Intercellular signaling as a cause of cell death in cyclically impacted cartilage explants

A single dose of P188 prevents cell death in meniscal explants following impact injury

OBJECTIVE

To determine the efficacy of single and multiple administrations of Poloxamer 188 (P188) in saving meniscal cells following an injurious impact.

METHODS

Meniscal explants were harvested from both the lateral and medial menisci of Flemish Giant rabbits. After a 24-h incubation period, explants were subjected to 50% impact strain to simulate traumatic joint injury, and the explants were then placed in media with or without supplemented P188. Temporal administrations of P188 over a 14-day period were given based on one of 6 different treatments regimes. Over the 14-day period, explants were cyclically loaded to 10% strain at 1 Hz for 1 h per day, five days a week. Cell viability was assessed on day 14, with the remainder of the tissue being fixed to determine cell apoptosis levels and proteoglycan changes via histology.

RESULTS

The injurious impact proved to produce significant levels of cell death in meniscal explants. The ability of P188 to prevent cell death was not affected by the number of P188 doses (single versus multiple). P188 treatment proved to maintain cell viability levels comparable to those from unimpacted explants. There were no significant changes in cell apoptosis or proteoglycan coverage in the tissues over a 14-day period for any group, all treatment groups were statistically similar to the unimpacted explants.

CONCLUSION

A single dose of P188 following impact is all that is necessary to inhibit cell death in the meniscus following a traumatic impact. Thus, orthopaedic surgeons may choose to administer P188 in addition to treating any other acute damage due to a traumatic load to the knee, such as anterior cruciate ligament rupture, although more in depth in vivo studies are necessary.

Evidence that reduction in volume protects in situ articular chondrocytes from mechanical impact

Chondrocytes, the resident cells in articular cartilage, carry the burden of producing and maintaining the extracellular matrix (ECM). However, as these cells have a low proliferative capacity and are not readily replaced, chondrocyte death due to extreme forces may contribute to the pathogenesis of osteoarthritis (OA) after injury or may inhibit healing after osteochondral transplantation, a restorative procedure for damaged cartilage that requires a series of mechanical impacts to insert the graft. Consequently, there is a need to understand what factors influence the vulnerability of in situ chondrocytes to mechanical trauma. To this end, the objective of this study was to investigate how altering cell volume by different means (hydrostatic pressure, uniaxial load, and osmotic challenge with and without inhibition of regulatory volume decrease) affects the vulnerability of in situ chondrocytes to extreme mechanical forces. Using a custom experimental platform enabling testing of viable and intact murine cartilage-on-bone explants, we established a strong correlation between chondrocyte volume and vulnerability to impact injury wherein reduced volume was protective. Moreover, we found that the volume-perturbing interventions did not affect cartilage ECM mechanical properties, suggesting that their effects on chondrocyte vulnerability occurred at the cellular level. The findings of this study offer new avenues for novel strategies aimed at preventing chondrocyte loss during osteochondral grafting or to halting the progression of cell death after a joint destabilizing injury.

Osteoarthritis year in review 2018: biology

This Year in Review highlights a selection of articles published between the 2017 and 2018 Osteoarthritis Research Society International (OARSI) World Congress meetings within the field of osteoarthritis biology, presented at OARSI 2018. Selected articles were obtained from a PubMed search covering cartilage, subchondral bone, inflammation, ageing, pain and animal models. Studies focused on biomechanics, biomarkers, genetics and epigenetics, imaging and clinical studies were excluded due to their coverage in other articles within the OARSI Year in Review series. Significant themes including the role of progenitor cells in cartilage homeostasis and repair, novel signalling mechanisms controlling chondrocyte phenotypic stability and the influence of disrupted or senescent chondrocytes were identified and are discussed in this review. Overarching conclusions derived from these study areas indicate that promising avenues of intervention are on the horizon, however further understanding is required in order to target therapeutic treatments to suitable patient subgroups and disease stages.

Comparative Biomechanical Analysis of Stress-Strain State of the Elbow Joint After Displaced Radial Head Fractures

Radial head fractures are becoming a major public health problem and are an increasingly important target for both clinical and mechanical researchers. In this work, comparative biomechanical analyses of the stress–strain state of a healthy elbow joint and elbow joints with radial head compression from 2 to 5 mm due to injury are performed. Three-dimensional models of the elbow joint with cartilage surfaces and ligaments were constructed based on the results of computed tomography. This study is focused on an elbow joint range of motion ranging from 0° to 120° flexion. Analysis of the stress–strain state of cartilage and ligaments under the influence of functional loads is conducted using a finite element method (FEM) and the ABAQUS software package. The results show that with increasing compression of the radial head, contact stress increases at the olecranon, which can lead to cartilage damage. Analysis of displacement shows that compression of the radial head during full extension of the elbow joint leads to an increased humeral shift from 1.14° ± 0.22 in the healthy joint to 10.3° ± 2.13 during 5-mm compression of the radial head. Mathematical modeling performed in this study proved that reducing the height of the radial head and the contact area between the radial head and the humeral head led to increased medial collateral ligament stresses of up to 36 ± 3.8 MPa. This work confirmed that the head of the radius is the main stabilizing structure of the elbow joint and that the medial collateral ligament is the second structure responsible for valgus stability of the elbow joint.

In vitro models for the study of osteoarthritis

Osteoarthritis (OA) is a prevalent disease of most mammalian species and is a significant cause of welfare and economic morbidity in affected individuals and populations. In vitro models of osteoarthritis are vital to advance research into the causes of the disease, and the subsequent design and testing of potential therapeutics. However, a plethora of in vitro models have been used by researchers but with no consensus on the most appropriate model. Models attempt to mimic factors and conditions which initiate OA, or dissect the pathways active in the disease. Underlying uncertainty as to the cause of OA and the different attributes of isolated cells and tissues used mean that similar models may produce differing results and can differ from the naturally occurring disease. This review article assesses a selection of the in vitro models currently used in OA research, and considers the merits of each. Particular focus is placed on the more prevalent cytokine stimulation and load-based models. A brief review of the mechanism of these models is given, with their relevance to the naturally occurring disease. Most in vitro models have used supraphysiological loads or cytokine concentrations (compared with the natural disease) in order to impart a timely response from the cells or tissue assessed. Whilst models inducing OA-like pathology with a single stimulus can answer important biological questions about the behaviour of cells and tissues, the development of combinatorial models encompassing different physiological and molecular aspects of the disease should more accurately reflect the pathogenesis of the naturally occurring disease.

Cartilage and chondrocyte response to extreme muscular loading and impact loading: Can in vivo pre-load decrease impact-induced cell death?

BACKGROUND

Impact loading causes cartilage damage and cell death. Pre-loading prior to impact loading may protect cartilage and chondrocytes. However, there is no systematic evidence and understanding of the effects of pre-load strategies on cartilage damage and chondrocyte death. This study aimed at determining the effects of the pre-load history on impact-induced chondrocyte death in an intact joint.

METHODS

Patellofemoral joints from 42 rabbits were loaded by controlled quadriceps muscle contractions and an external impacter. Two extreme muscular loading conditions were used: (i) a short-duration, high intensity, static muscle contraction, and (ii) a long-duration, low-intensity, cyclic muscle loading protocol. A 5-Joule centrally-oriented, gravity-accelerated impact load was applied to the joints. Chondrocyte viability was quantified following the muscular loading protocols, following application of the isolated impact loads, and following application of the impact loads that were preceded by the muscular pre-loads. Joint contact pressures were measured for all loading conditions by a pressure-sensitive film.

FINDINGS

Comparing to cartilage injured by impact loading alone, cartilage pre-loaded by static, maximal intensity, short-term muscle loads had lower cell death, while cartilage pre-loaded by repetitive, low-intensity, long-term muscular loads has higher cell death. The locations of peak joint contact pressures were not strongly correlated with the locations of greatest cell death occurrence.

INTERPRETATION

Static, high intensity, short muscular pre-load protected cells from impact injury, whereas repetitive, low intensity, prolonged muscular pre-loading to the point of muscular fatigue left the chondrocytes vulnerable to injury. However, cell death seems to be unrelated to the peak joint pressures.

