee varus and valgus angles, the areas of the weight-bearing acetabulum dome, average stress and peak stress are different. The knee varus and valgus at 20° were statistically significant, which may be the key pathogenesis of hip arthritis.

Articular cartilage plays an important role in organism due to its excellent shock absorbing and buffering functions. Increasing problems about damages of articular cartilage are making a great deal of trouble to human beings. The damage mechanism of articular cartilage is very complicated and keeps unclear. In this research, the damage mechanism was investigated from the perspective of micro-particle attrition by nanoindentation experiments. The micro-particle was simulated by the indenter in experiments. The experimental results demonstrated that the load from micro-particle could not maintain when water content was adequate. However, the load could maintain and increase after dehydration. It was found that the partial surface of articular cartilage was crushed and adhered to the indenter. The plastic energy was bigger than elastic energy in the nanoindentation process. Therefore, water content was the crucial factor to protect the articular cartilage from damage. And the recurring partial dehydration owing to ongoing compression enhanced the damage of micro-particle to articular cartilage. This research may provide a new understanding to the damage mechanism of articular cartilage.

It has great guiding significance for the prevention of osteoarthritis and the mechanical state of cartilage after tissue engineering repair to study the relationship between the mechanical properties of cartilage and its structure. This paper considered both the consideration of the solid phase, liquid phase, fiber-reinforced phase in the cartilage and the influence of the contents of major fibers and minor fibers near the cartilage surface. Based on these, a tangential zone of cartilage was established, and a certain improvement and optimization of the fiber-reinforced porous elastic model was performed. The Abaqus software and the Fortran language were used to complete simulation. Simulation results were compared with experiment’s results to verify the validity of the model. Finally, the model was used to perform finite element analysis of different degrees of repairable depth under sliding conditions. Several results were obtained. When the indenter is farther from the interface at the repair site, the mechanical changes in the cartilage are relatively stable. The contact stress of the tangential layer repair and the full-layer repair is small. The volume fraction of the liquid phase in the tangential layer and the full layer repair is lower than that in the other layer regions. The liquid flow rate and the Von Mises stress at the junction of the tangential layer repair are very high. Simulation results were used to explore differences in cartilage mechanical properties of different repairable depths, so as to select the best repairable depth for cartilage.

In this paper, the tangential zone of cartilage is introduced into the fiber-reinforced model of articular cartilage. Considering the distribution content of the main fiber and the secondary fiber in the tangential layer of cartilage, the permeability and fiber stiffness of the layer are set in parallel and perpendicular directions, respectively, to more accurately reflect the mechanical behavior of cartilage. The parameters are set to reflect the mechanical behavior of the cartilage more realistically. We use a modified articular cartilage model to simulate the mechanical properties of implanted cartilage with different elastic modulus. The simulation results show that the selection of implants with different elastic modulus will affect the repair of cartilage. Appropriately increasing the elastic modulus of implanted cartilage, can increase the bearing capacity of the repaired area and reduce the stress concentration at the junction. The elastic modulus of the implant should be moderate, not too large or too small, and the damage of stress concentration on the repair surface should be considered. Through simulation, the mechanical state of the repaired cartilage under pressure can be obtained comprehensively, which provides a theoretical basis for clinical pathology.

Solute transport is one of the important aspects involved in maintaining the physiological activity of tissues. The mechanical environment drives nutrition in and waste out in articular cartilage due to its avascularity, which plays a key role in the biological activity of articular cartilage. The human knee joint motion is a complex interaction between different bones including relative rolling and/or sliding movements. Rolling-compression process is a typical physiological load in knee joint motion. To investigate solute transport behavior in articular cartilage under rolling-compression load, fluorescence tracers with molecular weights of 40kDa and 0.43kDa were used respectively to mark the transport in fresh articular cartilage of mature pigs. Solute fluorescence intensity changing with time and depth of cartilage layer was measured under rolling-compression load and static state, respectively, and the distribution of corresponding relative concentration was calculated by the fluorescence microscope imaging method. The experiment results show that the solute relative concentration in articular cartilage under rolling-compression load increases significantly, even up to 62.4%, comparing with that under static state, and the changes of concentration vary in different layers and that small molecular weight solute is easier to transport than relatively large molecular weight solute in articular cartilage. Therefore, rolling-compression load can promote the solute transport in cartilage, and the mechanical loading may have application in functional cartilage tissue engineering.

