Effect of the variation of loading frequency on surface failure of bovine articular cartilage

Failure in articular cartilage: Finite element predictions of stress, strain, and pressure under micro-indentation induced fracture

Articular cartilage is found at the distal end of long bones and is responsible for assisting in joint articulation. While articular cartilage has remarkable resistance to failure, once initially damaged, degeneration is nearly irreversible. Thus, understanding damage initiation is important. There are a few proposed mechanisms for articular cartilage failure initiation: (A) a single collagen fibril stress-based regime; (B) a rate-dependent regime captured by brittle failure at slow displacement rates (SDR) and ductile failure at fast displacement rates (FDR); and (C) a rate-dependent regime where failure is governed by pressurization fragmentation at SDR and governed by strain at FDR. The objective of this study was to use finite element (FE) models to provide evidence to support or refute these proposed failure mechanisms. Models were developed of microfracture experiments that investigated osmolarity (hypo-osmolar, normal osmolarity, and hyper-osmolar) and displacement rate (FDR and SDR) effects. Cartilage was modeled with a neo-Hookean ground matrix, strain-dependent permeability, nonlinear fibril reinforcement with viscoelastic fibril terms, and Donnan equilibrium swelling. Total stress, solid matrix stress, Lagrange strain, and fluid pressure were determined under the indenter tip at the moment of microfracture. Results indicated significant rate dependence across multiple outputs, which does not support (A) a single failure regime. Larger solid and fluid pressures at FDR than SDR did not support (C) a rate-dependent regime split by pressurization at SDR and strain at FDR. Consistent solid shear stresses at SDR and consistent third principal solid stresses at FDR support (B) the ductile-brittle failure regime. These findings help to shed light on the underlying mechanisms of articular cartilage failure, which have implications for the development of osteoarthritis.

Contribution of joint tissue properties to load-induced osteoarthritis

Objective

Clinical evidence suggests that abnormal mechanical forces play a major role in the initiation and progression of osteoarthritis (OA). However, few studies have examined the mechanical environment that leads to disease. Thus, using a mouse tibial loading model, we quantified the cartilage contact stresses and examined the effects of altering tissue material properties on joint stresses during loading.

Design

Using a discrete element model (DEA) in conjunction with joint kinematics data from a murine knee joint compression model, the magnitude and distribution of contact stresses in the tibial cartilage during joint loading were quantified at levels ranging from 0 to 9 N in 1 N increments. In addition, a simplified finite element (FEA) contact model was developed to simulate the knee joint, and parametric analyses were conducted to investigate the effects of altering bone and cartilage material properties on joint stresses during compressive loading.

Results

As loading increased, the peak contact pressures were sufficient to induce fibrillations on the cartilage surfaces. The computed areas of peak contact pressures correlated with experimentally defined areas of highest cartilage damage. Only alterations in cartilage properties and geometry caused large changes in cartilage contact pressures. However, changes in both bone and cartilage material properties resulted in significant changes in stresses induced in the bone during compressive loading.

Conclusions

The level of mechanical stress induced by compressive tibial loading directly correlated with areas of biological change observed in the mouse knee joint. These results, taken together with the parametric analyses, are the first to demonstrate both experimentally and computationally that the tibial loading model is a useful preclinical platform with which to predict and study the effects of modulating bone and/or cartilage properties on attenuating OA progression. Given the direct correlation between computational modeling and experimental results, the effects of tissue-modifying treatments may be predicted prior to in vivo experimentation, allowing for novel therapeutics to be developed.

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.

Ultrasonographic Evaluation of the Metacarpal Cartilage Thicknesses in Weightlifters and Volleyball Players

OBJECTIVE

The aim of the study was to evaluate the possible effects of impact and loading on the metacarpal cartilage and hand functions in young elite athletes.

DESIGN

In this cross-sectional study athletes with at least 3 yrs of sport background and who have been under a scheduled training program were enrolled. The second to fifth fingers' metacarpal head cartilage thicknesses were measured bilaterally by using ultrasonography. Handgrip and pinch strengths were measured. Michigan Hand Outcomes Questionnaire was also completed for every participant.

RESULTS

A total of 42 male athletes (19 weightlifters, 23 volleyball players) and 46 healthy control subjects were enrolled. Metacarpal cartilage thicknesses of the athletes were thicker than those of the healthy controls (all P < 0.001). There were no differences between the dominant and nondominant hands (all P > 0.05). In the weightlifting group, Michigan Hand Outcomes Questionnaire work performance and pain scores were worse than the other groups (both P < 0.001).

CONCLUSIONS

The presence of increased cartilage thickness measurements in the athletes suggests that sports activities might affect the metacarpal articular cartilage. Highest pain scores and lowest work performance scores in the weightlifters with highest metacarpal cartilage thickness might suggest that impact and loading during their sports play could lead to cartilage edema.

Structural clues to articular calcified cartilage function: A descriptive review of this crucial interface tissue

Articular calcified cartilage (ACC) has been dismissed, by some, as a remnant of endochondral ossification without functional relevance to joint articulation or weight-bearing. Recent research indicates that morphologic and metabolic ACC features may be important, reflecting knee joint osteoarthritis (OA) predisposition. ACC is less investigated than neighbouring joint tissues, with its component chondrocytes and mineralised matrix often being either ignored or integrated into analyses of hyaline articular cartilage and subchondral bone tissue respectively. Anatomical variation in ACC is recognised between species, individuals and age groups, but the selective pressures underlying this variation are unknown. Consequently, optimal ACC biomechanical features are also unknown as are any potential locomotory roles. This review collates descriptions of ACC anatomy and biology in health and disease, with a view to revealing its structure/function relationship and highlighting potential future research avenues. Mouse models of healthy and OA joint ageing have shown disparities in ACC load-induced deformations at the knee joint. This raises the hypothesis that ACC response to locomotor forces over time may influence, or even underlie, the bony and hyaline cartilage symptoms characteristic of OA. To effectively investigate the ACC, greater resolution of joint imaging and merging of hierarchical scale data will be required. An appreciation of OA as a 'whole joint disease' is expanding, as is the possibility that the ACC may be a key player in healthy ageing and in the transition to OA joint pathology.

