Characterization of Tissue Response to Impact Loads Delivered Using a Hand-Held Instrument for Studying Articular Cartilage Injury

Synovial fluid mitochondrial DNA concentration reflects the degree of cartilage damage after naturally occurring articular injury

OBJECTIVE

To evaluate mitochondrial DNA (mtDNA) release from injured chondrocytes and investigate the utility of synovial fluid mtDNA concentration in early detection of posttraumatic osteoarthritis.

METHOD

We measured mtDNA release using four models of osteoarthritis: in vitro interleukin-1β stimulation of cultured equine chondrocytes, ex vivo mechanical impact of bovine cartilage explants, in vivo mechanical impact of equine articular cartilage, and naturally occurring equine intraarticular fracture. In our in vivo model, one group was treated with an intraarticular injection of the mitoprotective peptide SS-31 following cartilage injury. mtDNA content was quantified using qPCR. For naturally occurring cases of joint injury, clinical data (radiographs, arthroscopic video footage) were scored for criteria associated with degenerative joint disease.

RESULTS

Chondrocytes released mtDNA in the acute time frame following inflammatory and mechanical cellular stress in vitro. mtDNA was increased in equine synovial fluid following experimental and naturally occurring injury to the joint surface. In naturally occurring posttraumatic osteoarthritis, we found a strong positive correlation between the degree of cartilage damage and mtDNA concentration (r = 0.80, P = 0.0001). Finally, impact-induced mtDNA release was mitigated by mitoprotective treatment.

CONCLUSION

Changes in synovial fluid mtDNA occur following joint injury and correlate with the severity of cartilage damage. Mitoprotection mitigates increases in synovial fluid mtDNA suggesting that mtDNA release may reflect mitochondrial dysfunction. Further investigation of mtDNA as a potentially sensitive marker of early articular injury and response to mitoprotective therapy is warranted.

Finite Element Analysis of Different Osseocartilaginous Reconstruction Techniques in Animal Model Knees

Lesions of the articular cartilage are frequent in all age populations and lead to functional impairment. Multiple surgical techniques have failed to provide an effective method for cartilage repair. The aim of our research was to evaluate the effect of two different compression forces on three types of cartilage repair using finite element analysis (FEA). Initially, an in vivo study was performed on sheep. The in vivo study was prepared as following: Case 0-control group, without cartilage lesion; Case 1-cartilage lesion treated with macro-porous collagen implants; Case 2-cartilage lesion treated with collagen implants impregnated with bone marrow concentrate (BMC); Case 3-cartilage lesion treated with collagen implants impregnated with adipose-derived stem cells (ASC). Using the computed tomography (CT) data, virtual femur-cartilage-tibia joints were created for each Case. The study showed better results in bone changes when using porous collagen implants impregnated with BMC or ASC stem cells for the treatment of osseocartilaginous defects compared with untreated macro-porous implant. After 7 months postoperative, the presence of un-resorbed collagen influences the von Mises stress distribution, total deformation, and displacement on the Z axis. The BMC treatment was superior to ASC cells in bone tissue morphology, resembling the biomechanics of the control group in all FEA simulations.

A High-Throughput Mechanical Activator for Cartilage Engineering Enables Rapid Screening of in vitro Response of Tissue Models to Physiological and Supra-Physiological Loads

Articular cartilage is crucially influenced by loading during development, health, and disease. However, our knowledge of the mechanical conditions that promote engineered cartilage maturation or tissue repair is still incomplete. Current in vitro models that allow precise control of the local mechanical environment have been dramatically limited by very low throughput, usually just a few specimens per experiment. To overcome this constraint, we have developed a new device for the high throughput compressive loading of tissue constructs: the High Throughput Mechanical Activator for Cartilage Engineering (HiT-MACE), which allows the mechanoactivation of 6 times more samples than current technologies. With HiT-MACE we were able to apply cyclic loads in the physiological (e.g., equivalent to walking and normal daily activity) and supra-physiological range (e.g., injurious impacts or extensive overloading) to up to 24 samples in one single run. In this report, we compared the early response of cartilage to physiological and supra-physiological mechanical loading to the response to IL-1β exposure, a common but rudimentary in vitro model of cartilage osteoarthritis. Physiological loading rapidly upregulated gene expression of anabolic markers along the TGF-β1 pathway. Notably, TGF-β1 or serum was not included in the medium. Supra-physiological loading caused a mild catabolic response while IL-1β exposure drove a rapid anabolic shift. This aligns well with recent findings suggesting that overloading is a more realistic and biomimetic model of cartilage degeneration. Taken together, these findings showed that the application of HiT-MACE allowed the use of larger number of samples to generate higher volume of data to effectively explore cartilage mechanobiology, which will enable the design of more effective repair and rehabilitation strategies for degenerative cartilage pathologies.