Evaluation of Single-Impact-Induced Cartilage Degeneration by Optical Coherence Tomography

Posttraumatic osteoarthritis constitutes a major cause of disability in our increasingly elderly population. Unfortunately, current imaging modalities are too insensitive to detect early degenerative changes of this disease. Optical coherence tomography (OCT) is a promising nondestructive imaging technique that allows surface and subsurface imaging of cartilage, at near-histological resolution, and is principally applicable in vivo during arthroscopy. Thirty-four macroscopically normal human cartilage-bone samples obtained from total joint replacements were subjected to standardized single impacts in vitro (range: 0.25 J to 0.98 J). 3D OCT measurements of impact area and adjacent tissue were performed prior to impaction, directly after impaction, and 1, 4, and 8 days later. OCT images were assessed qualitatively (DJD classification) and quantitatively using established parameters (OII, Optical Irregularity Index; OHI, Optical Homogeneity Index; OAI, Optical Attenuation Index) and compared to corresponding histological sections. While OAI and OHI scores were not significantly changed in response to low- or moderate-impact energies, high-impact energies significantly increased mean DJD grades (histology and OCT) and OII scores. In conclusion, OCT-based parameterization and quantification are able to reliably detect loss of cartilage surface integrity after high-energy traumatic insults and hold potential to be used for clinical screening of early osteoarthritis.

A high-throughput model of post-traumatic osteoarthritis using engineered cartilage tissue analogs

OBJECTIVE

A number of in vitro models of post-traumatic osteoarthritis (PTOA) have been developed to study the effect of mechanical overload on the processes that regulate cartilage degeneration. While such frameworks are critical for the identification therapeutic targets, existing technologies are limited in their throughput capacity. Here, we validate a test platform for high-throughput mechanical injury incorporating engineered cartilage.

METHOD

We utilized a high-throughput mechanical testing platform to apply injurious compression to engineered cartilage and determined their strain and strain rate dependent responses to injury. Next, we validated this response by applying the same injury conditions to cartilage explants. Finally, we conducted a pilot screen of putative PTOA therapeutic compounds.

RESULTS

Engineered cartilage response to injury was strain dependent, with a 2-fold increase in glycosaminoglycan (GAG) loss at 75% compared to 50% strain. Extensive cell death was observed adjacent to fissures, with membrane rupture corroborated by marked increases in lactate dehydrogenase (LDH) release. Testing of established PTOA therapeutics showed that pan-caspase inhibitor [Z-VAD-FMK (ZVF)] was effective at reducing cell death, while the amphiphilic polymer [Poloxamer 188 (P188)] and the free-radical scavenger [N-Acetyl-L-cysteine (NAC)] reduced GAG loss as compared to injury alone.

CONCLUSIONS

The injury response in this engineered cartilage model replicated key features of the response of cartilage explants, validating this system for application of physiologically relevant injurious compression. This study establishes a novel tool for the discovery of mechanisms governing cartilage injury, as well as a screening platform for the identification of new molecules for the treatment of PTOA.

Role of miR-146a in human chondrocyte apoptosis in response to mechanical pressure injury in vitro

MicroRNA (miR)-146a is known to be overexpressed in osteoarthritis (OA). However, the role of miR-146a in OA has not yet been fully elucidated. In the present study, we applied mechanical pressure of 10 MPa to human chondrocytes for 60 min in order to investigate the expression of miR-146a and apoptosis following the mechanical pressure injury. Normal human chondrocytes were transfected with an miR-146a mimic or an inhibitor to regulate miR-146a expression. Potential target genes of miR-146a were predicted using bioinformatics. Moreover, luciferase reporter assay confirmed that Smad4 was a direct target of miR-146a. The expression levels of miR-146a, Smad4 and vascular endothelial growth factor (VEGF) were quantified by quantitative reverse transcription PCR and/or western blot analysis. The effects of miR-146a on apoptosis were detected by Annexin V-fluorescein isothiocyanate (FITC)/propidium iodide (PI) flow cytometry. The results indicated that mechanical pressure affected chondrocyte viability and induced the early apoptosis of chondrocytes. Mechanical pressure injury increased the expression levels of miR-146a and VEGF and decreased the levels of Smad4 in the chondrocytes. In the human chondrocytes, the upregulation of miR-146a induced apoptosis, upregulated VEGF expression and downregulated Smad4 expression. In addition, the knockdown of miR-146a reduced cell apoptosis, upregulated Smad4 expression and downregulated VEGF expression. Smad4 was identified as a direct target of miR-146a by harboring a miR‑146a binding sequence in the 3'-untranslated region (3'-UTR) of its mRNA. Furthermore, the upregulation of VEGF induced by miR‑146a was mediated by Smad4 in the chondrocytes subjected to mechanical pressure injury. These results demonstrated that miR-146a was overexpressed in our chondrocyte model of experimentally induced human mechanical injury, accompanied by the upregulation of VEGF and the downregulation of Smad4 in vitro. Moreover, our data suggest that miR-146a is involved in human chondrocyte apoptosis in response to mechanical injury, and may contribute to the mechanical injury of chondrocytes, as well as to the pathogenesis of OA by increasing the levels of VEGF and damaging the transforming growth factor (TGF)-β signaling pathway through the targeted inhibition of Smad4 in cartilage.

Posttraumatic Chondrocyte Apoptosis in the Murine Xiphoid

OBJECTIVE

To demonstrate posttraumatic chondrocyte apoptosis in the murine xiphoid after a crush-type injury and to ultimately determine the pathway (i.e., intrinsic or extrinsic) by which chondrocytes undergo apoptosis in response to mechanical injury.

DESIGN

The xiphoids of adult female wild-type mice were injured with the use of a modified Kelly clamp. Postinjury xiphoid cartilage was analyzed via 3 well-described independent means of assessing apoptosis in chondrocytes: hematoxylin and eosin staining, terminal deoxynucleotidyl transferase dUTP nick end labeling assay, and activated caspase-3 staining.

RESULTS

Injured specimens contained many chondrocytes with evidence of apoptosis, which is characterized by cell shrinkage, chromatin condensation, nuclear fragmentation, and the liberation of apoptotic bodies. There was a statistically significant increase in the number of chondrocytes undergoing apoptosis in the injured specimens as compared with the uninjured specimens.

CONCLUSIONS

Chondrocytes can be stimulated to undergo apoptosis as a result of mechanical injury. These experiments involving predominantly cartilaginous murine xiphoid in vivo establish a baseline for future investigations that employ the genetic and therapeutic modulation of chondrocyte apoptosis in response to mechanical injury.

Effects of cartilage impact with and without fracture on chondrocyte viability and the release of inflammatory markers

Post-traumatic arthritis (PTA) frequently develops after intra-articular fracture of weight bearing joints. Loss of cartilage viability and post-injury inflammation have both been implicated as possible contributing factors to PTA progression. To further investigate chondrocyte response to impact and fracture, we developed a blunt impact model applying 70%, 80%, or 90% surface-to-surface compressive strain with or without induction of an articular fracture in a cartilage explant model. Following mechanical loading, chondrocyte viability, and apoptosis were assessed. Culture media were evaluated for the release of double-stranded DNA (dsDNA) and immunostimulatory activity via nuclear factor kappa B (NF-κB) activity in Toll-like receptor (TLR) -expressing Ramos-Blue reporter cells. High compressive strains, with or without articular fracture, resulted in significantly reduced chondrocyte viability. Blunt impact at 70% strain induced a loss in viability over time through a combination of apoptosis and necrosis, whereas blunt impact above 80% strain caused predominantly necrosis. In the fracture model, a high level of primarily necrotic chondrocyte death occurred along the fracture edges. At sites away from the fracture, viability was not significantly different than controls. Interestingly, both dsDNA release and NF-κB activity in Ramos-Blue cells increased with blunt impact, but was only significantly increased in the media from fractured cores. This study indicates that the mechanism of trauma determines the type of chondrocyte death and the potential for post-injury inflammation.

A novel impaction technique to create experimental articular fractures in large animal joints

OBJECTIVE

A novel impaction fracture insult technique, developed for modeling post-traumatic osteoarthritis in porcine hocks in vivo, was tested to determine the extent to which it could replicate the cell-level cartilage pathology in human clinical intra-articular fractures.

DESIGN

Eight fresh porcine hocks (whole-joint specimens with fully viable chondrocytes) were subjected to fracture insult. From the fractured distal tibial surfaces, osteoarticular fragments were immediately sampled and cultured in vitro for 48 h. These samples were analyzed for the distribution and progression of chondrocyte death, using the Live/Dead assay. Five control joints, in which "fractures" were simulated by means of surgical osteotomy, were also similarly analyzed.