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.

Shoulder structure and function: The impact of osteoarthritis and rehabilitation strategies

STUDY DESIGN

Invited review.

BACKGROUND

Shoulder osteoarthritis can result in significant functional deficits. To improve diagnosis and treatment, we must better understand the impact of osteoarthritis on shoulder biomechanics and the known mechanical benefits of currently available treatments.

PURPOSE

The purpose of this paper is to present up-to-date data on the effects of osteoarthritis and rehabilitation on the biomechanical parameters contributing to shoulder function. With this goal, we also reviewed the anatomy and the ranges of motion of the shoulder.

METHODS

A search of electronic databases was conducted. All study designs were included to inform this qualitative, narrative literature review.

RESULTS

This review describes the biomechanics of the shoulder, the impact of osteoarthritis on shoulder function, and the treatment of shoulder osteoarthritis with an emphasis on rehabilitation.

CONCLUSIONS

The shoulder is important for the completion of activities of daily living, and osteoarthritis of the shoulder can significantly reduce shoulder motion and arm function. Although shoulder rehabilitation is an integral treatment modality to improve pain and function in shoulder osteoarthritis, few high-quality studies have investigated the effects and benefits of shoulder physical and occupational therapies. To advance the fields of therapy and rehabilitation, future studies investigating the effects of therapy intensity, therapy duration, and the relative benefits of therapy subtypes on shoulder biomechanics and function are necessary.

Analysis of hydration and subchondral bone density on the viscoelastic properties of bovine articular cartilage

Background

Articular cartilage is known to be a viscoelastic material, however little research has explored the impact of cartilage water content and bone density on its viscoelasticity. This study aimed to isolate subchondral bone density and hydration of articular cartilage and analyse their effects on the viscoelastic properties of articular cartilage.

Methods

Dynamic mechanical analysis was used to test samples at frequencies of 1, 8, 12, 29, 49, 71, and 88 Hz. Synthetic bone material with densities of 663.7 kg/m³ and 156.8 kg/m³ were used to mimic the bone mineral density (BMD). Dehydration occurred in a stepwise manner at relative humidity (RH) levels of 100%, 30%, and 1%. These relative humidity levels led to water contents of approximately 76%, 8.5%, and ≈ 0% by mass, respectively.

Results

Samples from eight bovine femoral heads were tested under a sinusoidal load. Storage stiffness was lower on the lower substrate density. Storage stiffness, though, increased as cartilage samples were dehydrated from a water content of 76% to 8.5%; decreasing again as the water content was further reduced. Loss stiffness was lower on a lower density substrate and decreased as the water content decreased.

Conclusions

In conclusions, a decrease in hydration decreases the loss stiffness, but a non-linear relationship between hydration and storage stiffness may exist. Additionally, higher BMD values led to greater storage and loss stiffnesses.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-022-05169-0.

Contrary response of porcine articular cartilage below and over 1000 s-1

BACKGROUND

Knee joints experience excessive loads quite frequently during sports activities, and these shocks could accelerate progressive degeneration in articular cartilage.

METHODS

Quasi-static and dynamic response of porcine knee articular cartilages were investigated in this research. Split Hopkinson Pressure Bars (SHPB) were utilized to examine the articular cartilage properties at strain rates between 0.01-2000 s^-1.

FINDINGS

The results showed that strain rate is an important factor for articular cartilages, distinctively divided into above and below 1000 s^-1. The articular cartilages exhibit a strain hardening phenomenon when shock loaded at strain rates under 1000 s^-1. When loaded at strain rates over 1000 s^-1, their ultimate strength and elastic modulus decreased with increasing strain rates.

INTERPRETATION

The biphasic structure of the cartilage explained the change of modulus. At the lower strain rates, fibers realigned and solidified the structure, while at higher strain rates, there is not enough time for the tissue fluid to move inside the cartilage, leading to a reduction in the deformability of the specimen and raising of Young's modulus. The results can be utilized to provide some useful data for biomaterial and computational works in the future.

The effect of hemiarthroplasty implant modulus on contact mechanics: an experimental investigation

BACKGROUND

Hemiarthroplasties cause damage to the cartilage that they articulate against, which is a major limitation to their use. This study investigated the use of lower-stiffness materials to determine whether they improve hemiarthroplasty contact mechanics and thus reduce the risk of cartilage damage.

METHODS

Eleven fresh-frozen cadaveric upper extremities were disarticulated and fixed in a custom-built jig that applied a static load of 50 N to the radiocapitellar joint. Flexion angles of 0°, 45°, 90°, and 135° were tested with radial head implants made of cobalt-chrome (CoCr) and ultrahigh-molecular-weight polyethylene (UHMWPE) compared with the native radial head. A Tekscan thin-film sensor was used to measure the contact area and contact pressure between the radius and capitellum.

RESULTS

UHMWPE and CoCr were too stiff in the application of hemiarthroplasty, resulting in lower contact areas and higher contact pressures relative to the native joint. The native contact area was, on average, 42 ± 20 mm² larger than that of UHMWPE (P < .001) and 55 ± 24 mm² larger than that of CoCr (P < .001). UHMWPE had a contact area 13 ± 10 mm² greater than that of CoCr (P = .014).

DISCUSSION AND CONCLUSION

This study shows that even though UHMWPE has a stiffness several times lower than CoCr, the use of this material in hemiarthroplasty led to only a minor improvement in contact mechanics. Neither implant restored contact similar to the native articulation. Investigations into new materials to improve the contact mechanics of hemiarthroplasty should focus on materials with a lower stiffness than UHMWPE.

