Mechanical Behavior Of Articular Cartilage Research Articles

Optimization of In Situ Indentation Protocol to Map the Mechanical Properties of Articular Cartilage.

Tissue engineering aims at developing complex composite scaffolds for articular cartilage repair. These scaffolds must exhibit a mechanical behavior similar to the whole osteochondral unit. In situ spherical indentation allows us to map the mechanical behavior of articular cartilage, avoiding removal of the underlying bone tissue. Little is known about the impact of grid spacing, indenter diameter, and induced deformation on the cartilage response to indentation. We investigated the impact of grid spacing (range: a to 3a, where a is the radius of the contact area between cartilage and indenter), indenter diameter (range: 1 to 8 mm), and deformation induced by indentation (constant indentation depth versus constant nominal deformation) on cartilage response. The bias induced by indentations performed in adjacent grid points was minimized with a 3a grid spacing. The cartilage response was indenter-dependent for diameters ranging between 1 and 6 mm with a nominal deformation of 15%. No significant differences were found using 6 mm and 8 mm indenters. Six mm and 8 mm indenters were used to map human articular cartilage with a grid spacing equal to 3a. Instantaneous elastic modulus E0 was calculated for constant indentation depth and constant nominal deformation. E0 value distribution did not change significantly by switching the two indenters, while dispersion decreased by 5–6% when a constant nominal deformation was applied. Such an approach was able to discriminate changes in tissue response due to doubling the indentation rate. The proposed procedure seems to reduce data dispersion and properly determine cartilage mechanical properties to be compared with those of complex composite scaffolds.

The effect of highly inhomogeneous biphasic properties on mechanical behaviour of articular cartilage

Background and objectiveInvestigating the biomechanics of cartilage could help to understand the unique load-bearing property of the cartilage and optimize the scaffold design in tissue-engineering. It is important to model the cartilage as a highly inhomogeneous fibril-reinforced biphasic material to represent its complex composition and structure. The depth-dependent and strain-dependent properties of the cartilage would also play an important role in its mechanical behaviour. However, the differences in representing the cartilage as a highly inhomogeneous model or as simplified models still remain unclear. Hence, in this study, a highly inhomogeneous fibril-reinforced biphasic cartilage model considering both the depth-dependent and strain-dependent properties was constructed; the effect of highly inhomogeneous properties on the mechanical behaviour of articular cartilage was investigated. MethodsA finite element model of the cartilage was developed based on a flat-ended indentation test. Compressive forces were applied to four various inhomogeneous layered models through a porous indenter (Model 1: nine layers with strain-dependent permeability; Model 2: three layers with strain-dependent permeability; Model 3: single layer with strain-dependent permeability; Model 4: nine layers with constant permeability). ResultsModels 1 and 2 provided similar results with less than 3% difference in the peak effective stress, contact pressure, fluid pressure as well as fluid support ratio. However, Model 1 to Model 3 differed in stress and strain distribution patterns along depth over prolonged loads, which may provide an important insight into the highly inhomogeneous depth-dependent properties of cartilage. In addition, Model 1 with strain-dependent permeability demonstrated an enhanced capability on fluid pressurisation as compared with Model 4 which had constant permeability. ConclusionsA highly inhomogeneous fibril-reinforced biphasic model considering both depth-dependent and strain-dependent properties was developed in this study, in order to illustrate the effect of highly inhomogeneous properties on the mechanical behaviour of the articular cartilage. The number of layers in the models with depth-dependent properties should be selected according to the research questions and clinical demands. The model with strain-dependent permeability offers an enhanced capability on fluid pressurisation. In future studies, the proposed model could be adopted in cell-models to provide more in-depth information or in tissue-engineering to optimize the depth-dependent scaffold structure.