RESULTS

In the impaction-fractured joints, chondrocyte death was concentrated in regions adjacent to fracture lines (near-fracture regions), as evidenced by fractional cell death significantly higher (P < 0.0001) than in central non-fracture (control) regions. Although nominally similar spatial distribution patterns were identified in the osteotomized joints, fractional cell death in the near-osteotomy regions was nine-fold lower (P < 0.0001) than in the near-fracture regions. Cell death in the near-fracture regions increased monotonically during 48 h after impaction, dominantly within 1 mm from the fracture lines.

CONCLUSION

The impaction-fractured joints exhibited chondrocyte death characteristics reasonably consistent with those in human intra-articular fractures, but were strikingly different from those in "fractures" simulated by surgical osteotomy. These observations support promise of this new impaction fracture technique as a mechanical insult modality to replicate the pathophysiology of human intra-articular fractures in large animal joints in vivo.

Response of cartilage and meniscus tissue explants to in vitro compressive overload

OBJECTIVE

To examine the relative susceptibility of cartilage and meniscus tissues to mechanical injury by applying a single, controlled overload and observing cellular, biochemical, and mechanical changes.

DESIGN

Cartilage and meniscus tissue explants in radial confinement were subjected to a range of injury by indenting to 40% strain at three different strain rates: 0.5%/s (slow), 5%/s (medium), or 50%/s (fast). Following injury, samples were cultured for either 1 or 9 days. Explants were assayed for cell metabolic activity, water content, and sulfated glycosaminoglycan (sGAG) content. Mechanical properties of explants were determined in torsional shear and unconfined compression. Conditioned medium was assayed for sGAG and lactate dehydrogenase (LDH) release.

RESULTS

Peak injury force increased with strain rate but both tissues displayed little to no macroscopic damage. Cell metabolism was lowest in medium and fast groups on day 1. Cell lysis increased with peak injury force and loading rate in both tissues. In contrast, sGAG content and release did not significantly vary with loading rate. Additionally, mechanical properties did not significantly vary with loading rate in either tissue.

CONCLUSION

By use of a custom confinement chamber, large peak forces were obtained without macroscopic destruction of the explants. At the loads achieved in this studied, cell damage was induced without detectable physical or compositional changes. These results indicate that sub-failure injury can induce biologic damage that may not be readily detected and could be an early event in osteoarthritis genesis.

Repeated measurement of mechanical properties in viable osteochondral explants following a single blunt impact injury

The objective of this work was to develop a method for repeated same-site measurement of mechanical properties suitable for the detection of degenerative changes in a biologically active explant model after a single blunt impact injury. Focal blunt impact injuries to articular surfaces lead to local cartilage degeneration and loss of mechanical properties. We employed a repeated measurement methodology to determine variations in mechanical same-site properties before and after injury in living cartilage, with the hypothesis that normalization with initial mechanical properties may provide a clearer evaluation of impact effects and improve our understanding of the biologic responses to impact injury. Bovine osteochondral explants were cultured for up to 14 days after impact injury. Indentation tests were performed before and after impact injury to assess relative changes in mechanical properties. Creep strain increased significantly in impacted explants after 7 days and in both impacted and control explants after 14 days. Further analysis at 14 days revealed decreases in stretch factor beta, creep time constant and local compressive modulus. A repeated measures methodology reliably detected changes in the mechanical behaviour of viable osteochondral explants after a single impact injury.

Streaming potential-based arthroscopic device is sensitive to cartilage changes immediately post-impact in an equine cartilage injury model

Models of post-traumatic osteoarthritis where early degenerative changes can be monitored are valuable for assessing potential therapeutic strategies. Current methods for evaluating cartilage mechanical properties may not capture the low-grade cartilage changes expected at these earlier time points following injury. In this study, an explant model of cartilage injury was used to determine whether streaming potential measurements by manual indentation could detect cartilage changes immediately following mechanical impact and to compare their sensitivity to biomechanical tests. Impacts were delivered ex vivo, at one of three stress levels, to specific positions on isolated adult equine trochlea. Cartilage properties were assessed by streaming potential measurements, made pre- and post-impact using a commercially available arthroscopic device, and by stress relaxation tests in unconfined compression geometry of isolated cartilage disks, providing the streaming potential integral (SPI), fibril modulus (Ef), matrix modulus (Em), and permeability (k). Histological sections were stained with Safranin-O and adjacent unstained sections examined in polarized light microscopy. Impacts were low, 17.3 ± 2.7 MPa (n = 15), medium, 27.8 ± 8.5 MPa (n = 13), or high, 48.7 ± 12.1 MPa (n = 16), and delivered using a custom-built spring-loaded device with a rise time of approximately 1 ms. SPI was significantly reduced after medium (p = 0.006) and high (p<0.001) impacts. Ef, representing collagen network stiffness, was significantly reduced in high impact samples only (p < 0.001 lateral trochlea, p = 0.042 medial trochlea), where permeability also increased (p = 0.003 lateral trochlea, p = 0.007 medial trochlea). Significant (p < 0.05, n = 68) moderate to strong correlations between SPI and Ef (r = 0.857), Em (r = 0.493), log(k) (r = -0.484), and cartilage thickness (r = -0.804) were detected. Effect sizes were higher for SPI than Ef, Em, and k, indicating greater sensitivity of electromechanical measurements to impact injury compared to purely biomechanical parameters. Histological changes due to impact were limited to the presence of superficial zone damage which increased with impact stress. Non-destructive streaming potential measurements were more sensitive to impact-related articular cartilage changes than biomechanical assessment of isolated samples using stress relaxation tests in unconfined compression geometry. Correlations between electromechanical and biomechanical methods further support the relationship between non-destructive electromechanical measurements and intrinsic cartilage properties.

Distribution and progression of chondrocyte damage in a whole-organ model of human ankle intra-articular fracture

BACKGROUND

Despite the best current treatments, intra-articular fractures commonly cause posttraumatic osteoarthritis. In this disorder, death and dysfunction of chondrocytes associated with acute cartilage injury presumably plays an important role in triggering the pathomechanical cascade that eventually leads to whole-joint degeneration. Information regarding this cell-level cartilage injury, particularly at the whole-organ level in actual human joints, has been lacking. In this study, the distribution and progression of fracture-associated cell-level cartilage damage were assessed using a novel whole-organ model of human ankle intra-articular fracture.

METHODS

Seven normal human ankles harvested immediately following amputation were subjected to a transarticular compressive impaction insult that mimicked an injury mechanism typical of tibial plafond fractures. For each ankle, site-specific, time-dependent changes in chondrocyte viability in the fractured tibial surface were studied by means of live-dead assay, using a confocal laser-scanning microscope. Fractional chondrocyte death was measured at several time points, in the superficial zone of the cartilage in "fracture-edge" regions within 1 mm of the fracture lines, as well as in "non-fracture" regions more than 3 mm centrally away from the fracture lines.

RESULTS

All seven experimental fractures morphologically replicated tibial plafond fractures. Immediately post-fracture, superficial-zone chondrocyte death was significantly greater (p = 0.001) in fracture-edge regions (fractional cell death = 7.6%) than in non-fracture regions (1.6%). Progression of cell death over the next forty-eight hours was significantly faster in fracture-edge regions (p = 0.007), with the fractional cell death reaching 25.9%, which was again significantly higher (p < 0.001) than in non-fracture regions (8.6%).

CONCLUSIONS

Cell-level cartilage damage in human intra-articular fractures was characterized by acute chondrocyte death that predominated along fracture lines and that spontaneously progressed in the forty-eight hours following injury.

CLINICAL RELEVANCE

Progressive chondrocyte damage along fracture lines appears to be a reasonable target of therapeutic treatment to preserve the whole-joint cartilage metabolism in intra-articular fractures, eventually to mitigate the risk of posttraumatic osteoarthritis.

Response of engineered cartilage to mechanical insult depends on construct maturity

Injury to articular cartilage leads to degenerative changes resulting in a loss of mechanical and biochemical properties. In engineered cartilage, the injury response of developing constructs is unclear.

OBJECTIVE

To characterize the cellular response of tissue-engineered constructs cultured in chemically-defined medium after mechanical insult, either by compression-induced cracking, or by cutting, as a function of construct maturity.