Mesenchymal stromal cells donate mitochondria to articular chondrocytes exposed to mitochondrial, environmental, and mechanical stress

Articular cartilage has limited healing capacity and no drugs are available that can prevent or slow the development of osteoarthritis (OA) after joint injury. Mesenchymal stromal cell (MSC)-based regenerative therapies for OA are increasingly common, but questions regarding their mechanisms of action remain. Our group recently reported that although cartilage is avascular and relatively metabolically quiescent, injury induces chondrocyte mitochondrial dysfunction, driving cartilage degradation and OA. MSCs are known to rescue injured cells and improve healing by donating healthy mitochondria in highly metabolic tissues, but mitochondrial transfer has not been investigated in cartilage. Here, we demonstrate that MSCs transfer mitochondria to stressed chondrocytes in cell culture and in injured cartilage tissue. Conditions known to induce chondrocyte mitochondrial dysfunction, including stimulation with rotenone/antimycin and hyperoxia, increased transfer. MSC-chondrocyte mitochondrial transfer was blocked by non-specific and specific (connexin-43) gap-junction inhibition. When exposed to mechanically injured cartilage, MSCs localized to areas of matrix damage and extended cellular processes deep into microcracks, delivering mitochondria to chondrocytes. This work provides insights into the chemical, environmental, and mechanical conditions that can elicit MSC-chondrocyte mitochondrial transfer in vitro and in situ, and our findings suggest a new potential role for MSC-based therapeutics after cartilage injury.

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.

Pathogenesis of Osteoarthritis: Risk Factors, Regulatory Pathways in Chondrocytes, and Experimental Models

As the most common chronic degenerative joint disease, osteoarthritis (OA) is the leading cause of pain and physical disability, affecting millions of people worldwide. Mainly characterized by articular cartilage degradation, osteophyte formation, subchondral bone remodeling, and synovial inflammation, OA is a heterogeneous disease that impacts all component tissues of the articular joint organ. Pathological changes, and thus symptoms, vary from person to person, underscoring the critical need of personalized therapies. However, there has only been limited progress towards the prevention and treatment of OA, and there are no approved effective disease-modifying osteoarthritis drugs (DMOADs). Conventional treatments, including non-steroidal anti-inflammatory drugs (NSAIDs) and physical therapy, are still the major remedies to manage the symptoms until the need for total joint replacement. In this review, we provide an update of the known OA risk factors and relevant mechanisms of action. In addition, given that the lack of biologically relevant models to recapitulate human OA pathogenesis represents one of the major roadblocks in developing DMOADs, we discuss current in vivo and in vitro experimental OA models, with special emphasis on recent development and application potential of human cell-derived microphysiological tissue chip platforms.

Integrin α10β1-Selected Mesenchymal Stem Cells Mitigate the Progression of Osteoarthritis in an Equine Talar Impact Model

BACKGROUND

Early intervention with mesenchymal stem cells (MSCs) after articular trauma has the potential to limit progression of focal lesions and prevent ongoing cartilage degeneration by modulating the joint environment and/or contributing to repair. Integrin α10β1 is the main collagen type II binding receptor on chondrocytes, and MSCs that are selected for high expression of the α10 subunit have improved chondrogenic potential. The ability of α10β1-selected (integrin α10^high) MSCs to protect cartilage after injury has not been investigated.

PURPOSE

To investigate integrin α10^high MSCs to prevent posttraumatic osteoarthritis in an equine model of impact-induced talar injury.

STUDY DESIGN

Controlled laboratory study.