On Osmotic Pressure in Hyperelastic Biphasic Fiber‐Reinforced Articular Cartilage

AbstractArticular cartilage belongs to the group of connective tissues and ensures a low‐friction performance of the human musculoskeletal system. It is a complex multiphase material with two main components: a collagen fiber‐reinforced porous solid and a liquid phase. The mechanical behavior of articular cartilage is decisively determined by the interaction between the highly anisotropic, inhomogeneously distributed collagen fiber network, the isotropically distributed proteoglycans and the interstitial fluid. The electrically negatively charged proteoglycans generate an electrical gradient that creates an osmotic negative pressure on the fluid which causes swelling of the cartilage. This leads to mechanically prestressed collagen fibers resulting in a decrease of total pressure occurring inside the cartilage. An increasingly common joint disease is osteoarthritis (OA), which severely impairs everyday life, especially in the elderly population. It is characterized by a loss of matrix stiffness as well as by a reduced tension stiffness and viscoelasticity of the collagen fibers. The aim of the developed model is to describe the previously mentioned material properties of cartilage as precisely as possible. The biphasic model uses the Theory of Porous Media for homogenization and contains an incompressible poroelastic solid matrix reinforced with collagen fibers and an incompressible pore fluid. By including the occurring osmotic pressure, the correct initial stress state of the imaged configuration is considered.

A depth dependent transversely isotropic micromechanic model of articular cartilage.

Articular cartilage owing to the variation of collagen fibers orientation through its zones has been indicated to have depth dependent mechanical properties. The aim of this study was to present an innovative micromechanics model to predict the depth dependent mechanical properties of articular cartilage as a function of collagen fibers and proteoglycan matrix mechanical properties, collagen fibers volume fraction as well as angle toward cartilage surface. The variation of collagen fibers angle toward the cartilage surface as a function of cartilage depth was computed using the micromechanics model. This function showed that the collagen fibers parallel to the cartilage surface in the superficial zone have a nonlinear angle variation in the transition zone and become perpendicular to cartilage surface in the deep zone. Depth dependent elastic modulus in perpendicular to cartilage surface plane direction was calculated using presented micromechanics model and variation function of the collagen fibers' angle. The results revealed a suitable agreement with that of the experimental measurements in different samples at different ages and races (R2=0.944). The results also showed that the elastic and aggregate modules perpendicular to the cartilage surface plane in the deep zone were 25.8 and 26.3 times higher than that of the superficial zone, respectively. These findings have implications not only for computing the depth dependent mechanical properties of any type of articular cartilage at different ages and races, but also of potential ability for developing a depth dependent transversely isotropic biphasic model to predict the accurate mechanical behavior of articular cartilage.

A biphasic finite element study on the role of the articular cartilage superficial zone in confined compression

The aim of this study was to investigate the role of the superficial zone on the mechanical behavior of articular cartilage. Confined compression of articular cartilage was modeled using a biphasic finite element analysis to calculate the one-dimensional deformation of the extracellular matrix (ECM) and movement of the interstitial fluid through the ECM and articular surface. The articular cartilage was modeled as an inhomogeneous, nonlinear hyperelastic biphasic material with depth and strain-dependent material properties. Two loading conditions were simulated, one where the superficial zone was loaded with a porous platen (normal test) and the other where the deep zone was loaded with the porous platen (upside down test). Compressing the intact articular cartilage with 0.2MPa stress reduced the surface permeability by 88%. Removing the superficial zone increased the rate of change for all mechanical parameters and decreased the fluid support ratio of the tissue, resulting in increased tissue deformation. Apparent permeability linearly increased after superficial removal in the normal test, yet it did not change in the upside down test. Orientation of the specimen affected the time-dependent biomechanical behavior of the articular cartilage, but not equilibrium behavior. The two tests with different specimen orientations resulted in very different apparent permeabilities, suggesting that in an experimental study which quantifies material properties of an inhomogeneous material, the specimen orientation should be stated along with the permeability result. The current study provides new insights into the role of the superficial zone on mechanical behavior of the articular cartilage.