Microindentation Technique to Create Localized Cartilage Microfractures

Articular cartilage is a multiphasic, anisotropic, and heterogeneous material. Although cartilage possesses excellent mechanical and biological properties, it can undergo mechanical damage, resulting in osteoarthritis. Thus, it is important to understand the microscale failure behavior of cartilage in both basic science and clinical contexts. Determining cartilage failure behavior and mechanisms provides insight for improving treatment strategies to delay osteoarthritis initiation or progression and can also enhance the value of cartilage as bioinspiration for material fabrication. To investigate microscale failure behavior, we developed a protocol to initiate fractures by applying a microindentation technique using a well-defined tip geometry that creates localized cracks across a range of loading rates. The protocol includes extracting the tissue from the joint, preparing samples, and microfracture. Various aspects of the experiment, such as loading profile and solvent, can be adjusted to mimic physiological or pathological conditions and thereby further clarify phenomena underlying articular cartilage failure. © 2021 Wiley Periodicals LLC. Basic Protocol 1: Harvesting and dissection of the joint surfaces Basic Protocol 2: Preparation of samples for microindentation and fatigue testing Basic Protocol 3: Microfracture using microindentation Basic Protocol 4: Crack propagation under cyclic loading.

The dynamic viscoelastic characterisation and magnetic resonance imaging of poly(vinyl alcohol) cryogel: Identifying new attributes and opportunities

Poly(vinyl alcohol) (PVA) cryogel is a biocompatible, synthetic hydrogel, compatible with magnetic resonance (MR) imaging. It is widely used as a biomaterial in tissue scaffolds and mimics to test various diagnostic techniques. The aim of this study is to characterise the effect of varying PVA concentration, molecular weight (MW) and manufacturing protocol on the viscoelastic mechanical properties and MR T₂ relaxation time. Further to this MR imaging (MRI) was investigated as a method to quantify material homogeneity. Cylindrical samples of PVA, of varying MW, concentration and number of freeze thaw cycles (FTCs), were manufactured. Dynamic mechanical analysis was performed to evaluate the storage and loss moduli between frequencies of 0.5 and 10 Hz. MR T₂ relaxation maps were imaged using a 7 T MRI instrument. Storage and loss moduli were shown to increase with MW, concentration, or the number of FTCs; with storage modulus ranging from 55 kPa to 912 kPa and loss modulus ranging from 6 kPa to 103 kPa. MR T₂ relaxation time was shown to increase linearly with PVA concentration. The qualitative and quantitative heterogeneity of the PVA sample were identified through MR T₂ relaxation time maps. Excitingly, PVA demonstrated a composition-dependent casual correlation between the viscoelastic mechanical properties and MR T₂ relaxation time. In conclusion, this research thoroughly characterised the viscoelastic mechanical properties of PVA to support its extensive use as a biomaterial, and demonstrated the use of MRI to non-invasively identify sample heterogeneity and to predict the composition-dependent viscoelastic properties of PVA.

The orthotropic viscoelastic characterisation of sub-zero 3D-printed poly(vinyl alcohol) cryogel

Abstract

Poly(vinyl alcohol) cryogel (PVA) is a versatile biomaterial used to replicate the biomechanics of tissues. Additive manufacture (AM) at sub-zero (°C) temperatures enables the manufacture of PVA with complex geometry; however, the effect of processing parameters on the mechanical properties of PVA has not been evaluated. The aim of this study is to understand the impact of print nozzle diameter and orientation on the viscoelastic mechanical properties of PVA. Samples of sub-zero AM PVA, with different filament thicknesses, were tested under tension relative to the print direction, to calculate the storage and loss moduli. As the nozzle size was decreased, AM PVA exhibited more pronounced orthotropic properties; the smallest size showed a 33% decrease in storage moduli when tested perpendicular to the print direction, as opposed to parallel. This study has demonstrated the ability of sub-zero AM to tailor the orthotropic properties of PVA.

Graphic abstract

First Tarsometatarsal Joint Loading After Sequential Correction of Hallux Valgus Using a Proximal Opening Wedge Metatarsal Osteotomy and Distal Soft Tissue Procedure

Background:

The proximal opening wedge osteotomy (POWO) of the first metatarsal (TMT-1) is commonly performed in the operative treatment of hallux valgus. Limited work has been dedicated to study POWO’s effect on the TMT-1 joint, however. The purpose of this study is to evaluate the changes in TMT-1 joint contact stress following POWO of the first metatarsal.

Methods:

Five fresh-frozen cadaveric below-knee specimens (mean age: 73 years) with hallux valgus deformities (mean hallux valgus angle [HVA]: 37.4 ± 8.5 degrees) were studied. The specimens were loaded to 400 N on an MTS servohydraulic load frame. Joint contact characteristics at TMT-1 joint were measured with a Tekscan pressure sensor (Model 6900, 1100 psi; Tekscan Inc, Boston, MA) with various opening wedge sizes of 3, 5, and 7 mm both without and with a distal soft tissue release (DSTR). The contact force, area, and peak contact stress were compared among groups using analysis of variance and post hoc multiple comparisons over the untreated (Dunnett test, P < .05).

Results:

The mean contact force was 47.7 ± 33.5 N for untreated specimens. This increased sequentially with opening wedge size and reached statistical significance for 7-mm opening wedge (129.7 ± 62.3 N, P = .01) and 7-mm wedge + DSTR (134.8 ± 60.5 N, P = .008). The mean peak contact stress was 2.8 ± 1.3 MPa for the untreated specimens and increased incrementally with wedge size to 5.7 ± 3.0 MPa for 7-mm wedge only (P = .03) and 5.6 ± 2.5 MPa for 7-mm wedge + DSTR (P = .05). The contact area increased with corrections, but none reached significance.