METHODS

Primary immature bovine articular chondrocytes (4-6 weeks) were encapsulated in agarose hydrogel (2%, 30 millioncells/mL) and cultured in chemically-defined medium supplemented with Transforming growth factor (TGF)-β3 (10ng/mL, first 2 weeks). At early (5 days) and late (35 days) times in culture, subsets of constructs were exposed to mechanical overload to produce a crack in the tissue or were exposed to a sharp wound with a perpendicular cut. Constructs were returned to culture and allowed to recover in static conditions. Mechanical and biochemical properties were evaluated at 2-week intervals to day 70, and cellular viability was assessed at 2-week intervals to day 85.

RESULTS

Constructs injured early in culture recovered their mechanical stiffness back to control values, regardless of the mode of injury. Later in culture, when constructs exhibited properties similar to those of native cartilage, compression-induced cracking catastrophically damaged the bulk matrix of the tissue and resulted in permanent mechanical failure with persistent cell death. No such detrimental outcomes were observed with cutting. Biochemical content was similar across all groups irrespective of mode or time of injury.

CONCLUSIONS

Unlike native cartilage, engineered cartilage constructs exhibit a reparative capacity when the bulk integrity of the developing tissue is preserved after injury.

Articular cartilage: structure and regeneration

Articular cartilage (AC) has no or very low ability of self-repair, and untreated lesions may lead to the development of osteoarthritis. One method that has been proven to result in long-term repair or isolated lesions is autologous chondrocyte transplantation. However, first generation of these cells' implantation has limitations, and introducing new effective cell sources can improve cartilage repair. AC provides a resilient and compliant articulating surface to the bones in diarthrodial joints. It protects the joint by distributing loads applied to it, so preventing potentially damaging stress concentrations on the bone. At the same time it provides a low-friction-bearing surface to enable free movement of the joint. AC may be considered as a visco- or poro-elastic fiber-composite material. Fibrils of predominantly type II collagen provide tensile reinforcing to a highly hydrated proteoglycan gel. The tissue typically comprises 70% water and it is the structuring and retention of this water by the proteoglycans and collagen that is largely responsible for the remarkable ability of the tissue to support compressive loads.

New developments in osteoarthritis. Posttraumatic osteoarthritis: pathogenesis and pharmacological treatment options

Joint trauma can lead to a spectrum of acute lesions, including osteochondral fractures, ligament or meniscus tears and damage to the articular cartilage. This is often associated with intraarticular bleeding and causes posttraumatic joint inflammation. Although the acute symptoms resolve and some of the lesions can be surgically repaired, joint injury triggers a chronic remodeling process in cartilage and other joint tissues that ultimately manifests as osteoarthritis in a majority of cases. The objective of the present review is to summarize information on pathogenetic mechanisms involved in the acute and chronic consequences of joint trauma and discuss potential pharmacological interventions. The focus of the review is on the early events that follow joint trauma since therapies for posttraumatic joint inflammation are not available and this represents a unique window of opportunity to limit chronic consequences.

Acute repair of chondrocytes in the rabbit tibiofemoral joint following blunt impact using P188 surfactant and a preliminary investigation of its long-term efficacy

Recent studies have indicated that there may be a correlation between acute chondrocyte damage and joint degeneration reminiscent of early-stage osteoarthritis (OA). P188 surfactant has been shown to acutely restore the integrity of damaged chondrocytes; however, its long-term efficacy is unknown. The hypothesis of this study was that a single injection of P188 into a traumatized joint would acutely repair damaged cell membranes and maintain their viability in the long term. Twelve rabbits were divided into two groups, with and without P188, and sacrificed 4 days after tibiofemoral (TF) impact. Another six rabbits were sacrificed after 6 weeks and divided into two groups, with and without P188 treatment immediately posttrauma. Treatment with P188 increased the viable cell density 4 days posttrauma. A higher density of viable cells was also documented 6 weeks posttrauma in the treated versus untreated limb. The results of the current study confirm the acute efficacy of P188 treatment, and may suggest long-term efficacy of treatment, but additional studies are still needed to investigate the chronic implications of the acute repair of cells in the traumatized joint.

Topographic Patterns of Cartilage Lesions in Knee Osteoarthritis

OBJECTIVE: Treatments for articular cartilage lesions could benefit from characterization of lesion patterns and their progression to end-stage osteoarthritis. The objective of this study was to identify, quantitatively, topographic patterns of cartilage lesions in the human knee. DESIGN: Photographs were taken of 127 unilateral distal femora (from 109 cadavers and 18 arthroplasty remnants) with full-thickness cartilage lesions. Using digital image analysis, the lesions were localized and normalized lesion size was determined for the patellofemoral groove (PFG) and the lateral and medial femoral condyles (LFC, MFC). Samples were classified into patterns using cluster analysis of the lesion size at each compartment. For each pattern, maps showing the extent and frequency of lesions were created. RESULTS: Four main patterns (a-d) were identified (each p<0.001), with the lesion size varying from small (a) to large in distinct regions (b-d). Pattern b had a predominant lesion (23% area) in the MFC, and smaller (<3%) lesions elsewhere. Pattern c had predominant lesions in the LFC (19%) and MFC (10%). Pattern d had a predominant lesion in the PFG (15%) and smaller lesions in the MFC (6%) and LFC (2%). The sub-patterns of a (a1, a2, a3) had relatively small lesions, with similarity between a2 and b, and a3 and d. CONCLUSION: The present methods facilitated quantitative identification of distinct topographic patterns of full-thickness cartilage lesions, based on lesion size and location. These results have implications for stratifying OA patients using precise quantitative methods and, with additional longitudinal data, targeting cartilage treatments.

Anti-apoptotic treatments prevent cartilage degradation after acute trauma to human ankle cartilage

OBJECTIVES

To investigate the effect of anti-apoptotic agents on cartilage degradation after a single impact to ankle cartilage.

DESIGN

Ten human normal tali were impacted with the impulse of 1 Ns generating peak forces in the range of 600 N using a 4 mm diameter indenter. Eight millimeter cartilage plugs containing the 4 mm diameter impacted core and a 4 mm adjacent ring were removed and cultured with or without P188 surfactant (8 mg/ml), caspase-3 (10 uM), or caspase-9 (2 uM) inhibitors for 48 h. Results were assessed in the superficial and middle-deep layers immediately after injury at day 0 and at 2, 7 and 14 days after injury by live/dead cell and Tunel assays and by histology with Safranin O/fast green staining.

RESULTS

A single impact to human articular cartilage ex vivo resulted in cell death, cartilage degeneration, and radial progression of apoptosis to the areas immediately adjacent to the impact. The P188 was more effective in preventing cell death than the inhibitors of caspases. It reduced cell death by more than 2-fold (P<0.05) in the core and by about 30% in the ring in comparison with the impacted untreated control at all time points. P188 also prevented radial expansion of apoptosis in the ring region especially in the first 7 days post-impaction (7.5% Tunel-positive cells vs 46% in the untreated control; P<0.01). Inhibitors of caspase-3 or -9 were effective in reducing cell death in the impacted core only at early time points, but were ineffective in doing so in the ring. Mankin score was significantly improved in the P188 and caspase-3 treated groups.

CONCLUSIONS

Early intervention with the P188 and caspase-3 inhibitor may have therapeutic potential in the treatment of cartilage defects immediately after injury.

Material properties of fresh cold-stored allografts for osteochondral defects at 1 year

Little is known about the long-term properties of fresh cold-stored osteochondral allograft tissue. We hypothesized fresh cold-stored tissue would yield superior material properties in an in vivo ovine model compared to those using freeze-thawed acellular grafts. In addition, we speculated that a long storage time would yield less successful grafts. We created 10-mm defects in medial femoral condyles of 20 sheep. Defects were reconstructed with allograft plugs stored at 4 degrees C for 1, 14, and 42 days; control specimens were freeze-thawed or defect-only. At 52 weeks, animals were euthanized and retrieved grafts were analyzed for cell viability, gross morphology, histologic grade, and biomechanical and biochemical analysis. Explanted cold-stored tissue had superior histologic scores over freeze-thawed and defect-only grafts. Specimens stored for 1 and 42 days had higher equilibrium moduli and proteoglycan content than freeze-thawed specimens. We observed no difference among any of the cold-stored specimens for chondrocyte viability, histology, equilibrium aggregate modulus, proteoglycan content, or hypotonic swelling. Reconstructing cartilage defects with cold-stored allograft resulted in superior histologic and biomechanical properties compared with acellular freeze-thawed specimens; however, storage time did not appear to be a critical factor in the success of the transplanted allograft.