METHODS

Focal cartilage injuries were created on the tali of horses (2-5 years, n = 8) by using an impacting device equipped to measure impact stress. Joints were treated with 20 × 10⁶ allogenic adipose-derived α10^high MSCs or saline vehicle (control) 4 days after injury. Synovial fluid was collected serially and analyzed for protein content, cell counts, markers of inflammation (prostaglandin E2, tumor necrosis factor α) and collagen homeostasis (procollagen II C-propeptide, collagen type II cleavage product), and glycosaminoglycan content. Second-look arthroscopy was performed at 6 weeks, and horses were euthanized at 6 months. Joints were imaged with radiographs and quantitative 3-T magnetic resonance imaging. Postmortem examinations were performed, and India ink was applied to the talar articular surface to identify areas of cartilage fibrillation. Synovial membrane and osteochondral histology was performed, and immunohistochemistry was used to assess type I and II collagen and lubricin. A mixed effect model with Tukey post hoc and linear contrasts or paired t tests were used, as appropriate.

RESULTS

Integrin α10^high MSC-treated joints had less subchondral bone sclerosis on radiographs (P = .04) and histology (P = .006) and less cartilage fibrillation (P = .04) as compared with control joints. On gross pathology, less India ink adhered to impact sites in treated joints than in controls, which may be explained by the finding of more prominent lubricin immunostaining in treated joints. Prostaglandin E2 concentration in synovial fluid and mononuclear cell synovial infiltrate were increased in treated joints, suggesting possible immunomodulation by integrin α10^high MSCs.

CONCLUSION

Intra-articular administration of integrin α10^high MSCs is safe, and evidence suggests that the cells mitigate the effects of joint trauma.

CLINICAL RELEVANCE

This preclinical study indicates that intra-articular therapy with integrin α10^high MSCs after joint trauma may be protective against posttraumatic osteoarthritis.

Effects of mechanical injury on the tribological rehydration and lubrication of articular cartilage

Healthy articular cartilage is crucial to joint function, as it provides the low friction and load bearing surface necessary for joint articulation. Nonetheless, joint injury places patients at increased risk of experiencing both accelerated cartilage degeneration and wear, and joint dysfunction due to post-traumatic osteoarthritis (PTOA). In this study, we used our ex vivo convergent stationary contact area (cSCA) explant testing configuration to demonstrate that high-speed sliding of healthy tissues against glass could drive consistent and reproducible recovery of compression-induced cartilage deformation, through the mechanism of 'tribological rehydration'. In contrast, the presence of physical cartilage damage, mimicking those injuries known to precipitate PTOA, could compromise tribological rehydration and the sliding-driven recovery of cartilage function. Full-thickness cartilage injuries (i.e. fissures and chondral defects) markedly suppressed sliding-driven tribological rehydration. In contrast, impaction to cartilage, which caused surface associated damage, had little effect on the immediate tribomechanical response of explants to sliding (deformation/strain, tribological rehydration, and friction/lubricity). By leveraging the unique ability of the cSCA configuration to support tribological rehydration, this study permitted the first direct ex vivo investigation of injury-dependent strain and friction outcomes in cartilage under testing conditions that replicate and maintain physiologically-relevant levels of fluid load support and frictional outcomes under high sliding speeds (80 mm/s) and moderate compressive stresses (~0.3 MPa). Understanding how injury alters cartilage tribomechanics during sliding sheds light on mechanisms by which cartilage's long-term resilience and low frictional properties are maintained, and can guide studies investigating the functional consequences of physical injury and joint articulation on cartilage health, disease, and rehabilitation.

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.

Biomechanics of osteochondral impact with cushioning and graft Insertion: Cartilage damage is correlated with delivered energy

Articular cartilage is susceptible to impact injury. Impact may occur during events ranging from trauma to surgical insertion of an OsteoChondral Graft (OCG) into an OsteoChondral Recipient site (OCR). To evaluate energy density as a mediator of cartilage damage, a specialized drop tower apparatus was used to impact adult bovine samples while measuring contact force, cartilage surface displacement, and OCG advancement. When a single impact was applied to an isolated (non-inserted) OCG, force and surface displacement each rose monotonically and then declined. In each of five sequential impacts of increasing magnitude, applied to insert an OCG into an OCR, force rose rapidly to an initial peak, with minimal OCG advancement, and then to a second prolonged peak, with distinctive oscillations. Energy delivered to cartilage was confirmed to be higher with larger drop height and mass, and found to be lower with an interposed cushion or OCG insertion into an OCR. For both single and multiple impacts, the total energy density delivered to the articular cartilage correlated to damage, quantified as total crack length. The corresponding fracture toughness of the articular cartilage was 12.0 mJ/mm². Thus, the biomechanics of OCG insertion exhibits distinctive features compared to OCG impact without insertion, with energy delivery to the articular cartilage being a factor highly correlated with damage.

