Growth responses of cartilage to static and dynamic compression

Role of mitochondria in mediating chondrocyte response to mechanical stimuli

As the most common form of arthritis, osteoarthritis (OA) has become a major cause of severe joint pain, physical disability, and quality of life impairment in the affected population. To date, precise pathogenesis of OA has not been fully clarified, which leads to significant obstacles in developing efficacious treatments such as failures in finding disease-modifying OA drugs (DMOADs) in the last decades. Given that diarthrodial joints primarily display the weight-bearing and movement-supporting function, it is not surprising that mechanical stress represents one of the major risk factors for OA. However, the inner connection between mechanical stress and OA onset/progression has yet to be explored. Mitochondrion, a widespread organelle involved in complex biological regulation processes such as adenosine triphosphate (ATP) synthesis and cellular metabolism, is believed to have a controlling role in the survival and function implement of chondrocytes, the singular cell type within cartilage. Mitochondrial dysfunction has also been observed in osteoarthritic chondrocytes. In this review, we systemically summarize mitochondrial alterations in chondrocytes during OA progression and discuss our recent progress in understanding the potential role of mitochondria in mediating mechanical stress-associated osteoarthritic alterations of chondrocytes. In particular, we propose the potential signaling pathways that may regulate this process, which provide new views and therapeutic targets for the prevention and treatment of mechanical stress-associated OA.

A compression system for studying depth-dependent mechanical properties of articular cartilage under dynamic loading conditions

The biological activities of chondrocytes are influenced by the mechanical characteristics of their environment. The overall real-time mechanical response of cartilage has been investigated earlier. However, the instantaneous local mechano-biology of cartilage has not been investigated in detail under dynamic loading conditions. In order to address this gap in the literature, we designed a compression testing device and implemented a dual photon microscopy technique with the goal of measuring local mechanical and biological responses of articular cartilage under dynamic loading conditions. The details of the compression system and results of a pilot study are presented here. A 15% ramp compression at a rate of 0.003/s with a subsequent stress relaxation phase was applied to the cartilage explant samples. The extra cellular matrix was imaged throughout the entire thickness of the cartilage sample, and local tissue strains were measured during the compression and relaxation phase. The axial compressive strains in the middle and superficial zones of cartilage were observed to increase during the relaxation phase: this was a new finding, suggesting the importance of further investigations on the real-time local behavior of cartilage. The compression system showed promising results for investigating the dynamic, real-time mechanical response of articular cartilage, and can now be used to reveal the instantaneous mechanical and biological responses of chondrocytes in response to dynamic loading conditions.

The regional sensitivity of chondrocyte gene expression to coactive mechanical load and exogenous TNF-α stimuli

Both mechanical load and elevated levels of proinflammatory cytokines have been associated with the risk for developing osteoarthritis (OA), yet the potential interaction of these mechanical and biological factors is not well understood. The purpose of this study was to evaluate the response of chondrocytes to the effects of dynamic unconfined compression, TNF-α, and the simultaneous effects of dynamic unconfined compression and TNF-α. The response to these three treatments was markedly different and, taken together, the response in the gene expression of chondrocytes to the different treatment conditions suggest a complex interaction between structure, biology, and mechanical loading.

A novel bioreactor for the dynamic stimulation and mechanical evaluation of multiple tissue-engineered constructs

Systematic advancements in the field of musculoskeletal tissue engineering require clear communication about the mechanical environments that promote functional tissue growth. To support the rapid discovery of effective mechanostimulation protocols, this study developed and validated a mechanoactive transduction and evaluation bioreactor (MATE). The MATE provides independent and consistent mechanical loading of six specimens with minimal hardware. The six individual chambers accurately applied static and dynamic loads (1 and 10 Hz) in unconfined compression from 0.1 to 10 N. The material properties of poly(ethylene glycol) diacrylate hydrogels and bovine cartilage were measured by the bioreactor, and these values were within 10% of the values obtained from a standard single-chamber material testing system. The bioreactor was able to detect a 1-day 12% reduction (2 kPa) in equilibrium modulus after collagenase was added to six collagenase sensitive poly(ethylene glycol) diacrylate hydrogels (p = 0.03). By integrating dynamic stimulation and mechanical evaluation into a single batch-testing research platform, the MATE can efficiently map the biomechanical development of tissue-engineered constructs during long-term culture.

