Effect of frequency on crack growth in articular cartilage

Articular cartilage fatigue causes frequency-dependent softening and crack extension

Soft biological polymers, such as articular cartilage, possess exceptional fracture and fatigue resistance, offering inspiration for the development of novel materials. However, we lack a detailed understanding of changes in cartilage material behavior and of crack propagation following cyclic compressive loading. We investigated the structure and mechanical behavior of cartilage as a function of loading frequency and number of cycles. Microcracks were initiated in cartilage samples using microindentation, then cracks were extended under cyclic compression. Thickness, apparent stiffness, energy dissipation, phase angle, and crack length were measured to determine the effects of cyclic loading at two frequencies (1 Hz and 5 Hz). To capture the fatigue-induced material response (thickness, stiffness, energy dissipation, and phase angle), material properties were compared between pre-and-post diagnostic tests. The findings indicate that irreversible structural damage (reduced thickness), cartilage softening (reduced apparent stiffness), and reduced energy dissipation (including phase angle) increased with an increase in the number of cycles. Higher frequency loading resulted in less reduction in energy dissipation, phase angle, and thickness change. Crack lengths, quantified through brightfield imaging, increased with number of cycles and frequency. This study sheds light on the complex response of cartilage under cyclic loading resulting in softening, structural damage, and altered dynamic behavior. The findings provide better understanding of failure mechanisms in cartilage and thus may help in diagnosis and treatment of osteoarthritis.

Dependence of crack shape in loaded articular cartilage on the collagenous structure

Cartilage cracks disrupt tissue mechanics, alter cell mechanobiology, and often trigger tissue degeneration. Yet, some tissue cracks heal spontaneously. A primary factor determining the fate of tissue cracks is the compression-induced mechanics, specifically whether a crack opens or closes when loaded. Crack deformation is thought to be affected by tissue structure, which can be probed by quantitative polarized light microscopy (PLM). It is unclear how the PLM measures are related to deformed crack morphology. Here, we investigated the relationship between PLM-derived cartilage structure and mechanical behavior of tissue cracks by testing if PLM-derived structural measures correlated with crack morphology in mechanically indented cartilages.

METHODS

Knee joint cartilages harvested from mature and immature animals were used for their distinct collagenous fibrous structure and composition. The cartilages were cut through thickness, indented over the cracked region, and processed histologically. Sample-specific birefringence was quantified as two-dimensional (2D) maps of azimuth and retardance, two measures related to local orientation and degree of alignment of the collagen fibers, respectively. The shape of mechanically indented tissue cracks, measured as depth-dependent crack opening, were compared with azimuth, retardance, or "PLM index," a new parameter derived by combining azimuth and retardance.

RESULTS

Of the three parameters, only the PLM index consistently correlated with the crack shape in immature and mature tissues.

CONCLUSION

In conclusion, we identified the relative roles of azimuth and retardance on the deformation of tissue cracks, with azimuth playing the dominant role. The applicability of the PLM index should be tested in future studies using naturally-occurring tissue cracks.

Effect of osmolarity and displacement rate on cartilage microfracture clusters failure into two regimes

Articular cartilage is a poroviscoelastic (PVE) material with remarkable resistance to fracture and fatigue failure. Cartilage failure mechanisms and material properties that govern failure are incompletely understood. Because cartilage is partially comprised of negatively charged glycosaminoglycans, altering solvent osmolarity can influence PVE relaxations. Therefore, this study aims to use osmolarity as a tool to provide additional data to interpret the role of PVE relaxations and identify cartilage failure regimes. Cartilage fracture was induced using a 100 μm radius spheroconical indenter at controlled displacement rates under three different osmolarity solvents. Secondarily, contact pressure (CP) and strain energy density (SED) were estimated to cluster data into two failure regimes with an expectation maximization algorithm. Critical displacement, critical load, critical time, and critical work to fracture increased with increasing osmolarity at a slow displacement rate whereas no significant effect was observed at a fast displacement rate. Clustering provided two distinct failure regimes, with regime (I) at lower normalized thickness (contact radius divided by sample thickness), and regime (II) at higher normalized thickness. Varied CP and SED in regime (I) suggest that failure in the regime is strain-governed. Constant CP and SED in regime (II) suggests that failure in the regime is dominantly governed by stress. These regimes can be interpreted as ductile versus brittle, or using a pressurized fragmentation interpretation. These findings demonstrated fundamental failure properties and postulate failure regimes for articular cartilage.