Fourier transform infrared imaging (FTIRI) was used to examine the depth-dependent content variations of macromolecular components, collagen and proteoglycan (PG), in osteoarthritic and healthy cartilages. Dried 6 μm thick sections of canine knee cartilages were imaged at 6.25 μm pixel-size in FTIRI. By analyzing the infrared (IR) images and spectra, the depth dependence of characteristic band (sugar) intensity of PG show obvious difference between the cartilage sections of (OA) and health. The result confirms that PG content decreases in the osteoarthritic cartilage. However, no clear change occurs to collagen, suggesting that the OA influences little on the collagen content at early stage of OA. This observation will be helpful to further understand PG loss associated with pathological conditions in OA, and demonstrates that FTIRI has the potential to become an important analytical tool to identify early clinical signs of tissue degradation, such as PG loss even collagen disruption.

Two discriminant methods, partial least squares-discriminant analysis (PLS-DA) and Fisher’s discriminant analysis (FDA), were combined with Fourier transform infrared imaging (FTIRI) to differentiate healthy and osteoarthritic articular cartilage in a canine model. Osteoarthritic cartilage had been developed for up to two years after the anterior cruciate ligament (ACL) transection in one knee. Cartilage specimens were sectioned into 10 $\mu$m thickness for FTIRI. A PLS-DA model was developed after spectral pre-processing. All IR spectra extracted from FTIR images were calculated by PLS-DA with the discriminant accuracy of 90%. Prior to FDA, principal component analysis (PCA) was performed to decompose the IR spectral matrix into informative principal component matrices. Based on the different discriminant mechanism, the discriminant accuracy (96%) of PCA-FDA with high convenience was higher than that of PLS-DA. No healthy cartilage sample was mis-assigned by these two methods. The above mentioned suggested that both integrated technologies of FTIRI-PLS-DA and, especially, FTIRI-PCA-FDA could become a promising tool for the discrimination of healthy and osteoarthritic cartilage specimen as well as the diagnosis of cartilage lesion at microscopic level. The results of the study would be helpful for better understanding the pathology of osteoarthritics.

We report two patients with reconstruction of osteochondral defects of the proximal interphalangeal joint (PIPJ) using a costal osteochondral graft (COG). A box-cut osteotomy was done at the end of the phalanx preserving the lateral cortices and the insertion of the collateral ligaments. A COG was harvested from the rib, moulded and press fit into the groove formed by the box-cut osteotomy. The COG was fixed with mini screws in the coronal plane (dorsal to palmar) and the fixation off-loaded with an external fixator. This technique maintained the collateral ligament in-situ and is useful in reconstruction of chondral defects of the PIPJ.

Level of Evidence: Level V (Therapeutic)

Articular cartilage provides functions of lubrication to shear stress and protection from compressive force, but it has poor ability to repair itself after suffering damage. The advanced method of tissue engineering is developed and used to maintain cell functions for tissue regeneration. In order to improve the ECM synthesis for the regeneration, many materials have been examined on chondrocytes or other cell sources. In this study, fibrinogen was concentrated from plasma cryoprecipitation and then polymerized by thrombin into fibrin. Gelatin/hyaluronic acid/chondroitin-6-sulfate (GHC6S) was prepared by the cross-linking reaction with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) and ground in liquid nitrogen to particles. The GHC6S particles were mixed with fibrin glue as the tissue engineering scaffold. Porcine articular cartilage chondrocytes were expanded and seeded into the scaffolds. The engineered constructs were cultured and harvested after cultured for 1 and 2 weeks. Morphology of GHC6S particle was examined by scanning electron microscopy (SEM). Total glycosaminoglycans (GAGs) and sulfated GAGs were quantified by p-dimethylaminobenzaldehyde reaction and 1,9-dimethymethylene blue (DMMB) assay, respectively. The results demonstrated that the total GAGs, especially the content of nonsulfated GAGs, hyaluronic acid, were increased with time in chondrocytes growing in fibrin glue with GHC6S particles. It suggested that the GHC6S in fibrin glue chondrocyte kept the GAGs synthesis, which could help resist the compressive force. Therefore, the GHC6S particles mixed within fibrin glue can be used as a promising scaffold for articular tissue engineering.

Tissue engineering is a promising solution to address articular cartilage pathology. The creation of strategies for functional replacement of diseased cartilage relies heavily on the knowledge of the physiology and development of articular cartilage, especially in terms of the influence of biomechanical forces on the tissue. This review will present the current knowledge of biomechanical structure–function relationships of native articular cartilage, and synthesize this knowledge with strategies to engineer the tissue in vitro.