Central and peripheral region tibial plateau chondrocytes respond differently to in vitro dynamic compression

OBJECTIVE

The objective of this study was to test the hypotheses that chondrocytes from distinct regions of the porcine tibial plateau: (1) display region-specific baseline gene expression, and (2) respond differently to in vitro mechanical loading.

METHODS

Articular cartilage explants were obtained from central (not covered by meniscus) and peripheral (covered by meniscus) regions of porcine tibial plateaus. For baseline gene expression analysis, samples were snap frozen. To determine the effect of mechanical loading, central and peripheral region explants were exposed to equivalent dynamic compression (0-100 kPa) and compared to site-matched free-swelling controls (FSCs). mRNA levels for type II collagen (CII), aggrecan (AGGR), matrix metalloproteinase 1 (MMP-1), MMP-3, MMP-13, A disintegrin and metalloproteinase with thrombospondin motifs 4 (ADAM-TS4), ADAM-TS5, tissue inhibitor of metalloproteinases 1 (TIMP-1), TIMP-2, and tumor necrosis factor alpha (TNFalpha) were quantified using real time polymerase chain reaction (RT-PCR).

RESULTS

At baseline, mRNA levels for the structural proteins CII and AGGR were approximately twofold greater in the central region compared with peripheral region explants. In vitro dynamic compression strongly affected expression levels for CII, AGGR, MMP-3, and TIMP-2 relative to FSCs. Response differed significantly by region, with greater upregulation of CII, AGGR, and MMP-3 in central region explants.

CONCLUSIONS

Chondrocytes from different regions of the porcine tibial plateau express mRNA for structural proteins at different levels and respond to equivalent in vitro mechanical loading with distinctive changes in gene expression. These regional biological variations appear to be related to the local mechanical environment in the normal joint, and thus may indicate a sensitivity of the joint to conditions that alter joint loading such as anterior cruciate ligament (ACL) injury, meniscectomy, or joint instability.

The dynamic mechanical environment of the chondrocyte: a biphasic finite element model of cell-matrix interactions under cyclic compressive loading

Cyclic mechanical loading of articular cartilage results in a complex biomechanical environment at the scale of the chondrocytes that strongly affects cellular metabolic activity. Under dynamic loading conditions, the quantitative relationships between macroscopic loading characteristics and solid and fluid mechanical variables in the local cellular environment are not well understood. In this study, an axisymmetric multiscale model of linear biphasic cell-matrix interactions in articular cartilage was developed to investigate the cellular microenvironment in an explant subjected to cyclic confined compressive loading. The model was based on the displacement-velocity-pressure (u-v-p) mixed-penalty weighted residual formulation of linear biphasic theory that was implemented in the COMSOL MULTIPHYSICS software package. The microscale cartilage environment was represented as a three-zone biphasic region consisting of a spherical chondrocyte with encapsulating pericellular matrix (PCM) that was embedded in a cylindrical extracellular matrix (ECM) subjected to cyclic confined compressive loading boundary conditions. Biphasic material properties for the chondrocyte and the PCM were chosen based on previous in vitro micropipette aspiration studies of cells or chondrons isolated from normal or osteoarthritic cartilage. Simulations performed at four loading frequencies in the range 0.01-1.0 Hz supported the hypothesized dual role of the PCM as both a protective layer for the cell and a mechanical transducer of strain. Time varying biphasic variables at the cellular scale were strongly dependent on relative magnitudes of the loading period, and the characteristic gel diffusion times for the ECM, the PCM, and the chondrocyte. The multiscale simulations also indicated that axial strain was significantly amplified in the range 0.01-1.0 Hz, with a decrease in amplification factor and frequency insensitivity at the higher frequencies. Simulations of matrix degradation due to osteoarthritis indicated that strain amplification factors were more significantly altered when loss of matrix stiffness was exclusive to the PCM. The findings of this study demonstrate the complex dependence of dynamic mechanics in the local cellular environment of cartilage on macroscopic loading features and material properties of the ECM and the chondron.

Mature runt cow lumbar intradiscal pressures and motion segment biomechanics

BACKGROUND CONTEXT

The optimal animal model for in vivo testing of spinal implants, particularly total or partial disc replacement devices, has not yet been determined. Mechanical and morphological similarities of calf and human spines have been reported; however, limitations of the calf model include open growth plates and oversized vertebrae with growth. Mature runt cows (Corrientes breed) may avoid these limitations.