Conclusion:

With increasing opening wedge size, loading of the TMT-1 joint increases. Joint stresses higher than 4.7 MPa have been shown to be chondrotoxic, potentially predisposing patients to arthritic joint changes following POWO.

Level of Evidence:

XXXXXX

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.

Collagen fibres determine the crack morphology in articular cartilage

Cracks in articular cartilage compromise tissue integrity and mechanical properties and lead to chondral lesions if untreated. An understanding of the mechanics of cracked cartilage may help in the prevention of cartilage deterioration and the development of tissue-engineered substitutes. The degeneration of cartilage in the presence of cracks may depend on the ultrastructure and composition of the tissue, which changes with aging, disease and habitual loading. It is unknown if the structural and compositional differences between immature and mature cartilage affect the mechanics of cartilage cracks, possibly predisposing one to a greater risk of degeneration than the other. We used a fibre-reinforced poro-viscoelastic swelling material model that accounts for large deformations and tension-compression non-linearity, and the finite element method to investigate the role of cartilage structure and composition on crack morphology and tissue mechanics. We demonstrate that the crack morphology predicted by our theoretical model agrees well with the histo-morphometric images of young and mature cracked cartilages under indentation loading. We also determined that the crack morphology was primarily dependent on collagen fibre orientation which differs as a function of cartilage depth and tissue maturity. The arcade-like collagen fibre orientation, first discussed by Benninghoff in his classical 1925 paper, appears to be beneficial for slowing the progression of tissue cracks by 'sealing' the crack and partially preserving fluid pressure during loading. Preservation of the natural load distribution between solid and fluid constituents of cartilage may be a key factor in slowing or preventing the propagation of tissue cracks and associated tissue matrix damage. STATEMENT OF SIGNIFICANCE: Cracks in articular cartilage can be detrimental to joint health if not treated, but it is not clear how they propagate and lead to tissue degradation. We used an advanced numerical model to determine the role of cartilage structure and composition on crack morphology under loading. Based on the structure and composition found in immature and mature cartilages, our model successfully predicts the crack morphology in these cartilages and determines that collagen fibre as the major determinant of crack morphology. The arcade-like Benninghoff collagen fibre orientation appears to be crucial in 'sealing' the tissue crack and preserves normal fluid-solid load distribution in cartilage. Inclusion of the arcade-like fibre orientation in tissue-engineered construct may help improve its integration within the host tissue.

Influence of the Mechanical Environment on the Regeneration of Osteochondral Defects

Articular cartilage is a highly specialised connective tissue of diarthrodial joints which provides a smooth, lubricated surface for joint articulation and plays a crucial role in the transmission of loads. In vivo cartilage is subjected to mechanical stimuli that are essential for cartilage development and the maintenance of a chondrocytic phenotype. Cartilage damage caused by traumatic injuries, ageing, or degradative diseases leads to impaired loading resistance and progressive degeneration of both the articular cartilage and the underlying subchondral bone. Since the tissue has limited self-repairing capacity due its avascular nature, restoration of its mechanical properties is still a major challenge. Tissue engineering techniques have the potential to heal osteochondral defects using a combination of stem cells, growth factors, and biomaterials that could produce a biomechanically functional tissue, representative of native hyaline cartilage. However, current clinical approaches fail to repair full-thickness defects that include the underlying subchondral bone. Moreover, when tested in vivo, current tissue-engineered grafts show limited capacity to regenerate the damaged tissue due to poor integration with host cartilage and the failure to retain structural integrity after insertion, resulting in reduced mechanical function. The aim of this review is to examine the optimal characteristics of osteochondral scaffolds. Additionally, an overview on the latest biomaterials potentially able to replicate the natural mechanical environment of articular cartilage and their role in maintaining mechanical cues to drive chondrogenesis will be detailed, as well as the overall mechanical performance of grafts engineered using different technologies.

Rheometer enabled study of cartilage frequency-dependent properties

Despite the well-established dependence of cartilage mechanical properties on the frequency of the applied load, most research in the field is carried out in either load-free or constant load conditions because of the complexity of the equipment required for the determination of time-dependent properties. These simpler analyses provide a limited representation of cartilage properties thus greatly reducing the impact of the information gathered hindering the understanding of the mechanisms involved in this tissue replacement, development and pathology. More complex techniques could represent better investigative methods, but their uptake in cartilage research is limited by the highly specialised training required and cost of the equipment. There is, therefore, a clear need for alternative experimental approaches to cartilage testing to be deployed in research and clinical settings using more user-friendly and financial accessible devices. Frequency dependent material properties can be determined through rheometry that is an easy to use requiring a relatively inexpensive device; we present how a commercial rheometer can be adapted to determine the viscoelastic properties of articular cartilage. Frequency-sweep tests were run at various applied normal loads on immature, mature and trypsinased (as model of osteoarthritis) cartilage samples to determine the dynamic shear moduli (G*, G′ G″) of the tissues. Moduli increased with increasing frequency and applied load; mature cartilage had generally the highest moduli and GAG depleted samples the lowest. Hydraulic permeability (K_H) was estimated from the rheological data and decreased with applied load; GAG depleted cartilage exhibited higher hydraulic permeability than either immature or mature tissues. The rheometer-based methodology developed was validated by the close comparison of the rheometer-obtained cartilage characteristics (G*, G′, G″, K_H) with results obtained with more complex testing techniques available in literature. Rheometry is relatively simpler and does not require highly capital intensive machinery and staff training is more accessible; thus the use of a rheometer would represent a cost-effective approach for the determination of frequency-dependent properties of cartilage for more comprehensive and impactful results for both healthcare professional and R&D.