Mitoprotective therapy preserves chondrocyte viability and prevents cartilage degeneration in an ex vivo model of posttraumatic osteoarthritis

No disease-modifying osteoarthritis (OA) drugs are available to prevent posttraumatic osteoarthritis (PTOA). Mitochondria (MT) mediate the pathogenesis of many degenerative diseases, and recent evidence indicates that MT dysfunction is a peracute (within minutes to hours) response of cartilage to mechanical injury. The goal of this study was to investigate cardiolipin-targeted mitoprotection as a new strategy to prevent chondrocyte death and cartilage degeneration after injury. Cartilage was harvested from bovine knee joints and subjected to a single, rapid impact injury (24.0 ±1.4 MPa, 53.8 ± 5.3 GPa/s). Explants were then treated with a mitoprotective peptide, SS-31 (1µM), immediately post-impact, or at 1, 6, or 12 h after injury, and then cultured for up to 7 days. Chondrocyte viability and apoptosis were quantified in situ using confocal microscopy. Cell membrane damage (lactate dehydrogenase activity) and cartilage matrix degradation (glycosaminoglycan loss) were quantified in cartilage-conditioned media. SS-31 treatment at all time points after impact resulted in chondrocyte viability similar to that of un-injured controls. This effect was sustained for up to a week in culture. Further, SS-31 prevented impact-induced chondrocyte apoptosis, cell membrane damage, and cartilage matrix degeneration.

CLINICAL SIGNIFICANCE

This study is the first investigation of cardiolipin-targeted mitoprotective therapy in cartilage. These results suggest that even when treatment is delayed by up to 12 h after injury, mitoprotection may be a useful strategy in the prevention of PTOA. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 9999:1-10, 2018.

Hypothermia Promotes Cell-Protective and Chondroprotective Effects After Blunt Cartilage Trauma

BACKGROUND

Cryotherapy is routinely administered after sports injuries of synovial joints. Although positive clinical effects on periarticular swelling and pain have been described, the effects on the cell biological activities of cartilage and synovial cells remain largely unknown so far.

HYPOTHESIS

Local hypothermia alleviates synovial reactions and prevents chondrocyte death as well as cartilage destructive processes after blunt cartilage trauma.

STUDY DESIGN

Controlled laboratory study.

METHODS

Human cartilage explants were impacted by a drop-tower apparatus (0.59 J) and cultured at 24 hours or 7 days in different temperature conditions (2 hours [short term], 16 hours [medium term], or throughout [long term] at 27°C; afterwards or throughout at 37°C). Besides, isolated human fibroblast-like synoviocytes (FLS) were stimulated with traumatized cartilage conditioned medium and cultured as mentioned above up to 4 days. The effects of hypothermia were evaluated by cell viability, gene expression, type II collagen synthesis and cleavage, as well as the release of matrix metalloproteinase (MMP)-2, MMP-13, and interleukin 6 (IL-6).

RESULTS

Seven days after trauma, hypothermic treatment throughout improved cell viability (short term: 10.1% [ P = .016]; medium term: 6% [ P = .0362]; long term: 12.5% [ P = .0039]). Short-term hypothermia attenuated the expression of catabolic MMP-13 (mRNA: -2.2-fold [ P = .0119]; protein: -2-fold [ P = .0238]). Whereas type II collagen synthesis (1.7-fold [ P = .0227]) was increased after medium-term hypothermia, MMP-13 expression (mRNA: -30.8-fold [ P = .0025]; protein: -10.3-fold [ P < .0001]) and subsequent cleavage of type II collagen (-1.1-fold [ P = .0489]) were inhibited. Long-term hypothermia further suppressed MMP release (pro-MMP-2: -3-fold [ P = .0222]; active MMP-2: -5.2-fold [ P = .0183]; MMP-13: -56-fold [ P < .0001]) and type II collagen breakdown (-1.6-fold [ P = .0036]). Four days after FLS stimulation, hypothermia significantly suppressed the gene expression of matrix-destructive enzymes after medium-term (MMP-3: -4.1-fold [ P = .0211]) and long-term exposure (a disintegrin and metalloproteinase with thrombospondin motifs 4 [ADAMTS4]: -4.3-fold [ P = .0045]; MMP-3: -25.8-fold [ P = .014]; MMP-13: -122-fold [ P = .0444]) and attenuated IL-6 expression by trend.