PURPOSE

This study compared vertebral morphology and biomechanical properties of human and runt cow lumbar motion segments.

STUDY DESIGN

In vivo disc pressure measurements were obtained in six mature runt cows at L4-L5. In vitro evaluation was performed on these same segments and repeated on 12 human motion segments.

METHODS

Disc pressures were measured in vivo in runt cow (Corrientes breed) L45 discs using a percutaneous transducer with the animal performing various activities. These motion segments were then harvested and morphologic and biomechanical evaluations (disc pressure in compression, flexibility tests to 7.5Nm) were performed on both cow and male human L23 and L45 segments.

RESULTS

The transverse lumbar disc dimensions were slightly smaller for (mixed gender) cow versus (male) humans, but were within the range of reported (mixed gender) human values. The mean+/-SD disc height was smaller for runt cow (7+/-1mm) versus human discs (13+/-2mm, p<.001). The vertebral bodies of the cow were approximately twice as tall as the human. In vitro testing revealed significantly greater disc pressure response to applied axial loading in the runt cow versus humans (1.27+/-0.18 vs. 0.84+/-0.15kPa/N, respectively) but similar overall stiffness (2.15+/-0.71 vs. 1.91+/-0.94kN/mm, respectively). Runt cow and human segment flexibility curves were similar with the following exceptions: runt cow stiffness was approximately 40% greater in torsion (p<.05), runt cow segment lateral bending motion was greater versus humans (range of motion by 30%, neutral zone by 100%; both p<.05), and flexion range of motion tended to be smaller in runt cow versus human specimens (by approximately 40%, p=NS). In vivo, the standing disc pressure in the runt cow was 0.80+/-0.24MPa.

CONCLUSIONS

Although no animal replicates the human motion segment, the runt cow lumbar spine had a number of biomechanical and morphological measurements within the range of human values. The closed physes and temporally stable morphology of the mature runt cow may make this model more suitable versus standard calf models for human intradiscal implant studies.

Mechanical vibrations increase the proliferation of articular chondrocytes in high-density culture

Tissue engineering is a promising approach for articular cartilage repair; however, it still has proven a challenge to produce tissue from the limited number of cells that can be extracted from a single individual. Relatively few cell expansion methods exist without the problems of dedifferentiation and/or loss of potency. Previously, it has been shown that mechanical vibrations can enhance chondrocyte proliferation in monolayer culture. Thus, it was hypothesized that chondrocytes grown in high-density culture would respond in a similar fashion while maintaining phenotypic stability. Isolated bovine articular chondrocytes were seeded in high-density culture on Millicell™ filters and subjected to mechanical vibrations 48 h after seeding. Mechanical vibrations enhanced chondrocyte proliferation at frequencies above 350 Hz, with the peak response occurring at a 1 g amplitude for a duration of 30 min. Under these conditions, the gene expression of cartilage-specific and dedifferentiation markers (collagen II, collagen I, and aggrecan) were unchanged by the imposed stimulus. To determine the effect of accumulated extracellular matrix (ECM) on this proliferative response, selected cultures were stimulated under the same conditions after varying lengths of preculture. The amount of accumulated ECM (collagen and proteoglycans) decreased this proliferative response, with the cultures becoming insensitive to the stimulus after 1 week of preculture. Thus, mechanical vibration can serve as an effective means preferentially to stimulate the proliferation of chondrocytes during culture, but its effects appear to be limited to the early stages where ECM accumulation is at a minimum.

The role of hydrogel structure and dynamic loading on chondrocyte gene expression and matrix formation