Deformation behaviors and mechanical impairments of tissue cracks in immature and mature cartilages

Degeneration of articular cartilage is often triggered by a small tissue crack. As cartilage structure and composition change with age, the mechanics of cracked cartilage may depend on the tissue age, but this relationship is poorly understood. Here, we investigated cartilage mechanics and crack deformation in immature and mature cartilage exposed to a full-thickness tissue crack using indentation testing and histology, respectively. When a cut was introduced, tissue cracks opened wider in the mature cartilage compared to the immature cartilage. However, the opposite occurred upon mechanical indentation over the cracked region. Functionally, the immature-cracked cartilages stress-relaxed faster, experienced increased tissue strain, and had reduced instantaneous stiffness, compared to the mature-cracked cartilages. Taken together, mature cartilage appears to withstand surface cracks and maintains its mechanical properties better than immature cartilage and these superior properties can be explained by the structure of their collagen fibrous network.

Shear strain and inflammation-induced fixed charge density loss in the knee joint cartilage following ACL injury and reconstruction: A computational study

Excessive tissue deformation near cartilage lesions and acute inflammation within the knee joint after anterior cruciate ligament (ACL) rupture and reconstruction surgery accelerate the loss of fixed charge density (FCD) and subsequent cartilage tissue degeneration. Here, we show how biomechanical and biochemical degradation pathways can predict FCD loss using a patient-specific finite element model of an ACL reconstructed knee joint exhibiting a chondral lesion. Biomechanical degradation was based on the excessive maximum shear strains that may result in cell apoptosis, while biochemical degradation was driven by the diffusion of pro-inflammatory cytokines. We found that the biomechanical model was able to predict substantial localized FCD loss near the lesion and on the medial areas of the lateral tibial cartilage. In turn, the biochemical model predicted FCD loss all around the lesion and at intact areas; the highest FCD loss was at the cartilage-synovial fluid-interface and decreased toward the deeper zones. Interestingly, simulating a downturn of an acute inflammatory response by reducing the cytokine concentration exponentially over time in synovial fluid led to a partial recovery of FCD content in the cartilage. Our novel numerical approach suggests that in vivo FCD loss can be estimated in injured cartilage following ACL injury and reconstruction. Our novel modeling platform can benefit the prediction of PTOA progression and the development of treatment interventions such as disease-modifying drug testing and rehabilitation strategies.

Cartilage articulation exacerbates chondrocyte damage and death after impact injury

Posttraumatic osteoarthritis (PTOA) is typically initiated by momentary supraphysiologic shear and compressive forces delivered to articular cartilage during acute joint injury and develops through subsequent degradation of cartilage matrix components and tissue remodeling. PTOA affects 12% of the population who experience osteoarthritis and is attributed to over $3 billion dollars annually in healthcare costs. It is currently unknown whether articulation of the joint post-injury helps tissue healing or exacerbates cellular dysfunction and eventual death. We hypothesize that post-injury cartilage articulation will lead to increased cartilage damage. Our objective was to test this hypothesis by mimicking the mechanical environment of the joint during and post-injury and determining if subsequent joint articulation exacerbates damage produced by initial injury. We use a model of PTOA that combines impact injury and repetitive sliding with confocal microscopy to quantify and track chondrocyte viability, apoptosis, and mitochondrial depolarization in a depth-dependent manner. Cartilage explants were harvested from neonatal bovine knee joints and subjected to either rapid impact injury (17.34 ± 0.99 MPa, 21.6 ± 2.45 GPa/s), sliding (60 min at 1 mm/s, under 15% axial compression), or rapid impact injury followed by sliding. Explants were then bisected and fluorescently stained for cell viability, caspase activity (apoptosis), and mitochondria polarization. Results show that compared to either impact or sliding alone, explants that were both impacted and slid experienced higher magnitudes of damage spanning greater tissue depths.