CONCLUSION

After blunt cartilage trauma, initial hypothermia for only 2 hours and/or 16 hours induced significant cell-protective and chondroprotective effects and promoted the anabolic activity of chondrocytes, while the expression of matrix-destructive enzymes by stimulated FLS was attenuated by prolonged hypothermia.

CLINICAL RELEVANCE

The findings of this preliminary ex vivo investigation indicate that optimized cryotherapy management after cartilage trauma might prevent matrix-degenerative processes associated with the pathogenesis of posttraumatic osteoarthritis.

Mitochondrial dysfunction is an acute response of articular chondrocytes to mechanical injury

Mitochondrial (MT) dysfunction is known to occur in chondrocytes isolated from end-stage osteoarthritis (OA) patients, but the role of MT dysfunction in the initiation and early pathogenesis of post-traumatic OA (PTOA) remains unclear. The objective of this study was to investigate chondrocyte MT function immediately following mechanical injury in cartilage, and to determine if the response to injury differed between a weight bearing region (medial femoral condyle; MFC) and a non-weight bearing region (distal patellofemoral groove; PFG) of the same joint. Cartilage was harvested from the MFC and PFG of 10 neonatal bovids, and subjected to injurious compression at varying magnitudes (5-17 MPa, 5-34 GPa/s) using a rapid single-impact model. Chondrocyte MT respiratory function, MT membrane polarity, chondrocyte viability, and cell membrane damage were assessed in situ. Cartilage impact resulted in MT depolarization and impaired MT respiratory function within 2 h of injury. Cartilage from a non-weight bearing region of the joint (PFG) was more sensitive to impact-induced MT dysfunction and chondrocyte death than cartilage from a weight-bearing surface (MFC). Our findings suggest that MT dysfunction is an acute response of chondrocytes to cartilage injury, and that MT may play a key mechanobiological role in the initiation and early pathogenesis of PTOA.

CLINICAL SIGNIFICANCE

Direct therapeutic targeting of MT function in the early post-injury time frame may provide a strategy to block perpetuation of tissue damage and prevent the development of PTOA. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:739-750, 2018.

Bioinspired Technologies to Connect Musculoskeletal Mechanobiology to the Person for Training and Rehabilitation

Musculoskeletal tissues respond to optimal mechanical signals (e.g., strains) through anabolic adaptations, while mechanical signals above and below optimal levels cause tissue catabolism. If an individual's physical behavior could be altered to generate optimal mechanical signaling to musculoskeletal tissues, then targeted strengthening and/or repair would be possible. We propose new bioinspired technologies to provide real-time biofeedback of relevant mechanical signals to guide training and rehabilitation. In this review we provide a description of how wearable devices may be used in conjunction with computational rigid-body and continuum models of musculoskeletal tissues to produce real-time estimates of localized tissue stresses and strains. It is proposed that these bioinspired technologies will facilitate a new approach to physical training that promotes tissue strengthening and/or repair through optimal tissue loading.

Changes in Joint Contact Mechanics in a Large Quadrupedal Animal Model After Partial Meniscectomy and a Focal Cartilage Injury

Acute mechanical damage and the resulting joint contact abnormalities are central to the initiation and progression of post-traumatic osteoarthritis (PTOA). Study of PTOA is typically performed in vivo with replicate animals using artificially induced injury features. The goal of this work was to measure changes in a joint contact stress in the knee of a large quadruped after creation of a clinically realistic overload injury and a focal cartilage defect. Whole-joint overload was achieved by excising a 5-mm wedge of the anterior medial meniscus. Focal cartilage defects were created using a custom pneumatic impact gun specifically developed and mechanically characterized for this work. To evaluate the effect of these injuries on joint contact mechanics, Tekscan (Tekscan, Inc., South Boston, MA) measurements were obtained pre-operatively, postmeniscectomy, and postimpact (1.2-J) in a nonrandomized group of axially loaded cadaveric sheep knees. Postmeniscectomy, peak contact stress in the medial compartment is increased by 71% (p = 0.03) and contact area is decreased by 35% (p = 0.001); the center of pressure (CoP) shifted toward the cruciate ligaments in both the medial (p = 0.004) and lateral (p = 0.03) compartments. The creation of a cartilage defect did not significantly change any aspect of contact mechanics measured in the meniscectomized knee. This work characterizes the mechanical environment present in a quadrupedal animal knee joint after two methods to reproducibly induce joint injury features that lead to PTOA.