Crosslinked poly(ethylene glycol) (PEG) hydrogels are attractive scaffolds for cartilage tissue engineering because of their ability to mimic the aqueous environment and mechanical properties of native cartilage. In this study, hydrogel crosslinking density was varied to study the influence of gel structure and the application of dynamic loading (continuous, 1 Hz, 15% amplitude strain) on chondrocyte gene expression over approximately 1 week culture. Gene expression was quantified using real-time RT-PCR for collagen II and aggrecan, the major cartilage extracellular matrix (ECM) components, and collagen I, an indicator of chondrocyte de-differentiation. When chondrocytes were encapsulated in PEG gels with low or high crosslinking, a high collagen II expression compared to collagen I expression (1000 or 100,000:1, respectively) indicated the native chondrocyte phenotype was retained. In the absence of loading, relative gene expression for collagen II and aggrecan was significantly higher (e.g., 2-fold and 4-fold, respectively, day 7) in the low crosslinked gels compared to gels with higher crosslinking. Dynamic loading, however, showed little effect on ECM gene expression in both crosslinked systems. To better understand the cellular environment, ECM production was qualitatively assessed using an in situ immunofluorescent technique and standard histology. A pericellular matrix (PCM) was observed as early as day 3 post-encapsulation and the degree of formation was dependent on gel crosslinking. These results suggest the PCM may protect the cells from sensing the applied loads. This study demonstrates that gel structure has a profound effect on chondrocyte gene expression, while dynamic loading has much less of an effect at early culture times.

Biologic response of the intervertebral disc to static and dynamic compression in vitro

STUDY DESIGN

This study attempts to determine the biologic response of the intervertebral disc to static and dynamic compression in vitro.

OBJECTIVE

To quantitate and compare the changes of histology, Type I and II collagens, aggrecan, interleukin (IL)-1beta, and tumor necrosis factor (TNF)-alpha expression, and apoptosis in rabbit intervertebral discs following static and dynamic compression in vitro.

SUMMARY OF BACKGROUND DATA

Compressive stimuli associated with weight-bearing and loading of the intervertebral disc are thought to be important regulators of disc cell metabolism. Very little is known about the different types of mechanical stimuli that may achieve regulation of intervertebral disc cell metabolism. We examined the biologic response of the rabbit intervertebral discs when exposed to static and dynamic compression in vitro.

METHODS

The rabbit intervertebral disc explants were in vitro subjected to unconfined uniaxial compression. Static compression of 0.5 and 1 MPa and dynamic compression of 0.5 and 1 MPa were applied at the frequency of 0.1 and 1 Hz for 6 hours, respectively. After loaded with different types of mechanical stimuli, disc explants were examined for histologic changes, collagen I, collagen II, aggrecan, IL-1beta and TNF-alpha expression, and apoptosis.

RESULTS

The static compressive load was found to suppress gene expression for collagens and aggrecan in the disc, whereas the disc under dynamic compression exhibited significant anabolic change with increase in gene expression for Type I and II collagen and aggrecan. The regional difference of the responses to mechanical loading in vitro was found between the anulus fibrosus and nucleus pulposus. All loading conditions caused marked histologic changes, up-regulation of IL-1beta and TNF-alpha expression, and increase in TUNEL-positive cells in the intervertebral discs, with the most significant from control was that when statically loaded.

CONCLUSION

Mechanical loading is involved in the physiology and pathology of disc degeneration. Static and dynamic compression may induce different biologic response of the intervertebral disc: static compression has catabolic role on the disc, whereas the dynamic load at appropriate level may benefit the synthetic activity and anabolic response of the disc.

Solute transport in cartilage undergoing cyclic deformation

There are no blood vessels in cartilage to transport nutrients and growth factors to chondrocytes dispersed throughout the cartilage matrix. Insulin-like growth factor-I (IGF-I) is a large molecule with an important role in cartilage growth and metabolism, however, it first must reach the chondrocytes to exert its effect. While diffusion of IGF-I through cartilage is possible, it has been speculated that cyclic loading can enhance the rate of solute transport within cartilage. To better understand this process, here a one-dimensional axisymmetric mathematical model is developed to examine the transport of solutes through a cylindrical plug of cartilage undergoing cyclic axial deformation in the range of 10(-3) -1 Hz. This study has revealed the role of timescales in interpreting transport results in cartilage. It is shown that dynamic strains can either enhance or inhibit IGF-I transport at small timescales (< 20 min after onset of loading), depending on loading frequency. However, on longer timescales it is found that dynamic loading has negligible effect on IGF-I transport. Most importantly, in all cases examined the steady state IGF-I concentration did not exceed the fixed boundary value, in contrast to the predictions of Mauk et al. (2003).