Relaxation capacity of cartilage is a critical factor in rate- and integrity-dependent fracture

Articular cartilage heals poorly but experiences mechanically induced damage across a broad range of loading rates and matrix integrity. Because loading rates and matrix integrity affect cartilage mechanical responses due to poroviscoelastic relaxation mechanisms, their effects on cartilage failure are important for assessing and preventing failure. This paper investigated rate- and integrity-dependent crack nucleation in cartilage from pre- to post-relaxation timescales. Rate-dependent crack nucleation and relaxation responses were obtained as a function of matrix integrity through microindentation. Total work for crack nucleation increased with decreased matrix integrity, and with decreased loading rates. Critical energy release rate of intact cartilage was estimated as 2.39 ± 1.39 to 2.48 ± 1.26 kJ m-2 in a pre-relaxation timescale. These findings showed that crack nucleation is delayed when cartilage can accommodate localized loading through poroviscoelastic relaxation mechanisms before fracture at a given loading rate and integrity state.

Prediction of local fixed charge density loss in cartilage following ACL injury and reconstruction: A computational proof-of-concept study with MRI follow-up

The purpose of this proof-of-concept study was to develop three-dimensional patient-specific mechanobiological knee joint models to simulate alterations in the fixed charged density (FCD) around cartilage lesions during the stance phase of the walking gait. Two patients with anterior cruciate ligament (ACL) reconstructed knees were imaged at 1 and 3 years after surgery. The magnetic resonance imaging (MRI) data were used for segmenting the knee geometries, including the cartilage lesions. Based on these geometries, finite element (FE) models were developed. The gait of the patients was obtained using a motion capture system. Musculoskeletal modeling was utilized to calculate knee joint contact and lower extremity muscle forces for the FE models. Finally, a cartilage adaptation algorithm was implemented in both FE models. In the algorithm, it was assumed that excessive maximum shear and deviatoric strains (calculated as the combination of principal strains), and fluid velocity, are responsible for the FCD loss. Changes in the longitudinal T_1ρ and T₂ relaxation times were postulated to be related to changes in the cartilage composition and were compared with the numerical predictions. In patient 1 model, both the excessive fluid velocity and strain caused the FCD loss primarily near the cartilage lesion. T_1ρ and T₂ relaxation times increased during the follow-up in the same location. In contrast, in patient 2 model, only the excessive fluid velocity led to a slight FCD loss near the lesion, where MRI parameters did not show evidence of alterations. Significance: This novel proof-of-concept study suggests mechanisms through which a local FCD loss might occur near cartilage lesions. In order to obtain statistical evidence for these findings, the method should be investigated with a larger cohort of subjects.

Mechanobiological model for simulation of injured cartilage degradation via pro-inflammatory cytokines and mechanical stimulus

Post-traumatic osteoarthritis (PTOA) is associated with cartilage degradation, ultimately leading to disability and decrease of quality of life. Two key mechanisms have been suggested to occur in PTOA: tissue inflammation and abnormal biomechanical loading. Both mechanisms have been suggested to result in loss of cartilage proteoglycans, the source of tissue fixed charge density (FCD). In order to predict the simultaneous effect of these degrading mechanisms on FCD content, a computational model has been developed. We simulated spatial and temporal changes of FCD content in injured cartilage using a novel finite element model that incorporates (1) diffusion of the pro-inflammatory cytokine interleukin-1 into tissue, and (2) the effect of excessive levels of shear strain near chondral defects during physiologically relevant loading. Cytokine-induced biochemical cartilage explant degradation occurs near the sides, top, and lesion, consistent with the literature. In turn, biomechanically-driven FCD loss is predicted near the lesion, in accordance with experimental findings: regions near lesions showed significantly more FCD depletion compared to regions away from lesions (p<0.01). Combined biochemical and biomechanical degradation is found near the free surfaces and especially near the lesion, and the corresponding bulk FCD loss agrees with experiments. We suggest that the presence of lesions plays a role in cytokine diffusion-driven degradation, and also predisposes cartilage for further biomechanical degradation. Models considering both these cartilage degradation pathways concomitantly are promising in silico tools for predicting disease progression, recognizing lesions at high risk, simulating treatments, and ultimately optimizing treatments to postpone the development of PTOA.