Low-energy impact of human cartilage: predictors for microcracking the network of collagen

OBJECTIVE

We aimed to determine the minimum mechanical impact to cause microstructural damage in the network of collagen (microcracking) within human cartilage and hypothesized that energies below 0.1 J or 1 mJ/mm³ would suffice.

DESIGN

We completed 108 low-energy impact tests (0.05, 0.07, or 0.09 J; 0.75 or 1.0 m/s²) using healthy cartilage specimens from six male donors (30.2 ± 8.8 yrs old). Before and after impact we acquired, imaging the second harmonic generation (SHG), ten images from each specimen (50 μm depth, 5 μm step size), resulting in 2160 images. We quantified both the presence and morphology of microcracks. We then correlated test parameters (predictors) impact energy/energy dissipation density, nominal stress/stress rate, and strain/strain rate to microcracking and tested for significance. Where predictors significantly correlated with microstructural outcomes we fitted binary logistic regression plots with 95% confidence intervals (CIs).

RESULTS

No specimens presented visible damage following impact. We found that impact energy/energy dissipation density and nominal stress/stress rate were significant (P < 0.05) predictors of microcracking while both strain and strain rate were not. In our test configuration, an impact energy density of 2.93 mJ/mm³, an energy dissipation density of 1.68 mJ/mm³, a nominal stress of 4.18 MPa, and a nominal stress rate of 689 MPa/s all corresponded to a 50% probability of microcracking in the network of collagen.

CONCLUSIONS

An impact energy density of 1.0 mJ/mm³ corresponded to a ∼20% probability of microcracking. Such changes may initiate a degenerative cascade leading to post-traumatic osteoarthritis.

The influence of early degenerative changes on the vulnerability of articular cartilage to impact-induced injury

BACKGROUND

Recently, the structural changes in a bovine model of early degeneration were validated by our research group to be analogous to that in early human osteoarthritis. The hypothesis of this study was that the structural changes associated with increasing levels of degeneration would lead to higher levels of tissue damage in response to impact induced injury.

METHODS

A total of forty bovine patellae were obtained for this study. Cartilage-on-bone samples were extracted from the distal lateral quarter, a region known to be affected by varying levels of degeneration. A single impact drop test was applied to these samples delivering 2.3J of energy. A dynamic load cell and image capture at 2000fps allowed for the calculation of the reaction stress and coefficient of restitution. The extent of tissue damage was examined from the micro to ultrastructural levels using differential interference contrast optical microscopy and scanning electron microscopy respectively.

FINDINGS

The impact mechanical properties of mildly degenerate articular cartilage were not significantly different but showed a significantly larger amount of structural damage. From comparing the mechanical and structural response of intact and mildly degenerate cartilage, to tissue showing increased macro-scale tissue degeneration, the significance of the surface layer and fibrillar scale transverse interconnectivity in effectively attenuating impact loads is demonstrated in this study.

INTERPRETATION

This study shows that even though articular cartilage can appear visibly normal under macroscopic observation, the micro-scale structural changes associated with very early stage osteoarthritis can have a significant effect on its vulnerability to impact damage.

Post-traumatic osteoarthritis of the ankle: A distinct clinical entity requiring new research approaches

The diagnosis of ankle osteoarthritis (OA) is increasing as a result of advancements in non-invasive imaging modalities such as magnetic resonance imaging, improved arthroscopic surgical technology and heightened awareness among clinicians. Unlike OA of the knee, primary or age-related ankle OA is rare, with the majority of ankle OA classified as post-traumatic (PTOA). Ankle trauma, more specifically ankle sprain, is the single most common athletic injury, and no effective therapies are available to prevent or slow progression of PTOA. Despite the high incidence of ankle trauma and OA, ankle-related OA research is sparse, with the majority of clinical and basic studies pertaining to the knee joint. Fundamental differences exist between joints including their structure and molecular composition, response to trauma, susceptibility to OA, clinical manifestations of disease, and response to treatment. Considerable evidence suggests that research findings from knee should not be extrapolated to the ankle, however few ankle-specific preclinical models of PTOA are currently available. The objective of this article is to review the current state of ankle OA investigation, highlighting important differences between the ankle and knee that may limit the extent to which research findings from knee models are applicable to the ankle joint. Considerations for the development of new ankle-specific, clinically relevant animal models are discussed. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:440-453, 2017.