Autologous chondrocyte implantation: superior biologic properties of hyaline cartilage repairs

Information regarding the quality of autologous chondrocyte implantation repair is needed to determine whether the current autologous chondrocyte implantation surgical technology and the subsequent biologic repair processes are capable of reliably forming durable hyaline or hyaline-like cartilage in vivo. We report and analyze the properties and qualities of autologous chondrocyte implantation repairs. We evaluated 66 autologous chondrocyte implantation repairs in 57 patients, 55 of whom had histology, indentometry, and International Cartilage Repair Society repair scoring at reoperation for mechanical symptoms or pain. International Knee Documentation Committee scores were used to address clinical outcome. Maximum stiffness, normalized stiffness, and International Cartilage Repair Society repair scoring were higher for hyaline articular cartilage repairs compared with fibrocartilage, with no difference in clinical outcome. Reoperations revealed 32 macroscopically abnormal repairs (Group B) and 23 knees with normal-looking repairs in which symptoms leading to arthroscopy were accounted for by other joint disorders (Group A). In Group A, 65% of repairs were either hyaline or hyaline-like cartilage compared with 28% in Group B. Autologous chondrocyte repairs composed of fibrocartilage showed more morphologic abnormalities and became symptomatic earlier than hyaline or hyaline-like cartilage repairs. The hyaline articular cartilage repairs had biomechanical properties comparable to surrounding cartilage and superior to those associated with fibrocartilage repairs.

Tailoring secretion of proteoglycan 4 (PRG4) in tissue-engineered cartilage

Articular cartilage provides a low-friction surface for joint articulation, with boundary lubrication facilitated by proteoglycan 4 (PRG4), which is secreted by chondrocytes of the superficial zone. Chondrocytes from different zones are phenotypically distinct, and their phenotypes in vitro are influenced by the system in which they are cultured. We hypothesized that culturing cells from the superficial (S) zone in two-dimensional monolayer or three-dimensional alginate would affect their synthesis of PRG4, and that subsequently seeding them atop alginate-recovered cells from the middle/ deep (M) zone in various proportions would result in tissue-engineered constructs with varying levels of PRG4 secretion and matrix accumulation. During monolayer culture, S cells retained their PRG4-secreting phenotype, whereas in alginate culture the percentage of cells secreting PRG4 decreased with time. Constructs formed with increasing percentages of S cells decreased in thickness and matrix accumulation, depending on both the culture conditions before construct formation and the S-cell density. PRG4-secreting cells were localized to the S-cell seeded construct surface, with secretion rates of 0.1-4 pg/cell/day or 0.1-1 pg/cell/day for constructs formed with monolayer-recovered or alginate-recovered S cells, respectively. Tailoring secretion of PRG4 in cartilage constructs may be useful for enhancing low-friction properties at the articular surface, while maintaining other surfaces free of PRG4 for enhancing integration with surrounding tissues.

Gene Regulation ex Vivo within a Wrap-Around Tendon

This study tested the hypothesis that physiologic tendon loading modulates the fibrous connective tissue phenotype in undifferentiated skeletal cells. Type I collagen sponges containing human bone marrow stromal cells (MSCs) were implanted into the midsubstance of excised sheep patellar tendons. An ex vivo loading system was designed to cyclically stretch each tendon from 0 to 5% at 1.0 Hz. The MSC-sponge constructs were implanted into 2 tendon sites: the first site subjected to tension only and a second site located at an artificially created wrap-around region in which an additional compressive stress was generated transverse to the longitudinal axis of the tendon. The induced contact pressure at the wraparound site was 0.55 +/- 0.12 MPa, as quantified by pressure-sensitive film. An MSC-sponge construct was maintained free swelling in the same bath as an unloaded control. After 2 h of tendon stretching, the MSC-sponge constructs were harvested and real-time PCR was used to quantify Fos, Sox9, Cbfa1 (Runx2), and scleraxis mRNA expression as markers of skeletal differentiation. Two hours of mechanical loading distinctly altered MSC differentiation in the wrap-around region and the tensile-only region, as evidenced by differences in Fos and Sox9 mRNA expression. Expression of Fos mRNA was 13 and 52 times higher in the tensile-only and wrap-around regions, respectively, compared to the free-swelling controls. Expression of Sox9 mRNA was significantly higher (2.5-3 times) in MSCs from the wraparound region compared to those from the tensile-only region or in free-swelling controls. In contrast, expression levels for Cbfa1 did not differ among constructs. Scleraxis mRNA was not detected in any construct. This study demonstrates that the physiologic mechanical environment in the wrap-around regions of tendons provides stimuli for upregulating early response genes and transcription factors associated with chondrogenic differentiation. These differentiation responses begin within as little as 2 h after the onset of mechanical stimulation and may be the basis for the formation of fibrocartilage that is typically found in the wrap-around region of mature tendons in vivo.