Model for analyzing the mechanical behavior of articular cartilage under creep indentation test

In this study, an innovative depth dependent biphasic transversely isotropic model (DBT) was proposed to study the mechanical behavior of Articular Cartilage (AC). To find a more precise model to address the mechanical behavior of AC, the vital role of collagen fibers in all zones of the AC has been taken into account and depth dependent elasticity mechanical properties of cartilage are calculated as a function of collagen fibers orientation and volume fraction. Material parameters of permeability function were calculated in such a way that the variations of indenter displacement with time predicted by Finite Element Method (FEM) simulation for creep indentation test of the AC sample based on DBT model. In addition, the test was simulated by an isotropic-biphasic model to compare the capabilities of these two models and difference in mechanical behaviors of biphasic-isotropic and depth dependent transversely isotropic materials. According to the calculations, the presence of collagen fibers triggers increasing of stresses in fibers direction and decreasing of stresses perpendicular to fiber direction in the superficial and deep zones of AC. The findings of this study may have implications not only for calculating stress distributions in AC components but also for developing progressive damage model of AC for predicting osteoarthritic cartilage behavior in different cartilage-related diseases.

Retraction notice

Background: Collagen fiber volume fraction of articular cartilage alters by race and age of samples and has a key role in the mechanical behavior of articular cartilage. Methods: This study presented innovative micromechanics models to predict the mechanical properties of superficial and deep zones of any kind of articular cartilage for different races and ages as a function of cartilage component mechanical properties and collagen fiber volume fraction. Results: Collagen fiber volume fraction as a function of normalized depth of an articular cartilage sample was calculated in this study by micromechanics models from variation measurements of its axial elastic modulus. Poisson’s ratio and aggregate modulus was also computed from this function and compared to the experimental data for verification. Since collagen fiber volume fraction was reported in a huge range for different articular cartilages with different races and ages, the effect of this variation is discussed in detail in this paper. The results showed that, as the collagen volume fraction increases, the mechanical properties of articular cartilage in all planes and directions increase as well. Conclusion: Aging reduces all the elasticity mechanical properties of articular cartilage in all directions and planes.

Depth-dependent strain fields of articular cartilage under rolling load by the optimized digital image correlation technique

It is significant to investigate the depth-dependent mechanical behaviors of articular cartilage under rolling load since considerable rolling occurs for cartilage joint in activities of daily living. In this study, the rolling experiments of articular cartilage were conducted by applying an optimized digital image correlation (DIC) technique for the first time and the depth-dependent normal strain and shear strain of cartilage were analyzed. It is found that the normal strain and shear strain values of different layers increase firstly and then decrease with rolling time, and they increase with increasing compressive strains. The normal strain and shear strain values decrease along cartilage depth with constant compressive strain. The normal strain values of different normalized depth decrease with increasing rolling rates. The shear strain values of superficial layer and middle layer decrease; however there are no major changes for the shear strain values of deep layer with increasing rolling rates. The normal strain values with different rolling time increase with increasing rolling numbers and the 30.6% increase in initial normal strain is observed from 1st to 99th cycle. The fitting relationship of the normal strain and normalized depth was obtained considering the effects of compressive strain and rolling rate and the fitting curves agree with the experimental results for cartilage very well.

An Augmented Lagrangian Method for Sliding Contact of Soft Tissue

Despite the importance of sliding contact in diarthrodial joints, only a limited number of studies have addressed this type of problem, with the result that the mechanical behavior of articular cartilage in daily life remains poorly understood. In this paper, a finite element formulation is developed for the sliding contact of biphasic soft tissues. The augmented Lagrangian method is used to enforce the continuity of contact traction and fluid pressure across the contact interface. The resulting method is implemented in the commercial software COMSOL Multiphysics. The accuracy of the new implementation is verified using an example problem of sliding contact between a rigid, impermeable indenter and a cartilage layer for which analytical solutions have been obtained. The new implementation's capability to handle a complex loading regime is verified by modeling plowing tests of the temporomandibular joint (TMJ) disc.