Effect of collagen fibril distributions on the crack profile in articular cartilage

BACKGROUND AND OBJECTIVE

Cartilage cracks and fissures may occur due to certain daily life activities such as sports practice, blunt trauma, and matrix fibrillation during early osteoarthritis. These cracks could further grow at the macroscopic level, alter the load distribution pattern in the matrix, limit the joint range of motion, and disturb chondrocytes synthesis. Cracks' shape and deformations in the loaded cartilage may affect the subsequent mechanobiological processes in the long term, likely because of the altered fluid exchange and excessive local deformations in the vicinity of the damage site. The fibrillar structure of the cartilage matrix appeared to have a protective effect against excessive deformations and tissue failure. Hence, in the present study, a fibril reinforced biphasic cartilage model was used to assess the potential role of different fibril orientations on the profile of a vertical crack in cartilage after applying a compressive load.

METHODS

A 20 × 20 × 1.5 mm³ cartilage model was developed with a 0.7 mm length V-shape cut at the center. Using an impermeable indenter, a 27% compression was applied to immature, mature, and isotropic cartilage models. Each of immature and mature groups had 4 different split line directions with respect to the cut edges, including 90°, 45°, 0°, and random orientation. The latter represented the disrupted collagen fibril orientations in early osteoarthritis. The model was verified with the experimental results in the literature.

RESULTS

In the superficial zones, the larger angle between the split lines and cut edges resulted in a wider cut opening. In the absence of collagen fibrils, the isotropic model resulted in a closed edge profile. Also, under a consistently applied compression, the OA model, with random collagen fibril distribution on its surface, had the smallest load-bearing capacity compared to the other models.

CONCLUSIONS

Findings highlighted a primary role of collagen fibrils on the cut profile, which was more pronounced at dynamic rather than static conditions. Split lines perpendicular to the cut edges had some protective effects against the large dislocation of cut edges. These findings could be utilized to develop engineered tissues less susceptible to rupture. Moreover, the outcome of the present study can explain the potential causes of the crack propagation path reported in the literature.

Glycosaminoglycan depletion increases energy dissipation in articular cartilage under high-frequency loading

High-frequency material behavior of cartilage at macroscopic lengths is not widely understood, despite a wide range of frequencies and contact lengths experienced in vivo. For example, cartilage at different stages of matrix integrity can experience high-frequency loading during traumatic impact, making high-frequency behavior relevant in the context of structural failure. Therefore, this study examined macroscopic dissipative and mechanical responses of intact and glycosaminoglycan (GAG)-depleted cartilage under previously unexplored high-frequency loading. These dynamic responses were complemented with the evaluation of quasi-static responses. A custom dynamic mechanical analyzer was used to obtain dynamic behavior, and stress relaxation testing was performed to obtain quasi-static behavior. Under high-frequency loading, cartilage energy dissipation increased with GAG depletion and decreased with strain; dynamic modulus exhibited opposite trends. Similarly, under quasi-static loading, equilibrium modulus and relaxation time of cartilage decreased with GAG depletion. The increased energy dissipation after GAG depletion under high-frequency loading was likely due to increased viscoelastic dissipation. These findings broaden our understanding of fundamental properties of cartilage as a function of solid matrix integrity in an unprecedented loading regime. They also provide a foundation for analyzing energy dissipation associated with cartilage failure induced by traumatic impact.

Clinical Contrast-Enhanced Computed Tomography With Semi-Automatic Segmentation Provides Feasible Input for Computational Models of the Knee Joint

Computational models can provide information on joint function and risk of tissue failure related to progression of osteoarthritis (OA). Currently, the joint geometries utilized in modeling are primarily obtained via manual segmentation, which is time-consuming and hence impractical for direct clinical application. The aim of this study was to evaluate the applicability of a previously developed semi-automatic method for segmenting tibial and femoral cartilage to serve as input geometry for finite element (FE) models. Knee joints from seven volunteers were first imaged using a clinical computed tomography (CT) with contrast enhancement and then segmented with semi-automatic and manual methods. In both segmentations, knee joint models with fibril-reinforced poroviscoelastic (FRPVE) properties were generated and the mechanical responses of articular cartilage were computed during physiologically relevant loading. The mean differences in the absolute values of maximum principal stress, maximum principal strain, and fibril strain between the models generated from semi-automatic and manual segmentations were <1 MPa, <0.72% and <0.40%, respectively. Furthermore, contact areas, contact forces, average pore pressures, and average maximum principal strains were not statistically different between the models (p >0.05). This semi-automatic method speeded up the segmentation process by over 90% and there were only negligible differences in the results provided by the models utilizing either manual or semi-automatic segmentations. Thus, the presented CT imaging-based segmentation method represents a novel tool for application in FE modeling in the clinic when a physician needs to evaluate knee joint function.

Etoricoxib decreases subchondral bone mass and attenuates biomechanical properties at the early stage of osteoarthritis in a mouse model

Etoricoxib, a selective Cyclooxygenase-2 (COX-2) inhibitor, is commonly used in osteoarthritis (OA) for pain relief, however, little is known about the effects on subchondral bone. In the current study, OA was induced via destabilization of the medial meniscus (DMM) in C57BL/6 mice. Two days after surgery, mice were treated with different concentrations of Etoricoxib. Four weeks after treatment, micro computed tomography (Micro-CT) analysis, histological analysis, atomic force microscopy (AFM) analysis, and scanning electron microscopy (SEM) were performed to evaluate OA progression. We demonstrated that Etoricoxib inhibited osteophyte formation in the subchondral bone. However, it also reduced the bone volume fraction (BV/TV), lowered trabecular thickness (Tb.Th), and more microfractures and pores were observed in the subchondral bone. Moreover, Etoricoxib reduced the elastic modulus of subchondral bone. Exposure to Etoricoxib further increased the empty/total osteocyte ratio of the subchondral bone. Etoricoxib did not show significant improvement in articular cartilage destruction and synovial inflammation in early OA. Together, our observations suggested that although Etoricoxib can relieve OA-induced pain and inhibit osteophyte formation in the subchondral bone, it can also change the microstructures and biomechanical properties of subchondral bone, promote subchondral bone loss, and reduce subchondral bone quality in early OA mice.