Dynamic viscoelastic characterisation of human osteochondral tissue: understanding the effect of the cartilage-bone interface

BACKGROUND

Despite it being known that subchondral bone affects the viscoelasticity of cartilage, there has been little research into the mechanical properties of osteochondral tissue as a whole system. This study aims to unearth new knowledge concerning the dynamic behaviour of human subchondral bone and how energy is transferred through the cartilage-bone interface.

METHODS

Dynamic mechanical analysis was used to determine the frequency-dependent (1-90 Hz) viscoelastic properties of the osteochondral unit (cartilage-bone system) as well as isolated cartilage and bone specimens extracted from human femoral heads obtained from patients undergoing total hip replacement surgery, with a mean age of 78 years (N = 5, n = 22). Bone mineral density (BMD) was also determined for samples using micro-computed tomography as a marker of tissue health.

RESULTS

Cartilage storage and loss moduli along with bone storage modulus were found to increase logarithmically (p < 0.05) with frequency. The mean cartilage storage modulus was 34.4 ± 3.35 MPa and loss modulus was 6.17 ± 0.48 MPa (mean ± standard deviation). In contrast, bone loss modulus decreased logarithmically between 1 and 90 Hz (p < 0.05). The storage stiffness of the cartilage-bone-core was found to be frequency-dependent with a mean value of 1016 ± 54.0 N.mm^- 1, while the loss stiffness was determined to be frequency-independent at 78.84 ± 2.48 N.mm^- 1. Notably, a statistically significant (p < 0.05) linear correlation was found between the total energy dissipated from the isolated cartilage specimens, and the BMD of the isolated bone specimens at all frequencies except at 90 Hz (p = 0.09).

CONCLUSIONS

The viscoelastic properties of the cartilage-bone core were significantly different to the tissues in isolation (p < 0.05). Results from this study demonstrate that the functionality of these tissues arises because they operate as a unit. This is evidenced through the link between cartilage energy dissipated and bone BMD. The results may provide insights into the functionality of the osteochondral unit, which may offer further understanding of disease progression, such as osteoarthritis (OA). Furthermore, the results emphasise the importance of studying human tissue, as bovine models do not always display the same trends.

The status and challenges of replicating the mechanical properties of connective tissues using additive manufacturing

The ability to fabricate complex structures via precise and heterogeneous deposition of biomaterials makes additive manufacturing (AM) a leading technology in the creation of implants and tissue engineered scaffolds. Connective tissues (CTs) remain attractive targets for manufacturing due to their "simple" tissue compositions that, in theory, are replicable through choice of biomaterial(s) and implant microarchitecture. Nevertheless, characterisation of the mechanical and biological functions of 3D printed constructs with respect to their host tissues is often limited and remains a restriction towards their translation into clinical practice. This review aims to provide an update on the current status of AM to mimic the mechanical properties of CTs, with focus on arterial tissue, articular cartilage and bone, from the perspective of printing platforms, biomaterial properties, and topological design. Furthermore, the grand challenges associated with the AM of CT replacements and their subsequent regulatory requirements are discussed to aid further development of reliable and effective implants.

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.

Rate-dependent crack nucleation in cartilage under microindentation

This study investigates rate-dependent crack nucleation in cartilage under microindentation using a poroviscoelastic framework and nano/microscopic images. Localized crack failure was induced at known locations and at different loading rates via microindentation with an axisymmetric sphero-conical indenter. Finite element (FE) modeling was used to reproduce results of microindentation tests within a poroviscoelastic framework. Scanning electron microscopy (SEM) was used to examine nano- and microscale structural features of crack surfaces. Microindentation results showed rate-dependent crack nucleation in cartilage. In particular, critical total work required for crack nucleation was larger at the slow loading rate compared to the fast loading rate. FE results suggested that viscoelastic relaxation of cartilage was a major contributor to the rate dependency and that tensile stresses localized at the indenter tip was a governing factor in crack nucleation. SEM images combined with microindentation and FE results suggested that the solid matrix in the vicinity of the tip experienced relatively large relaxation and kinematic fiber rearrangement at the slow loading rate in comparison to the fast loading rate. These findings extend current understanding of rate-dependent failure mechanisms in cartilage.