Osmolarity influences chondrocyte death in wounded articular cartilage

BACKGROUND

Mechanical injury results in chondrocyte death in articular cartilage. The purpose of the present study was to determine whether medium osmolarity affects chondrocyte death in injured articular cartilage.

METHODS

Osteochondral explants (n = 48) that had been harvested from the metacarpophalangeal joints of three-year-old cows were exposed to media with varying osmolarity (0 to 480 mOsm) for ninety seconds to allow in situ chondrocytes to respond to the altered osmotic environment. Explants were then wounded with a scalpel through the full thickness of articular cartilage, incubated in the same media for 2.5 hours, and transferred to 340-mOsm Dulbecco's Modified Eagle Medium (control medium) with further incubation for seven days. The spatial distribution of in situ chondrocyte death, percentage cell death, and marginal cell death at the wounded cartilage edge were compared as a function of osmolarity and time (2.5 hours compared with seven days) with use of confocal laser scanning microscopy.

RESULTS

In situ chondrocyte death was mainly localized to the superficial tangential zone of injured articular cartilage for the range of medium osmolarities (0 to 480 mOsm) at 2.5 hours and seven days. Therefore, a sample of articular cartilage from the superficial region (which included the scalpel-wounded cartilage edge) was studied with use of confocal laser scanning microscopy to compare the effects of osmolarity on percentage and marginal cell death in the superficial tangential zone. Compared with the control explants exposed to 340-mOsm Dulbecco's Modified Eagle Medium, percentage cell death in the superficial tangential zone was greatest for explants exposed to 0-mOsm (distilled water) and least for explants exposed to 480-mOsm Dulbecco's Modified Eagle Medium at 2.5 hours (13.0% at 340 mOsm [control], 35.5% at 0 mOsm, and 4.3% at 480 mOsm; p <or= 0.02 for paired comparisons) and seven days (9.9% at 340 mOsm [control], 37.7% at 0 mOsm, and 3.5% at 480 mOsm; p <or= 0.01 for paired comparisons). Marginal cell death in the superficial tangential zone decreased with increasing medium osmolarity at 2.5 hours (p = 0.001) and seven days (p = 0.002). There was no significant change in percentage cell death from 2.5 hours to seven days for explants initially exposed to any of the medium osmolarities.

CONCLUSIONS

Medium osmolarity significantly affects chondrocyte death in wounded articular cartilage. The greatest chondrocyte death occurs at 0 mOsm. Conversely, increased medium osmolarity (480 mOsm) is chondroprotective. The majority of cell death occurs within 2.5 hours, with no significant increase over seven days.

Cyclooxygenase inhibition lowers prostaglandin E2 release from articular cartilage and reduces apoptosis but not proteoglycan degradation following an impact load in vitro

This study investigated the release of prostaglandin E₂ (PGE₂) from cartilage following an impact load in vitro and the possible chondroprotective effect of cyclooxygenase-2 (COX-2) inhibition using non-steroidal anti-inflammatory drugs (NSAIDs).

Explants of human articular cartilage were subjected to a single impact load in a drop tower, and then cultured for 6 days in the presence of either a selective COX-2 inhibitor (celecoxib; 0.01, 0.1, 1.0 and 10 μM) or a non-selective COX inhibitor (indomethacin; 0.1 and 10 μM). The concentrations of PGE₂ and glycosaminoglycans (GAGs), a measure of cartilage breakdown, were measured in the explant culture medium at 3 and 6 days post-impact. Apoptotic cell death was measured in frozen explant sections by the terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling (TUNEL) method.

PGE₂ levels were increased by more than 20-fold in the medium of explants at both 3 (p = 0.012) and 6 days (p = 0.004) following impact, compared with unloaded controls. In the presence of celecoxib and indomethacin, the PGE₂ levels were reduced in a dose-related manner. These inhibitors, however, had no effect in reducing the impact-induced release of GAGs from the cartilage matrix. Addition of celecoxib and indomethacin significantly reduced the number of trauma-induced apoptotic chondrocytes in cartilage explant sections.

In this study, a marked increase in PGE₂ was measured in the medium following an impact load on articular cartilage, which was abolished by the selective COX-2 inhibitor, celecoxib, and non-selective indomethacin. These inhibitors reduced chondrocyte apoptosis but no change was observed in the release of GAGs from the explants, suggesting that the COX/PGE₂ pathway is not directly responsible for cartilage breakdown following traumatic injury. Our in vitro study demonstrates that it is unlikely that COX-2 inhibition alone would slow down or prevent the development of secondary osteoarthritis.

Temporal effects of impact on articular cartilage cell death, gene expression, matrix biochemistry, and biomechanics

Articular cartilage injury can cause post-traumatic osteoarthritis, but early processes leading to the disease are not well understood. The objective of this study was to characterize two levels of impact loading at 24 h, 1 week, and 4 weeks in terms of cell death, gene expression, extracellular matrix biochemistry, and tissue biomechanical properties. The data show cell death increased and tissue stiffness decreased by 24 h following High impact (2.8 J). These degradative changes persisted at 1 and 4 weeks, and were further accompanied by measurable changes in ECM biochemistry. Moreover, following High impact at 24 h there were specific changes in gene expression that distinguished injured tissue from adjacent tissue that was not loaded. In contrast, Low impact (1.1 J) showed little change from control specimens at 24 h or 1 week. However, at 4 weeks, a significant increase in cell death and significant decrease in tissue stiffness were present. The constellation of findings indicates Low impacted tissue exhibited a delayed biological response. The study characterizes a model system for examining the biology of articular cartilage post-impact, as well as identifies possible time points and success criteria to be used in future studies employing intervention agents.

Preventing chondrocyte programmed cell death caused by iatrogenic injury

Cartilage repair technology is advancing at a rapid pace. However, all techniques share a common weakness-unintentional chondrocyte cell death resulting from cartilage injury that occurs during preparation of the defect site. The loss of chondrocytes at the edge of host cartilage is likely to contribute to failed integration of regenerated tissue or grafts to the surrounding cartilage. Recent studies have demonstrated that "apoptosis", or programmed cell death (PCD), may be responsible for much of the cell death caused by cartilage injury. Theoretically, inhibitors of key pathways responsible for PCD could rescue chondrocytes and improve the results of cartilage repair surgery. The purpose of this study was to test the hypothesis that short-term, intra-articular PCD inhibitor treatment can limit chondrocyte death in vivo following simulated preparation of host cartilage for a repair procedure. A microcurette was used to create full-thickness articular cartilage injuries to the femoral condyles of adult New Zealand White rabbits. Animals received daily intra-articular injections either with a potent PCD inhibitor or with vehicle alone. Treatment with the inhibitor resulted in a significant reduction in the percentage of chondrocytes undergoing PCD compared to controls [treated=10.1+/-2.4%; controls=26.5+/-3.6%; (p=0.0013)]. These results provide proof of concept for the use of PCD inhibitors to enhance the results of cartilage repair surgeries.

Indentation probing of human articular cartilage: Effect on chondrocyte viability

BACKGROUND

Clinical arthroscopic probes based on indentation testing are being developed. However, the biological effects of certain design parameters (i.e., tip geometry and size) and loading protocols (i.e., indentation depth, rate, and repetition) on human articular cartilage are unclear.

OBJECTIVE

Determine if indenter design and indentation protocol modulate mechanical injury of probed cartilage samples.

METHODS

The objectives of this study were to determine the effects of indentation testing using clinically applicable tips (0.4mm radius, plane- or sphere-ended) and protocols (indentation depths of 100, 200, or 300 microm, applied at a rate of 50 or 500 microm/s) on the extent and the pattern of chondrocyte death, should it occur. Grossly normal osteochondral blocks were harvested from human talar dome, indented, stained with live/dead dyes, and imaged en face on a fluorescence microscope.