Cartilage-like mechanical properties of poly (ethylene glycol)-diacrylate hydrogels

Hydrogels prepared from poly-(ethylene glycol) (PEG) have been used in a variety of studies of cartilage tissue engineering. Such hydrogels may also be useful as a tunable mechanical material for cartilage repair. Previous studies have characterized the chemical and mechanical properties of PEG-based hydrogels, as modulated by precursor molecular weight and concentration. Cartilage mechanical properties vary substantially, with maturation, with depth from the articular surface, in health and disease, and in compression and tension. We hypothesized that PEG hydrogels could mimic a broad range of the compressive and tensile mechanical properties of articular cartilage. The objective of this study was to characterize the mechanical properties of PEG hydrogels over a broad range and with reference to articular cartilage. In particular, we assessed the effects of PEG precursor molecular weight (508 Da, 3.4 kDa, 6 kDa, and 10 kDa) and concentration (10–40%) on swelling property, equilibrium confined compressive modulus (HA0), compressive dynamic stiffness, and hydraulic permeability (kp0) of PEG hydrogels in static/dynamic confined compression tests, and equilibrium tensile modulus (Eten) in tension tests. As molecular weight of PEG decreased and concentration increased, hydrogels exhibited a decrease in swelling ratio (31.5–2.2), an increase in HA0 (0.01–2.46 MPa) and Eten (0.02–3.5 MPa), an increase in dynamic compressive stiffness (0.055–42.9 MPa), and a decrease in kp0 (1.2 × 10−15 to 8.5 × 10−15 m2/(Pa s)). The frequency-dependence of dynamic compressive stiffness amplitude and phase, as well as the strain-dependence of permeability, were typical of the time- and strain-dependent mechanical behavior of articular cartilage. HA0 and Eten were positively correlated with the final PEG concentration, accounting for swelling. These results indicate that PEG hydrogels can be prepared to mimic many of the static and dynamic mechanical properties of articular cartilage.

Evaluation of Viscoelastic Property of Articular Cartilage Based on Mechanical Model Considering Tissue Microstructure

The mechanical behavior of articular cartilage is strongly dependent on its microstructure characterized by three-dimensional, depth-dependent densities and anisotropic arrangements of cartilage cells and collagen fibres in extracellular matrix. The inclusion of cartilage microstructure to model analysis is essential for the accurate evaluation of the mechanical property. This study proposes a model-based evaluation method, which could predict tissue microstructure (collagen fibres, proteoglycans, and cartilage cells) based on a time history of reaction force obtained from indentation test, thereby estimating the depth-dependent cartilage mechanical property that is difficult to be estimated by experiment in conventional methods. The model was used for evaluating the viscoelasticities of cartilage at medial (load-supporting) and lateral (non load-supporting) regions on bovine femoral head. The present simulation allows fitting the time-history of reaction force measured from indentation test (R2 = 0.993 ± 0.0012 for lateral region and R2 = 0.990 ± 0.0032 for medial region). The differences of the elastic modulus of collagen fibre and the permeability between medial and lateral regions were consistent to those observed from the chemical composition analysis.

Analysis of biphasic lubrication of articular cartilage loaded by cylindrical indenter