Surface damage of bovine articular cartilage-off-bone: the effect of variations in underlying substrate and frequency

BACKGROUND

Changes in bone mineral density have been implicated with the onset of osteoarthritis, but its role in inducing failure of articular cartilage mechanically is unclear. This study aimed to determine the effect of substrate density, as the underlying bone, on the surface damage of cartilage-off-bone, at frequencies associated with gait, and above.

METHODS

Bovine articular cartilage samples were tested off-bone to assess induced damage with an indenter under a compressive sinusoidal load range of 5-50 N at frequencies of 1, 10 and 50 Hz, corresponding to normal and above normal gait respectively, for up to 10,000 cycles. Cartilage samples were tested on four underlying substrates with densities of 0.1556, 0.3222, 0.5667 and 0.6000 g/cm3. India ink was applied to identify damage as cracks, measured across their length using ImageJ software. Linear regression was performed to identify if statistical significance existed between substrate density, and surface damage of articular cartilage-off-bone, at all three frequencies investigated (p < 0.05).

RESULTS

Surface damage significantly increased (p < 0.05) with substrate density at 10 Hz of applied frequency. Crack length at this frequency reached the maximum of 10.95 ± 9.12 mm (mean ± standard deviation), across all four substrates tested. Frequencies applied at 1 and 50 Hz failed to show a significant increase (p > 0.05) in surface damage with an increase in substrate density, at which the maximum mean crack length were 3.01 ± 3.41 mm and 5.65 ± 6.54 mm, respectively. Crack formation at all frequencies tended to form at the periphery of the cartilage specimen, with multiple straight-line cracking observed at 10 Hz, in comparison to single straight-line configurations produced at 1 and 50 Hz.

CONCLUSIONS

The effect of substrate density on the surface damage of articular cartilage-off-bone is multi-factorial, with an above-normal gait frequency. At 1 Hz cartilage damage is not associated with substrate density, however at 10 Hz, it is. This study has implications on the effects of the factors that contribute to the onset of osteoarthritis.

A novel mechanobiological model can predict how physiologically relevant dynamic loading causes proteoglycan loss in mechanically injured articular cartilage

Cartilage provides low-friction properties and plays an essential role in diarthrodial joints. A hydrated ground substance composed mainly of proteoglycans (PGs) and a fibrillar collagen network are the main constituents of cartilage. Unfortunately, traumatic joint loading can destroy this complex structure and produce lesions in tissue, leading later to changes in tissue composition and, ultimately, to post-traumatic osteoarthritis (PTOA). Consequently, the fixed charge density (FCD) of PGs may decrease near the lesion. However, the underlying mechanisms leading to these tissue changes are unknown. Here, knee cartilage disks from bovine calves were injuriously compressed, followed by a physiologically relevant dynamic compression for twelve days. FCD content at different follow-up time points was assessed using digital densitometry. A novel cartilage degeneration model was developed by implementing deviatoric and maximum shear strain, as well as fluid velocity controlled algorithms to simulate the FCD loss as a function of time. Predicted loss of FCD was quite uniform around the cartilage lesions when the degeneration algorithm was driven by the fluid velocity, while the deviatoric and shear strain driven mechanisms exhibited slightly discontinuous FCD loss around cracks. Our degeneration algorithm predictions fitted well with the FCD content measured from the experiments. The developed model could subsequently be applied for prediction of FCD depletion around different cartilage lesions and for suggesting optimal rehabilitation protocols.

Matrix degradation in osteoarthritis primes the superficial region of cartilage for mechanical damage

Osteoarthritis (OA) is a degenerative disease that affects 25% of the world's population over fifty years of age. It is a chronic disease of the synovial joints, primarily the hip and knee. The main pathologies are degradation of the articular cartilage and changes to the subchondral bone, as a result of both mechanical wear and a locally elevated inflammatory state. This study compares the viscoelastic properties of cartilage that represents the biochemical changes in OA and age-matched healthy tissue. Further, the mechanical damage induced by this compressive loading cycle was characterised and the mechanism for it was investigated. The storage modulus of OA cartilage was shown to be significantly lower than that of healthy cartilage whilst having a higher capacity to hold water. Following mechanical testing, there was a significant increase in the surface roughness of OA cartilage. This change in surface structure occurred following a reduction in sulphated glycosaminoglycan content of the superficial region in OA, as seen by alcian blue staining and quantified by micro X-ray fluorescence. These findings are important in understanding how the chemical changes to cartilage matrix in OA influence its dynamic mechanical properties and structural integrity.