RESULTS

The occurrence and the extent of cell death generally increased with indentation depth, being undetected at an indentation depth of 100 microm but marked at 300 microm. In addition, tip geometry affected the pattern of cell death: ring- and solid circle-shaped areas of cell deaths were apparent when compressed to 300 microm using plane- and sphere-ended indenters.

CONCLUSION

Indenter design and indentation protocol modulated the extent and the pattern of chondrocyte death. These results have implications for designing indentation probes and protocols, as well as clinicians performing arthroscopic probing.

The Biophysical Effects of a Single Impact Load on Human and Bovine Articular Cartilage

Impact injury to a joint is a known risk factor for the subsequent development of secondary osteoarthritis. An in vitro model, employing a drop-tower loading machine, was used to examine the effect of an impact load on isolated articular cartilage explants from human and bovine femoral heads. Two different types of impact experiment were performed. In the first, 4 mm diameter explants were loaded using a plane-ended impactor. In the second, a modified impactor was developed that had a central 4 mm diameter plane-ended indentor which was used to load the centre of 8 mm diameter explants. This enabled the unloaded outer ring of each explant to be compared with the loaded central core. The modulus values measured using the impactor were found to be higher, compared with the indentor in both species. Scanning electron microscopy showed that cartilage surrounding the loaded central region of the 8 mm explants protected the indented tissue, and these explants showed less damage than the 4 mm samples that were fully impacted. In addition, human cartilage was found to be less damaged than bovine, possibly as a consequence of the different structure as well as of a greater thickness. Both the source of the tissue and the nature of the impact affected the type of damage observed.

Comparison of techniques for determination of chondrocyte viability after thermal injury

OBJECTIVE

To compare 2 methods of quantitating chondrocyte viability and to determine chondrocyte response to thermal injury over time.

SAMPLE POPULATION

108 stifle joints from 54 adult rats.

PROCEDURES

Cartilage from the distal aspect of the femur was treated ex vivo with radiofrequency energy at a probe setting that would result in immediate partial-thickness chondrocyte death; untreated sections served as controls. Explants were cultured, and cell viability was compared by use of lactate dehydrogenase (LDH) histochemical staining and calcein AM and ethidium homodimer-1 confocal laser microscopy (CLM) cell viability staining. Terminal deoxynucleotidyl transferase-mediated X-dUTP nick end labeling (TUNEL) was used to detect apoptosis. All labeling studies were performed 0, 1, 3, 7, 14, and 21 days after treatment.

RESULTS

In the treated tissues, a greater percentage of viable cells were found with CLM, compared with LDH staining. This result contrasted that of control tissues in which LDH staining indicated a greater percentage of live cells than CLM. The greatest number of TUNEL-positive chondrocytes was present at day 3, declining at later time intervals.

CONCLUSIONS AND CLINICAL RELEVANCE

CLM and LDH histochemistry techniques yield different absolute numbers of live and dead cells, resulting in differing percentages of live or dead cells with each technique. These differences may be related to the enzymes responsible for activation in each technique and the susceptibility of these enzymes to thermal injury. Results of TUNEL indicate that apoptosis contributes to chondrocyte death after thermal injury, with a peak signal identified 3 days after insult.

The effect of finite compressive strain on chondrocyte viability in statically loaded bovine articular cartilage

Recent studies have reported that certain regimes of compressive loading of articular cartilage result in increased cell death in the superficial tangential zone (STZ). The objectives of this study were (1) to test the prevalent hypothesis that preferential cell death in the STZ results from excessive compressive strain in that zone, relative to the middle and deep zones, by determining whether cell death correlates with the magnitude of compressive strain and (2) to test the corollary hypothesis that the viability response of cells is uniform through the thickness of the articular layer when exposed to the same loading environment. Live cartilage explants were statically compressed by approximately 65% of their original thickness, either normal to the articular surface (axial loading) or parallel to it (transverse loading). Cell viability after 12 h was compared to the local strain distribution measured by digital image correlation. Results showed that the strain distribution in the axially loaded samples was highest in the STZ (77%) and lowest in the deep zone (55%), whereas the strain was uniformly distributed in the transversely loaded samples (64%). In contrast, axially and transversely loaded samples exhibited very similar profiles of cell death through the depth, with a preferential distribution in the STZ. Unloaded control samples showed negligible cell death. Thus, under prolonged static loading, depth-dependent variations in chondrocyte death did not correlate with the local depth-dependent compressive strain, and the prevalent hypothesis must be rejected. An alternative hypothesis, suggested by these results, is that superficial zone chondrocytes are more vulnerable to prolonged static loading than chondrocytes in the middle and deep zones.

Beneficial effects of intra-articular caspase inhibition therapy following osteochondral injury

OBJECTIVE

Recent studies have demonstrated that articular cartilage injury leads to chondrocyte death through a mechanism termed "apoptosis", or programmed cell death (PCD). Inhibitors of caspases, key enzymatic mediators of apoptosis, have been shown to block chondrocyte PCD. We hypothesized that short-term intra-articular administration of a potent caspase inhibitor would decrease chondrocyte PCD and subsequent cartilage degeneration following experimental osteochondral injury in rabbits.

METHODS

Adult New Zealand white rabbits were subjected to osteochondral injuries of their femoral condyles. Knees in the treatment group received daily intra-articular injections of the broad-spectrum caspase inhibitor Z-VAD-fmk for 7 days, while the control group received injections of vehicle alone. Seven days postinjury, one group of rabbits was sacrificed to assess levels of chondrocyte PCD. A second group was sacrificed 42 days postinjury for histological evaluation to measure cartilage degeneration and cartilage repair.

RESULTS

Seven days postinjury, there was a 45% reduction in chondrocyte PCD in the caspase inhibitor treated knees as compared to controls (P=0.01). Forty-two days postinjury, treated knees were found to have 17.9% greater chondrocyte survival (P<0.01) and 7.6% greater articular cartilage thickness (P=0.01).

CONCLUSIONS

Intra-articular administration of the caspase inhibitor Z-VAD-fmk effectively blocks chondrocyte PCD following experimental osteochondral injury in this model. Inhibition of chondrocyte PCD rescues chondrocytes that would otherwise die, limiting subsequent cartilage loss. To our knowledge, this study is the first to demonstrate that short-term inhibition of chondrocyte PCD leads to long-term preservation of cartilage in vivo.

Treatment with the non-ionic surfactant poloxamer P188 reduces DNA fragmentation in cells from bovine chondral explants exposed to injurious unconfined compression

Excessive mechanical loading to a joint has been linked with the development of post-traumatic osteoarthritis (OA). Among the suspected links between impact trauma to a joint and associated degeneration of articular cartilage is an acute reduction in chondrocyte viability. Recently, the non-ionic surfactant poloxamer 188 (P188) has been shown to reduce by approximately 50% the percentage of non-viable chondrocytes 24 h post-injury in chondral explants exposed to 25 MPa of unconfined compression. There is a question whether these acutely 'saved' chondrocytes will continue to degrade over time, as P188 is only thought to act by acute repair of damaged cell membranes. In order to investigate the degradation of traumatized chondrocytes in the longer term, the current study utilized TUNEL staining to document the percentage of cells suffering DNA fragmentation with and without an immediate 24 h period of exposure of the explants to P188 surfactant. In the current study, as in the previous study by this laboratory, chondral explants were excised from bovine metacarpophalangeal joints and subjected to 25 MPa of unconfined compression. TUNEL staining was performed at 1 h, 4 days, and 7 days post-impact. The current study found that P188 was effective in reducing the percentage of cells with DNA fragmentation in impacted explants by approximately 45% at 4 and 7 days post-impact. These data suggest that early P188 intervention was effective in preventing DNA fragmentation of injured chondrocytes. The current hypothesis is that this process was mitigated by the acute repair of damaged plasma membranes by the non-ionic surfactant P188, and that most repaired cells did not continue to degrade as measured by the fragmentation of their DNA.

Subchondral bone trauma causes cartilage matrix degeneration: an immunohistochemical analysis in a canine model

Joint instability was believed to be the main cause of osteoarthritis following non-fracture articular trauma. However, sudden high impact load through articular cartilage onto subchondral bone may also cause osteoarthritic changes.

OBJECTIVE

We asked whether early osteoarthritic changes following transarticular impact may be depicted using immunofluorescence on unfixed cryosections to contribute to a more detailed understanding of degenerative processes of joint impaction.