Combination of theoretical biphasic analyses and corresponding experimental measurements for articular cartilage has successfully revealed the fundamental material properties and time-depending mechanical behaviors of articular cartilage containing plenty of water. The insight of load partitioning between solid and fluid phases advanced the prediction of the frictional behavior of articular cartilage. One of the recent concerns about biphasic finite element (FE) analysis seems to be a dynamic and physiological condition in terms of mechanical functionality as a load-bearing for articular joint system beyond material testing, which has mainly focused on time-dependent reaction force and deformation in relatively small and low speed compression. Recently, the biphasic FE model for reciprocating sliding motion was applied to confirm the frictional effect on the migrating contact area. The results indicated that the model of a cylindrical indenter sliding over the cartilage surface remarkably sustained the higher proportion of fluid load support than a condition without migrating contact area, but the effectiveness of constitutive material properties has not been sufficiently evaluated for sliding motion. In our present study, at the first stage, the compressive response of the articular cartilage was examined by high precision testing machine. Material properties for the biphasic FE model, which included inhomogeneous apparent Young's modulus of solid phase along depth, strain-dependent permeability and collagen reinforcement in tensile strain, were estimated in cylindrical indentation tests by the curve fitting between the experimental time-dependent behavior and FE model simulation. Then, the biphasic lubrication mechanism of the articular cartilage including migrating contact area was simulated to elucidate functionality as a load-bearing material. The results showed that the compaction effect on permeability of solid phase was functional particularly in the condition without the migrating contact area, whereas in sliding condition the compaction effect did not clearly show its role in terms of the proportion of fluid load support. The reinforcement of solid phase, which represented the collagen network in the tissue, improved the proportion of fluid load support especially in the sliding condition. Thus, a functional integration of constitutive mechanical properties as a load-bearing was evaluated by FE model simulation in this study.

Effects of the pH on the mechanical behavior of articular cartilage and corneal stroma

Electro-chemo-mechanical couplings in articular cartilages and corneal stroma are due to the presence of electric charges on proteoglycans. In addition, at non-physiological pH, collagen molecules become charged as well. Variation of the pH of the electrolyte has strong implications on the electric charge of these tissues, and by the same token, on their transport and mechanical properties. Indeed, articular cartilages and corneal stroma swell and shrink depending on the composition of the electrolyte, they are in contact with. Emphasis is laid here on the combined effects of pH, ionic strength, calcium and chloride binding on mechanical properties. The tissues are viewed as three-phase multi-species porous media. The constitutive framework is phrased in the theory of thermodynamics of deformable porous media. Acid–base reactions, as well as ion binding, are embedded in this framework. Although, macroscopic in nature, the approach accounts for a number of biochemical details defining collagen and proteoglycans. The model is used to simulate laboratory experiments where specimens of articular cartilages and corneal stroma are put in contact with a bath of controlled chemical composition. Chemical loadings, where the ionic composition and pH of the bath are varied, are intermingled with mechanical loadings. The variations of the stress and strain are observed to depend strongly on the ionic strength and ion type present in the bath: sodium chloride leads to a stiffer response than calcium chloride and hydrochloric acid. Moreover, when the bath changes from basic to acidic, the change of sign of the fixed charge across the isoelectric point has definite mechanical implications, and it gives rise to non-monotonous evolutions of the stress, strain and chemical content. While the chemo-mechanical effect is a key phenomenon that governs the behavior of tissues with fixed charges, the converse mechano-chemical effect is significant in corneal stroma due to its low stiffness.

A linearized formulation of triphasic mixture theory for articular cartilage, and its application to indentation analysis

The negative charges on proteoglycans significantly affect the mechanical behaviors of articular cartilage. Mixture theories, such as the triphasic theory, can describe quantitatively how this charged nature contributes to the mechano-electrochemical behaviors of such tissue. However, the mathematical complexity of the theory has hindered its application to complicated loading profiles, e.g., indentation or other multi-dimensional configurations. In this study, the governing equations of triphasic mixture theory for soft tissue were linearized and dramatically simplified by using a regular perturbation method and the use of two potential functions. We showed that this new formulation can be used for any axisymmetric problem, such as confined or unconfined compressions, hydraulic perfusion, and indentation. A finite difference numerical program was further developed to calculate the deformational, electrical, and flow behaviors inside the articular cartilage under indentation. The calculated tissue response was highly consistent with the data from indentation experiments (our own and those reported in the literature). It was found that the charged nature of proteoglycans can increase the apparent stiffness of the solid matrix and lessen the viscous effect introduced by fluid flow. The effects of geometric and physical properties of indenter tip, cartilage thickness, and that of the electro-chemical properties of cartilage on the resulting deformation and fluid pressure fields across the tissue were also investigated and presented. These results have implications for studying chondrocyte mechanotransduction in different cartilage zones and for tissue engineering designs or in vivo cartilage repair.