STATEMENT OF SIGNIFICANCE

Cartilage has a very specialised tissue structure which acts to resist compressive loading. In osteoarthritis (OA), there is both mechanically- and chemically-induced damage to cartilage, resulting in severe degradation of the tissue. In this study we have undertaken a detailed mechanical and chemical analysis of macroscopically undamaged OA and healthy cartilage tissue. We have demonstrated, for the first time in human tissue, that the mechanical degradation of the tissue is attributed to a chemical change across the structure. In macroscopically undamaged OA tissue, there is a reduction in the elastic response of cartilage tissue and an associated destabilisation of the matrix that leaves it susceptible to damage. Understanding this allows us to better understand the progression of OA to design better therapeutic interventions.

Formulation and viscoelasticity of mineralised hydrogels for use in bone-cartilage interfacial reconstruction

Articular cartilage is a viscoelastic tissue whose structural integrity is important in maintaining joint health. To restore the functionality of osteoarthritic joints it is vital that regenerative strategies mimic the dynamic loading response of cartilage and bone. Here, a rotating simplex model was employed to optimise the composition of agarose and gellan hydrogel constructs structured with hydroxyapatite (HA) with the aim of obtaining composites mechanically comparable to human cartilage in terms of their ability to dissipate energy. Addition of ceramic particles was found to reinforce both matrices up to a critical concentration (< 3w/v%). Beyond this, larger agglomerates were formed, as evidenced by micro computed tomography data, which acted as stress risers and reduced the ability of composites to dissipate energy demonstrated by a reduction in tan δ values. A maximum compressive modulus of 450.7±24.9 kPa was achieved with a composition of 5.8w/v% agarose and 0.5w/v% HA. Interestingly, when loaded dynamically (1-20Hz) this optimised formulation did not exhibit the highest complex modulus instead a sample with a higher concentration of mineral was identified (5.8w/v% agarose and 25w/v% HA). Thus, demonstrating the importance of examining the mechanical behaviour of biomaterials under conditions representative of physiological environments. While the complex moduli of the optimised gellan (1.0 ± 0.2MPa at 1Hz) and agarose (1.7 ± 0.2MPa at 1Hz) constructs did not match the complex moduli of healthy human cartilage samples (26.3 ± 6.5MPa at 1Hz), similar tan δ values were observed between 1 and 5Hz. This is promising since these frequencies represent the typical heel strike time of the general population. In summary, this study demonstrates the importance of considering more than just the strength of biomaterials since tissues like cartilage play a more complex role.

Viscoelasticity of articular cartilage: Analysing the effect of induced stress and the restraint of bone in a dynamic environment

The aim of this study was to determine the effect of the induced stress and restraint provided by the underlying bone on the frequency-dependent storage and loss stiffness (for bone restraint) or modulus (for induced stress) of articular cartilage, which characterise its viscoelasticity. Dynamic mechanical analysis has been used to determine the frequency-dependent viscoelastic properties of bovine femoral and humeral head articular cartilage. A sinusoidal load was applied to the specimens and out-of-phase displacement response was measured to determine the phase angle, the storage and loss stiffness or modulus. As induced stress increased, the storage modulus significantly increased (p < 0.05). The phase angle decreased significantly (p < 0.05) as the induced stress increased; reducing from 13.1° to 3.5°. The median storage stiffness ranged from 548N/mm to 707N/mm for cartilage tested on-bone and 544N/mm to 732N/mm for cartilage tested off-bone. On-bone articular cartilage loss stiffness was frequency independent (p > 0.05); however, off-bone, articular cartilage loss stiffness demonstrated a logarithmic frequency-dependency (p < 0.05). In conclusion, the frequency-dependent trends of storage and loss moduli of articular cartilage are dependent on the induced stress, while the restraint provided by the underlying bone removes the frequency-dependency of the loss stiffness.