DESIGN

Transarticular impacts were applied to patellofemoral joints of 12 skeletally mature beagle dogs (age: 15-16 months) using a drop tower. Biopsies of impact areas were sampled after 6 months and processed for standard light microscopy on formalin-fixed sections and for immunofluorescence for collagen type I (col I), type II (col II) and aggrecan (AC) on unfixed cryosections. Gross morphology and immunofluorescence on cryosections were documented using a semi-quantitative scaling system, compared to healthy controls and to standard light microscopy.

RESULTS

Four biopsies showed almost entirely fibrocartilaginous morphology, four appeared to be of preserved hyaline morphology with only minor signs of fibrocartilaginous remodelling and four showed preserved hyaline appearance. We found decrease in col II and AC expression in highly degenerative specimens as well as increase of col I expression. Increased col I expression in the pericellular matrix could even be depicted in specimens with intact hyaline morphology.

DISCUSSION/CONCLUSION

Observations suggest that joint impaction causes early osteoarthritic changes after 6 months. Collagen network disruption seems to lead to AC loss, although other researchers found isolated AC loss without denaturation of col II using immunofluorescence in formalin-fixed specimens. This is the first study on effects of transarticular impact using immunofluorescence on unfixed cryosections.

Prestrain decreases cartilage susceptibility to injury by ramp compression in vitro

BACKGROUND

Injurious mechanical loading of articular cartilage can be an initiating factor in the development of degenerative joint disease. The tissue response to compression depends on the loading conditions and matrix mechanical properties. The short-term loading history of cartilage can affect its water content and microstructural organization, and may thereby modify its susceptibility to injury. We therefore examined the role of prestrain on the response of articular cartilage to injurious compression.

METHODS

The full-thickness cartilage of bovine osteochondral explants was subjected to prestrains of 0, 5, 10, 25 or 50% before application of injurious ramp compression characterized by a strain rate of 7x10(-2) or 7x10(-3)s-1 and peak stress of 3.5 or 14 MPa. Effects of prestrain were evaluated in terms of fluid exudation, tissue mechanical stiffening, and the tissue response to injurious compression as characterized by macroscopic crack formation, cell viability and glycosaminoglycan release to culture media.

RESULTS

Macroscopic crack formation due to injurious compression decreased with increasing prestrain in association with lower cell mortality. Significantly decreased susceptibility to injury was already evident for 10% prestrain. In contrast, explant mechanical stiffness was unchanged up to 25% prestrain.

CONCLUSION

Findings demonstrate that compressive strains due to the short-term loading history of cartilage may strongly reduce its susceptibility to mechanical injury. Conversely, matrix swelling may render cartilage more vulnerable to injury. The cartilage response to injurious compression is therefore strongly influenced by matrix fluid content, and possibly also by other structural parameters such as collagen fiber orientation.

Impact loading of articular cartilage during transplantation of osteochondral autograft

Surgical reconstruction of articular surfaces by transplantation of osteochondral autografts has shown considerable promise in the treatment of focal articular lesions. During mosaicplasty, each cylindrical osteochondral graft is centred over the recipient hole and delivered by impacting the articular surface. Impact loading of articular cartilage has been associated with structural damage, loss of the viability of chondrocytes and subsequent degeneration of the articular cartilage. We have examined the relationship between single-impact loading and chondrocyte death for the specific confined-compression boundary conditions of mosaicplasty and the effect of repetitive impact loading which occurs during implantation of the graft on the resulting viability of the chondrocytes.

Fresh bovine and porcine femoral condyles were used in this experiment. The percentage of chondrocyte death was found to vary logarithmically with single-impact energy and was predicted more strongly by the mean force of the impact rather than by the number of impacts required during placement of the graft. The significance of these results in regard to the surgical technique and design features of instruments for osteochondral transplantation is discussed.

Increased MIG-6 mRNA transcripts in osteoarthritic cartilage

The biochemical mechanism for initiation of cartilage destruction in osteoarthritis (OA) is unknown but may involve as yet unidentified cartilage genes. The first evidence that MIG-6, a protein involved in signal transduction, is expressed in articular cartilage came from our recent in vitro microarray experiments using the Affymetrix canine GeneChip. Quantitative RT-PCR (q RT-PCR) confirmed a fourfold increase in MIG-6 mRNA in cartilage in response to mechanical impact in vitro. Our goal is to determine if MIG-6, which responds to mechanical impact, could have a role in the initiation of OA. We determined that mRNA transcript levels of MIG-6 were fourfold higher in degenerated cartilage from dogs with hip osteoarthritis than in disease-free cartilage from unaffected dogs and twofold higher than in the cartilage surrounding the lesion. This is the first report associating MIG-6 with OA. Additional studies will determine what role MIG-6 has in the origin of cartilage degeneration.

Effect of tissue maturity on cell viability in load-injured articular cartilage explants

OBJECTIVE

During joint maturation, articular cartilage undergoes compositional, structural, and biomechanical changes, which could affect how the chondrocytes within the cartilage matrix respond to load-induced injury. The objective of this study was to determine the effects of tissue maturity on chondrocyte viability when explanted cartilage was subjected to load-induced injury.

DESIGN

Cartilage explants from immature (4-8-week-old) and mature (1.5-2-year-old) bovine humeral heads were cyclically loaded at 0.5 hertz in confined compression with a stress of 1 or 5 megapascals for 0.5, 1, 3, 6 and 16 h. Cell death was assessed at 0, 24 and 48 h after load removal using cell viability dyes and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling assay. The organization of pericellular matrix (PCM), biochemical composition and biomechanical properties of the cartilage were also determined.

RESULTS

For the immature and mature cartilage, cell death began at the articular surface and increased in depth with loading time up to 6h. No increase of cell death was found after load removal for up to 48 h. In both groups, cell death increased at a faster rate with the increase of stress level. The depth of cell death in the immature cartilage was greater than the mature cartilage, despite the immature cartilage having a higher bulk aggregate modulus. A less organized PCM in immature cartilage was found as indicated by the weak staining of type VI collagen.

CONCLUSION

Cells in the mature cartilage are less vulnerable to load-induced injury than those in immature cartilage.

Caspase-3/7 inhibition alters cell morphology in mitomycin-c treated chondrocytes

Apoptosis may play a role in osteoarthritis (OA). Apoptosis can proceed via two different pathways depending on the stimulus. However, both pathways converge upon the effector caspases, caspases-3 and -7. In some systems inhibition of caspases-3 and -7 can prevent apoptosis and may therefore have important therapeutic implications. To confirm this, apoptosis was induced in canine chondrocytes with mitomycin-c (MMC), either in the presence or absence of the general caspase inhibitor, Z-VAD FMK, or a specific caspase-3/7 inhibitor. Z-VAD FMK prevented MMC induced cell death. In contrast, inhibition of caspases-3 and -7 in the presence of MMC induced morphological changes that could be described as necrotic-like or paraptotic-like but did not prevent cell death. The addition of an inhibitor of caspase-8 or caspase-9 along with inhibitor of caspase-3/7 was required to reduce cell death. The morphological changes did not occur in the presence of the caspase-3/7 inhibitor alone and could be prevented by addition of Z-VAD FMK. These data lead to the conclusion that, if the apoptotic program cannot be completed, the cells are pushed into a necrotic or other nonapoptotic mode of death which may involve caspase-8 and/or caspase-9.

Viability and volume of in situ bovine articular chondrocytes-changes following a single impact and effects of medium osmolarity

OBJECTIVE

Mechanical stress above the physiological range can profoundly influence articular cartilage causing matrix damage, changes to chondrocyte metabolism and cell injury/death. It has also been implicated as a risk factor in the development of osteoarthritis (OA). The mechanism of cell damage is not understood, but chondrocyte volume could be a determinant of the sensitivity and subsequent response to load. For example, in OA, it is possible that the chondrocyte swelling that occurs renders the cells more sensitive to the damaging effects of mechanical stress. This study had two aims: (1) to investigate the changes to the volume and viability of in situ chondrocytes near an injury to cartilage resulting from a single blunt impact, and (2) to determine if alterations to chondrocyte volume at the time of impact influenced cell viability.