Mechanical properties of bovine articular cartilage under microscale indentation loading from atomic force microscopy.

Atomic force microscopy (AFM) techniques have been increasingly used for investigating the mechanical properties of articular cartilage. According to the previous studies reporting the microscale Young's modulus under AFM indentation tests, the Hertz contact model has been employed with a sharp conical tip indenter. However, the non-linear microscale behaviour of articular cartilage could not be resolved by the standardized Hertz analysis using small and sharp atomic force microscope tips. Therefore, the objective of this study was to evaluate the microscale Young's modulus of articular cartilage more accurately through a non-Hertzian approach with a spherical tip of 5 microm diameter, and to characterize its microscale mechanical behaviour. This methodology adopted in the present study was proved by the consistent values between the microscale (2 per cent, about 9.3 kPa; 3 per cent, about 17.5kPa) and macroscale (2 per cent, about 8.3kPa; 3 per cent, about 18.3kPa) Young's moduli for 2 per cent and 3 per cent agarose gel (n = 100). Therefore, the microscale Young's modulus evaluated in this study is representative of more accurate measurements of cartilage stiffness at the 600 nm deformation level and corresponds to approximately 30.9 kPa (n = 100). Furthermore, on this level of the microscale deformation, articular cartilage showed depth-dependent and frequency-independent behaviour under AFM indentation loading. These findings reveal the microscale mechanical behaviour of articular cartilage more accurately and can be employed further to design microscale structures of chondrocyte-seeded scaffolds and tissue-engineered cartilage by evaluating their microscale properties.

Biomechanics of Articular Cartilage and Determination of Material Properties

Descriptions of the mechanical behaviors of articular cartilage and their correlations with collagen, proteoglycan, water, and ions are summarized, with particular emphasis on understanding the osmotic effect inside the tissue. First, a descriptive explanation is presented of the biphasic theory required to understand how interstitial water contributes toward the viscoelastic behavior of any hydrated soft tissue. Then, the famous osmotic effect in charged, hydrated soft tissue is interpreted in light of the triphasic mixture theory framework. In the introduction of mechanical testing methods, our emphasis is on the popular indentation technique, which can determine the material properties of cartilage in situ or in vivo. The widely accepted indentation analysis solutions in cartilage biomechanics history are summarized and evaluated. At the end of this paper, a new generalized correspondence principle between charged, hydrated soft tissue and linear, isotropic, elastic material (i.e., elasticity theory) is introduced. This principle makes the employment of triphasic theory as straightforward as using an elasticity theory to solve any equilibrium problem where the elasticity theory can be used to model the material. By using this generalized correspondence principle, the fixed charge density of bovine cartilage has been simply and conveniently calculated from the indentation testing data. The results of proteoglycan content from this mechanical test are remarkably consistent with those from standard biochemical assay. This new correspondence principle significantly improves the power of indentation tests in the determination of mechanoelectrochemical properties of articular cartilage.

Depth-Dependence and Time-Dependence in Mechanical Behaviors of Articular Cartilage in Unconfined Compression Test under Constant Total Deformation

Articular cartilage has high water content from 70 to 80% and biphasic property. The structures of the tissue are inhomogeneous and anisotropy. Consequently, cartilage tissue shows complicated viscoelastic behavior to mechanical stimuli because of depth-dependent and time-dependent interstitial fluid flux and stress-strain behaviors. Therefore it is necessary to consider not only the average tissue property but also the local one to explain mechanical and functional behaviors. The aim of this study is to consider the effect of elastic modulus distribution on the mechanical behavior of cartilage tissue using experiments with visualization and two-dimensional finite element method (FEM).In this study, we performed the compression test of the articular cartilage under the unconfined condition. First, on the basis of the distribution of Young's modulus in depth direction calculated from local strain at equilibrium in experiment, the stress and strain behaviors in articular cartilage were analyzed by biphasic theory.Immediately after loading, the FEM analysis for deformed profiles of the model with depth-dependent Young's modulus corresponded to actual profiles, while the model with average value for Young's modulus showed inadequate deformed profiles. However, the total load-carrying capacity estimated in FEM for the former model is about one order lower than the experimental one measured by a load cell. Therefore, we provided time-dependence to elastic property to understand the complicated viscoelastic behavior during stress relaxation. Thus, the deformed shape profiles of the model immediately after loading and the total load-carrying capacity could satisfactorily correspond to measured data by considering depth-dependent and time-dependent variation of elastic property.