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.

Depth-dependent ratcheting strains of young and adult articular cartilages by experiments and predictions

Background

Ratcheting strain is produced due to the repeated accumulation of compressive strain in cartilage and may be a precursor to osteoarthritis. The aim of this study was to investigate the ratcheting behaviors of young and adult articular cartilages under cyclic compression by experiments and theoretical predictions.

Methods

A series of uniaxial cyclic compression tests were conducted for young and adult cartilage, and the effects of different loading conditions on their ratcheting behaviors were probed. A theoretical ratcheting model was constructed and applied to predict the ratcheting strains of young and adult cartilages with different loading conditions.

Results

Ratcheting strains of young and adult cartilages rapidly increased at the initial stage, followed by a slower increase in subsequent stages. The strain accumulation value and its rate for young cartilage were greater than them for adult cartilage. The ratcheting strains of the two groups of cartilage samples decreased with increasing stress rate, while they increased with increasing stress amplitude. As the stress amplitude increased, the gap between the ratcheting strains of young and adult cartilages increased gradually. The ratcheting strains of young and adult cartilages decreased along the cartilage depth from the surface to the deep layer. The ratcheting strains of different layers increased with the compressive cycle, and the difference among the three layers was noticeable. Additionally, the theoretical predictions agreed with the experimental data.

Conclusions

Overall, the ratcheting behavior of articular cartilage is affected by the degree of articular cartilage maturation.

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.

Computational evaluation of altered biomechanics related to articular cartilage lesions observed in vivo

Chondral lesions provide a potential risk factor for development of osteoarthritis. Despite the variety of in vitro studies on lesion degeneration, in vivo studies that evaluate relation between lesion characteristics and the risk for the possible progression of OA are lacking. Here, we aimed to characterize different lesions and quantify biomechanical responses experienced by surrounding cartilage tissue. We generated computational knee joint models with nine chondral injuries based on clinical in vivo arthrographic computed tomography images. Finite element models with fibril-reinforced poro(visco)elastic cartilage and menisci were constructed to simulate physiological loading. Systematically, the lesions experienced increased peak values of maximum principal strain, maximum shear strain, and minimum principal strain in the surrounding chondral tissue (p < 0.01) compared with intact tissue. Depth, volume, and area of the lesion correlated with the maximum shear strain (p < 0.05, Spearman rank correlation coefficient ρ = 0.733-0.917). Depth and volume of the lesion correlated also with the maximum principal strain (p < 0.05, ρ = 0.767, and ρ = 0.717, respectively). However, the lesion area had non-significant correlation with this strain parameter (p = 0.06, ρ = 0.65). Potentially, the introduced approach could be developed for clinical evaluation of biomechanical risks of a chondral lesion and planning an intervention. Statement of Clinical Relevance: In this study, we computationally characterized different in vivo chondral lesions and evaluated their risk of cartilage degeneration. This information is vital in decision-making for intervention in order to prevent post-traumatic osteoarthritis. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res.

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.

The role of subchondral bone, and its histomorphology, on the dynamic viscoelasticity of cartilage, bone and osteochondral cores

OBJECTIVE

Viscoelastic properties of articular cartilage have been characterised at physiological frequencies. However, studies investigating the interaction between cartilage and subchondral bone and the influence of underlying bone histomorphometry on the viscoelasticity of cartilage are lacking.

METHOD

Dynamic Mechanical Analysis (DMA) has been used to quantify the dynamic viscoelasticity of bovine tibial plateau osteochondral cores, over a frequency sweep from 1 to 88 Hz. Specimens (approximately aged between 18 and 30 months) were neither osteoarthritic nor otherwise compromised. A maximum nominal stress of 1.7 MPa was induced. Viscoelastic properties of cores have been compared with that of its components (cartilage and bone) in terms of the elastic and viscous components of both structural stiffness and material modulus. Micro-computed tomography scans were used to quantify the histomorphological properties of the subchondral bone.

RESULTS

Opposing frequency-dependent loss stiffness, and modulus, trends were witnessed for osteochondral tissues: for cartilage it increased logarithmically (P < 0.05); for bone it decreased logarithmically (P < 0.05). The storage stiffness of osteochondral cores was logarithmically frequency-dependent (P < 0.05), however, the loss stiffness was typically frequency-independent (P > 0.05). A linear relationship between the subchondral bone plate (SBP) thickness and cartilage thickness (P < 0.001) was identified. Cartilage loss modulus was linearly correlated to bone mineral density (BMD) (P < 0.05) and bone volume (P < 0.05).

CONCLUSION

The relationship between the subchondral bone histomorphometry and cartilage viscoelasticity (namely loss modulus) and thickness, have implications for the initiation and progression of osteoarthritis (OA) through an altered ability of cartilage to dissipate energy.