METHODS

Explants of bovine articular cartilage were incubated with the fluorescent indicators calcein-AM and propidium iodide permitting the measurement of cell volume and viability, respectively, using confocal laser scanning microscopy (CLSM). Cartilage was then subjected to a single impact (optimally 100g from 10 cm) delivered from a drop tower which caused areas of chondrocyte injury/death within the superficial zone (SZ). The presence of lactate dehydrogenase (LDH; an enzyme released following cell injury) was used to determine the effects of medium osmolarity on the response of chondrocytes to a single impact.

RESULTS

A single impact caused discrete areas of chondrocyte injury/death which were almost exclusively within the SZ of cartilage. There appeared to be two phases of cell death, a rapid phase lasting approximately 3 min, followed by a slower progressive 'wave of cell death' away from the initial area lasting for approximately 20 min. The volume of the majority (88.1+/-5.99% (n=7) of the viable chondrocytes in this region decreased significantly (P<0.006). By monitoring LDH release, a single impact 5 min after changing the culture medium to hyper-, or hypo-osmolarity, reduced or stimulated chondrocyte injury, respectively.

CONCLUSIONS

A single impact caused temporal and spatial changes to in situ chondrocyte viability with cell shrinkage occurring in the majority of cells. However, chondrocyte shrinkage by raising medium osmolarity at the time of impact protected the cells from injury, whereas swollen chondrocytes were markedly more sensitive. These data showed that chondrocyte volume could be an important determinant of the sensitivity and response of in situ chondrocytes to mechanical stress.

The use of a non-ionic surfactant (P188) to save chondrocytes from necrosis following impact loading of chondral explants

While current injury criteria for the automotive industry are based on bone fracture, the majority of knee injuries suffered in collisions each year do not involve fracture of bone. Instead, clinical studies of traumatic joint injury often document early pain and development of chronic diseases, such as osteoarthritis. Previous studies suggest chronic disease can be initiated by cell death that occurs in articular cartilage during mechanical trauma to the joint. In the current investigation early necrosis of chondrocytes was investigated after blunt trauma to chondral explants. A non-ionic surfactant (P188) was explored as a potential tool for early intervention into the disease process, as this surfactant has been shown to repair damaged membranes in other cell lines. Three groups of adult bovine chondral explants were equilibrated for 48 h in culture media. Two groups were then loaded to 25 MPa in unconfined compression. Half the specimens in each group were incubated in media supplemented with 8 mg/ml P188 immediately after loading, while the other half was returned to standard media. At 1 and 24 h the percentages of live and dead cells in compressed and control groups were determined with a cell viability stain. At 1 h post-trauma, P188 incubated specimens had a significantly increased percentage of live cells in the superficial zone versus the no-P188 group. At 24 h the percentages of live cells in all three zones of the P188-treated explants were significantly greater than in the no treatment group. This study showed that P188 surfactant could help restore the integrity of cell membranes in cartilage damaged by blunt mechanical trauma. With the ability of P188 to "save" chondrocytes from early necrotic death using in vitro chondral explants, its role in prevention of a post-traumatic osteoarthritis in a diarthrodial joint should be further explored using in vivo animal models.

The spread of cell death from impact damaged cartilage: lack of evidence for the role of nitric oxide and caspases

Over 21 days in culture, cell death spreads, both radially and transversely, from loaded to surrounding cartilage. This spread was prevented by physical separation and separate culture post-impact.

OBJECTIVE

One aim was to determine if nitric oxide (NO) is the intercellular signal mediating cell death. Another aim was to clarify the nature of the cell death, whether caspase mediated apoptosis or necrosis.

DESIGN

Cyclic impacts were applied to the central 2 mm core of 4 mm canine articular cartilage discs. Post-impact culturing was for 21 days in the presence or absence of the iNOS inhibitor, L-NAME, or the broad-spectrum caspase inhibitor, Z-VAD FMK. Cell death was quantified using the TUNEL assay. Culture media were collected every 2 days for measurements of glycosaminoglycan (GAG) and NO release.

RESULTS

Cell death spread from the loaded core into the surrounding ring over 21 days in culture. Although L-NAME significantly reduced nitrite release into the culture media of both loaded and control cartilage, the spread of cell death was not prevented. Neither was the spread of cell death prevented by Z-VAD FMK.

CONCLUSIONS

These data indicate that NO is not acting as an intercellular signalling factor in this in vitro system and that the cell death post-impact is not caspase mediated.

Cartilage injury by ramp compression near the gel diffusion rate

The mechanics of cartilage injuries have implications for repair strategies. We examined the role of strain rate in cartilage injury under compression near the "gel diffusion" rate (the inherent tissue mechanical relaxation rate). Bovine osteochondral explant disks were subjected to one radially unconfined axial compression at approximately 0.1, 1, 10, 100, or 1000 times the gel diffusion rate to a peak stress of 3.5, 7, or 14 MPa. Effects were monitored in terms of axial strain, changes in water content, superficial cracks, chondrocyte viability, and proteoglycan release. Injury worsened monotonically with peak stress, but varied substantially with strain rate. High strain rates resulted in significant matrix fluid pressurization and impact-like surface cracking with cell death near the superficial zone. Below the gel diffusion rate, cells died throughout the tissue depth during extensive matrix consolidation without cracks. At approximately the gel diffusion rate, no measurable injury occurred, even for peak stresses of 14 MPa and axial compressive strains near 0.8. The gel diffusion rate therefore represented a threshold separating different biomechanical regimes of injury, but at which cartilage was relatively "safe" from injury. Findings may help identify strategies for prevention and treatment of cartilage injury and suggest loading guidelines for tissue engineering.

Time, stress, and location dependent chondrocyte death and collagen damage in cyclically loaded articular cartilage

We investigated the effect of light (0.1 MPa), moderate (1 MPa) or heavy (5 MPa) cyclical stresses applied continuously or intermittently for 0 to 72 h on cell death and collagen damage in adult bovine cartilage explants. No increase in cell death was observed in the cartilage loaded with a continuous cyclic stress at 0.1 MPa for up to 72 h. Cell death occurred in the uppermost superficial tangential zone (STZ) of explants after loading for 1 h at 1 MPa, and reached a maximum depth of 61+/-23 micro m by 6 h (at the rate of 9+/-6 micro m/h). At 5 MPa, cell death occurred in the STZ after as little as 1 min (30 cycles) of loading, and reached a maximum depth of 70+/-2 micro m by 60 min (47+/-8 micro m/h). When an intermittent (with 2 s on, 2 s off) stress of 5 MPa was applied, cell death appeared in the STZ after 2 min (30 cycles) and increased to a depth of 63+/-2 micro m at 60 min (45+/-11 micro m/h). No significant differences were observed between the continuous and intermittent loading conditions. Both collagenase-cleaved and denatured collagen fibers were found in the STZ of explants loaded at 1 and 5 MPa. We concluded that load-induced cell death depends on load duration and magnitude, and that the chondrocytes in the STZ are more vulnerable to load-induced injury than those in the middle and deep zones.

Proteoglycan metabolism and viability of articular cartilage explants as modulated by the frequency of intermittent loading

OBJECTIVE

This study was designed to systematically determine whether and to what extent the frequency of intermittent loading modulates the biosynthesis and release of proteoglycans (PGs), and to assess chondrocyte viability within mature bovine articular cartilage explants exposed to different loading patterns.

METHODS

Cultured full-thickness cartilage explants from the weight-bearing area of healthy bovine fetlock joints were exposed to intermittently applied, uniaxial cyclic loads by introducing a sinusoidal waveform of 0.1, 0.5 or 1.0Hz, frequency and a peak stress of 0.5MPa for a period of 6 days. The cyclic loads were applied for 5, 10 or 20s followed by a period of unloading lasting 10, 100 or 1000s. The incorporation of radiolabeled sulfate into glycosaminoglycans (GAGs) during the final 18h, the content of GAGs and DNA, the deformation of loaded explants as well as the viability of chondrocytes within the different zones of explants were determined.

RESULTS

PG synthesis and loss of endogenous PGs were non-linearly and independently regulated by the frequency of the chosen intermittent load, whereas the release of newly synthesized PGs remained unaffected. The viability of chondrocytes within the superficial zone decreased drastically under intermittent loading in a manner independent of the frequency applied.

CONCLUSIONS

Our results confirm the hypothesis that the frequency of intermittent loading is an important mechanical factor controlling the metabolic activities of chondrocytes. They also implicate that an initially healthy cartilage explant can be mechanically manipulated to generate an in vitro model of degenerative, osteoarthritic-like cartilage.