746 顕微鏡可視化力学試験機による関節軟骨の円筒部拘束条件の影響(S01-1 細胞と組織のバイオメカニクス(1)-ミクロからマクロまで-,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

The purpose of this study is to analyze the compressive behavior of articular cartilage in confined compression. Articular cartilage realizes very low friction and long durability in synovial joint. So, the mechanism and functional behavior of articular cartilage has been observed by many researchers for the insights of arthritic disease, regenerative medicine, and new tribological design. Articular cartilage has high water content of nearly 80% and behaves as biphasic material. To observe the mechanical behavior of articular cartilage, authors developed micro testing machine, which equipped on microscope and had the ability to visualize the strain in tissue. In this report, we constructed a confined compression chamber for the cartilage test piece, and compared the mechanical behavior between unconfined and confined compression. The morphology of subcondral bone was also observed from the view point of fluid transport.

Mechanical Behavior of Articular Cartilage after Osteochondral Autograft Transfer in an Ovine Model

Background Grafting of autologous hyaline cartilage and bone for articular cartilage repair is a well-accepted technique. Although encouraging midterm clinical results have been reported, no information on the mechanical competence of the transplanted joint surface is available. Hypothesis The mechanical competence of osteochondral autografts is maintained after transplantation. Study Design Controlled laboratory study. Methods Osteochondral defects were filled with autografts (7.45 mm in diameter) in one femoral condyle in 12 mature sheep. The ipsilateral femoral condyle served as the donor site, and the resulting defect (8.3 mm in diameter) was left empty. The repair response was examined after 3 and 6 months with mechanical and histologic assessment and histomorphometric techniques. Results Good surface congruity and plug placement was achieved. The Young modulus of the grafted cartilage significantly dropped to 57.5% of healthy tissue after 3 months (P < .05) but then recovered to 82.2% after 6 months. The aggregate and dynamic moduli behaved similarly. The graft edges showed fibrillation and, in some cases (4 of 6), hypercellularity and chondrocyte clustering. Subchondral bone sclerosis was observed in 8 of 12 cases, and the amount of mineralized bone in the graft area increased from 40% to 61%. Conclusions The mechanical quality of transplanted cartilage varies considerably over a short period of time, potentially reflecting both degenerative and regenerative processes, while histologically signs of both cartilage and bone degeneration occur. Clinical Relevance Both the mechanically degenerative and restorative processes illustrate the complex progression of regeneration after osteochondral transplantation. The histologic evidence raises doubts as to the long-term durability of the osteochondral repair.

0917 顕微鏡可視化力学試験機を用いた力制御による関節軟骨の圧縮挙動の観察(S04-1 関節のバイオメカニズム(1),S04 関節のバイオメカニズム)

Cartilage tissue has a high water content and complicated structure composed of extracellar matrix and chondrocytes. Therefore, the articular cartilage shows time dependent and depth dependent mechanical behaviors. The aim of this study is to explain the mechanical behavior of articular cartilage in constant load compression with force control. In this study, we performed compression test under the constant load, in which the fluorescent images of cartilage tissue in compression and the displacement of the indenter were obtained. The bulk Young's modulus at equilibrium was calculated using experimental result. When we applied the step-up load to sample, the bulk Young's modulus at equilibrium responded at constant value.