Mechanically stimulated osteochondral organ culture for evaluation of biomaterials in cartilage repair studies

Surgical procedures such as microfracture or autologous chondrocyte implantation have been used to treat articular cartilage lesions; however, repair often fails in terms of matrix organization and mechanical behaviour. Advanced biomaterials and tissue engineered constructs have been developed to improve cartilage repair; nevertheless, their clinical translation has been hampered by the lack of reliable in vitro models suitable for pre-clinical screening of new implants and compounds. In this study, an osteochondral defect model in a bioreactor that mimics the multi-axial motion of an articulating joint, was developed. Osteochondral explants were obtained from bovine stifle joints, and cartilage defects of 4 mm diameter were created. The explants were used as an interface against a ceramic ball applying dynamic compressive and shear loading. Osteochondral defects were filled with chondrocytes-seeded fibrin-polyurethane constructs and subjected to mechanical stimulation. Cartilage viability, proteoglycan accumulation and gene expression of seeded chondrocytes were compared to free swelling controls. Cells within both cartilage and bone remained viable throughout the 10-day culture period. Loading did not wear the cartilage, as indicated by histological evaluation and glycosaminoglycan release. The gene expression of seeded chondrocytes indicated a chondrogenic response to the mechanical stimulation. Proteoglycan 4 and cartilage oligomeric matrix protein were markedly increased, while mRNA ratios of collagen type II to type I and aggrecan to versican were also enhanced. This mechanically stimulated osteochondral defect culture model provides a viable microenvironment and will be a useful pre-clinical tool to screen new biomaterials and biological regenerative therapies under relevant complex mechanical stimuli. STATEMENT OF SIGNIFICANCE: Articular cartilage lesions have a poor healing capacity and reflect one of the most challenging problems in orthopedic clinical practice. The aim of current research is to develop a testing system to assess biomaterials for implants, that can permanently replace damaged cartilage with the original hyaline structure and can withstand the mechanical forces long term. Here, we present an osteochondral ex vivo culture model within a cartilage bioreactor, which mimics the complex motion of an articulating joint in vivo. The implementation of mechanical forces is essential for pre-clinical testing of novel technologies in the field of cartilage repair, biomaterial engineering and regenerative medicine. Our model provides a unique opportunity to investigate healing of articular cartilage defects in a physiological joint-like environment.

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.

New algorithm for simulation of proteoglycan loss and collagen degeneration in the knee joint: Data from the osteoarthritis initiative

Osteoarthritis is a harmful joint disease but prediction of disease progression is problematic. Currently, there is only one modeling framework which can be applied to predict the progression of knee osteoarthritis but it only considers degenerative changes in the collagen fibril network. Here, we have developed the framework further by considering all of the major tissue changes (proteoglycan content, fluid flow, and collagen fibril network) occurring in osteoarthritis. While excessive levels of tissue stresses controlled degeneration of the collagen fibril network, excessive levels of tissue strains controlled the decrease in proteoglycan content and the increase in permeability. We created four knee joint models with increasing degrees of complexity based on the depth-wise composition. Models were tested for normal and abnormal, physiologically relevant, loading conditions in the knee. Finally, the predicted depth-wise compositional changes from each model were compared against experimentally observed compositional changes in vitro. The model incorporating the typical depth-wise composition of cartilage produced the best match with experimental observations. Consistent with earlier in vitro experiments, this model simulated the greatest proteoglycan depletion in the superficial and middle zones, while the collagen fibril degeneration was located mostly in the superficial zone. The presented algorithm can be used for predicting simultaneous collagen degeneration and proteoglycan loss during the development of knee osteoarthritis. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1673-1683, 2018.

Comparison between in vitro and in vivo cartilage overloading studies based on a systematic literature review

Methodological differences between in vitro and in vivo studies on cartilage overloading complicate the comparison of outcomes. The rationale of the current review was to (i) identify consistencies and inconsistencies between in vitro and in vivo studies on mechanically-induced structural damage in articular cartilage, such that variables worth interesting to further explore using either one of these approaches can be identified; and (ii) suggest how the methodologies of both approaches may be adjusted to facilitate easier comparison and therewith stimulate translation of results between in vivo and in vitro studies. This study is anticipated to enhance our understanding of the development of osteoarthritis, and to reduce the number of in vivo studies. Generally, results of in vitro and in vivo studies are not contradicting. Both show subchondral bone damage and intact cartilage above a threshold value of impact energy. At lower loading rates, excessive loads may cause cartilage fissuring, decreased cell viability, collagen network de-structuring, decreased GAG content, an overall damage increase over time, and low ability to recover. This encourages further improvement of in vitro systems, to replace, reduce, and/or refine in vivo studies. However, differences in experimental set up and analyses complicate comparison of results. Ways to bridge the gap include (i) bringing in vitro set-ups closer to in vivo, for example, by aligning loading protocols and overlapping experimental timeframes; (ii) synchronizing analytical methods; and (iii) using computational models to translate conclusions from in vitro results to the in vivo environment and vice versa. © 2018 The Authors. Journal of Orthopaedic Research® Published by Wiley Periodicals, Inc. J Orthop Res 9999:1-11, 2018.

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.

Effect of frequency on crack growth in articular cartilage

Cracks can occur in the articular cartilage surface due to the mechanical loading of the synovial joint, trauma or wear and tear. However, the propagation of such cracks under different frequencies of loading is unknown. The objective of this study was to determine the effect of frequency of loading on the growth of a pre-existing crack in cartilage specimens subjected to cyclic tensile strain. A 2.26 mm crack was introduced into cartilage specimens and crack growth was achieved by applying a sinusoidally varying tensile strain at frequencies of 1, 10 and 100 Hz (i.e. corresponding to normal, above normal and up to rapid heel-strike rise times, respectively). These frequencies were applied with a strain of between 10–20% and the crack length was measured at 0, 20, 50, 100, 500, 1000, 5000 and 10,000 cycles of strain. Crack growth increased with increasing number of cycles. The maximum crack growth was 0.6 ± 0.3 (mean ± standard deviation), 0.8 ± 0.2 and 1.1 ± 0.4 mm at frequencies of 1, 10 and 100 Hz, respectively following 10,000 cycles. Mean crack growth were 0.3 ± 0.2 and 0.4 ± 0.2 at frequencies of 1 and 10 Hz, respectively. However, this value increased up to 0.6 ± 0.4 mm at a frequency of 100 Hz. This study demonstrates that crack growth was greater at higher frequencies. The findings of this study may have implications in the early onset of osteoarthritis. This is because rapid heel-strike rise times have been implicated in the early onset of osteoarthritis.

Engineered cartilage regeneration from adipose tissue derived-mesenchymal stem cells: A morphomolecular study on osteoblast, chondrocyte and apoptosis evaluation

The poor self-repair capacity of cartilage tissue in degenerative conditions, such as osteoarthritis (OA), has prompted the development of a variety of therapeutic approaches, such as cellular therapies and tissue engineering based on the use of mesenchymal stem cells (MSCs). The aim of this study is to demonstrate, for the first time, that the chondrocytes differentiated from rat adipose tissue derived-MSCs (AMSCs), are able to constitute a morphologically and biochemically healthy hyaline cartilage after 6 weeks of culture on a Collagen Cell Carrier (CCC) scaffold. In this study we evaluated the expression of some osteoblasts (Runt-related transcription factor 2 (RUNX2) and osteocalcin), chondrocytes (collagen I, II and lubricin) and apoptosis (caspase-3) biomarkers in undifferentiated AMSCs, differentiated AMSCs in chondrocytes cultured in monolayer and AMSCs-derived chondrocytes seeded on CCC scaffolds, by different techniques such as immunohistochemistry, ELISA, Western blot and gene expression analyses. Our results showed the increased expression of collagen II and lubricin in AMSCs-derived chondrocytes cultured on CCC scaffolds, whereas the expression of collagen I, RUNX2, osteocalcin and caspase-3 resulted decreased, when compared to the controls. In conclusion, this innovative basic study could be a possible key for future therapeutic strategies for articular cartilage restoration through the use of CCC scaffolds, to reduce the morbidity from acute cartilage injuries and degenerative joint diseases.

Fatigue strength of bovine articular cartilage-on-bone under three-point bending: the effect of loading frequency

BACKGROUND

The objective of this study was to determine the influence of loading frequency on the failure of articular cartilage-on-bone specimens under three-point bending.

METHODS

In this study, cyclic three-point bending was used to introduce failure into cartilage-on-bone specimens at varying loading frequencies. Sinusiodally varying maximum compressive loads in the range 40-130 N were applied to beam-shaped cartilage-on-bone specimens at frequencies of 1, 10, 50 and 100 Hz.

RESULTS

The number of cycles to failure decreased when loading frequency increased from normal and above gait (1 and 10 Hz) to impulsive loading frequencies (50 and 100 Hz). It was found that 67 and 27% of the specimens reached run-out at loading of 10,000 cycles at frequencies of 1 and 10 Hz, respectively. However, 0% of the specimens reached run-out at loading frequencies of 50 and 100 Hz.

CONCLUSION

The results indicate that increasing the loading frequency reduces the ability of specimens to resist fracture during bending. The findings underline the importance of the loading frequency concerning the failure of articular cartilage-on-bone and it may have implications in the early onset of osteoarthritis.

Viscoelastic properties of human and bovine articular cartilage: a comparison of frequency-dependent trends

Background

The purpose of this study was to compare the frequency-dependent viscoelastic properties of human and bovine cartilage.

Methods

Full-depth cartilage specimens were extracted from bovine and human femoral heads. Using dynamic mechanical analysis, the viscoelastic properties of eight bovine and six human specimens were measured over the frequency range 1 Hz to 88 Hz. Significant differences between bovine and human cartilage viscoelastic properties were assessed using a Mann–Whitney test (p < 0.05).

Results

Throughout the range of frequencies tested and for both species, the storage modulus was greater than the loss modulus and both were frequency-dependent. The storage and loss moduli of all human and bovine cartilage specimens presented a logarithmic relationship with respect to frequency. The mean human storage modulus ranged from 31.9 MPa to 43.3 MPa, while the mean bovine storage modulus ranged from 54.0 MPa to 80.5 MPa; bovine storage moduli were 1.7 to 1.9 times greater than the human modulus. Similarly, the loss modulus of bovine cartilage was 2.0 to 2.1 times greater than human. The mean human loss modulus ranged from 5.3 MPa to 8.5 MPa while bovine moduli ranged from 10.6 MPa to 18.1 MPa.

Conclusion

Frequency-dependent viscoelastic trends of bovine articular cartilage were consistent with those of human articular cartilage; this includes a similar frequency dependency and high-frequency plateau. Bovine cartilage was, however, ‘stiffer’ than human by a factor of approximately 2. With these provisos, bovine articular cartilage may be a suitable dynamic model for human articular cartilage.

The effect of loading rate on the development of early damage in articular cartilage

Experimental reports suggest that cartilage damage depends on strain magnitude. Additionally, because of its poro-viscoelastic nature, strain magnitude in cartilage can depend on strain rate. The present study explores whether cartilage damage may develop dependent on strain rate, even when the presented damage numerical model is strain-dependent but not strain-rate-dependent. So far no experiments have been distinguished whether rate-dependent cartilage damage occurs in the collagen or in the non-fibrillar network. Thus, this research presents a finite element analysis model where, among others, collagen and non-fibrillar matrix are incorporated as well as a strain-dependent damage mechanism for these components. Collagen and non-fibrillar matrix stiffness decrease when a given strain is reached until complete failure upon reaching a maximum strain. With such model, indentation experiments at increasing strain rates were simulated on cartilage plugs and damage development was monitored over time. Collagen damage increased with increasing strain rate from 21 to 42 %. In contrast, damage in the non-fibrillar matrix decreased with increasing strain rates from 72 to 34 %. Damage started to develop at a depth of approximately 20 % of the sample height, and this was more pronounced for the slow and modest loading rates. However, the most severe damage at the end of the compression step occurred at the surface for the plugs subjected to 120 mm/min strain rate. In conclusion, the present study confirms that the location and magnitude of damage in cartilage may be strongly dependent on strain rate, even when damage occurs solely through a strain-dependent damage mechanism.