A Direct Compression Stimulator for Articular Cartilage and Meniscal Explants

This paper describes the development and use of a direct compression stimulator for culturing explants from the meniscus of the knee and articular cartilage. Following design and fabrication of the instrument along with its data acquisition system, the function of the machine was verified by both mechanical means and tissue effect. The loading chamber can hold up to 45 5 mm diameter samples. While designed to stimulate samples up to 4 mm thick, axial displacements as little as 0.127 microm are within the theoretical capacity of the stimulator. In gene expression studies, collagen II and aggrecan expression were examined in explants from articular cartilage as well as medial and lateral menisci subjected to dynamic stimulation and static compression. These results were then compared to free swelling samples. It was found that static compression to cut thickness down-regulated aggrecan and collagen II expression in articular cartilage explants compared to free swelling controls by 94% and 90%, respectively. The application of a dynamic, intermittent, 2% oscillation around the cut thickness returned expression levels to those of free swelling controls at 4 h but not at 76 h. In medial meniscus samples, dynamic compression up-regulated aggrecan expression by 108%, but not collagen II expression, at 4 and 76 h compared to static controls. No difference in gene expression was observed for lateral meniscal explants. Thus, effects of direct compression seen in articular cartilage may not necessarily translate to the knee meniscus. The design of this stimulator will allow a variety of tissues and loading regimens to be examined. It is hoped that regimens can be found that not only return samples to the production levels of free swelling controls, but also surpass them in terms of gene expression, protein synthesis, and functional properties.

In vitro organ culture of the bovine intervertebral disc: effects of vertebral endplate and potential for mechanobiology studies

STUDY DESIGN

Whole bovine coccygeal discs were cultured under static load, with or without vertebral endplates (VEPs), and assessed for cell viability, biochemical stability, biosynthetic activity, and biosynthetic responsiveness to changes in mechanical load.

OBJECTIVES

To assess the effects of VEPs on biochemical and cellular stability of disc cells during in vitro culture of large disc explants. To determine whether cultured discs could respond to mechanical perturbation.

SUMMARY OF BACKGROUND DATA

Previous methods for culturing the intervertebral disc have focused on rabbit and rat discs, but the small size of these discs limits the relevance of these culture systems to the human condition. Bovine coccygeal discs have similar dimensions to the human lumbar disc (i.e., similar size and nominal stresses), but long-term culture of these discs has not been reported.

METHODS

Bovine coccygeal discs were harvested with or without VEPs, cultured under static load (5 kg, approximately 0.25 MPa, in situ swelling pressure) for up to 1 week, and evaluated for changes in hydration, glycosaminoglycan content, cell viability, and biosynthetic activity. Additionally, the biochemical and biosynthetic response of discs cultured without VEP to increasing the load to a 20-kg (approximately 1 MPa, the estimated stress in human lumbar disc during heavy lifting) static load for 6 hours was assessed.

RESULTS

During the first 24 hours, culturing discs with endplates was moderately better with regards to maintaining in situ anulus hydration and nucleus glycosaminoglycan levels. The endplates, however, obstructed media flow to the disc, resulting in a marked decrease in cell viability after 1 week of culture. Nucleus pulposus cell viability was maintained in discs cultured without endplates, but there was a significant drop in biosynthetic activity within 2 days of culture. Despite this drop, the disc cells in the discs without VEP remained biosynthetically responsive to changes in mechanical loading.

CONCLUSIONS

It is possible to maintain cell viability and the biosynthetic responsiveness of large discs for up to 1 week in vitro when the discs are cultured under static load and without VEP.

Cartilage anlagen adapt in response to static deformation

Connective tissue adaptation, including the development of cartilaginous anlagen into bones, is widely believed to be related to dynamic, intermittent load and stress histories. Static stresses, on the other hand, are generally believed deleterious in tissue adaptation. Using serial MRI in a natural human experiment (manipulation and corrective casting of infant clubfoot), we have observed casting produces two effects: (1) the well recognized change in relative positions of the hindfoot anlagen; (2) a newly recognized immediate shape change in the anlagen. These changes seemingly enhance the rate of growth of the anlagen and of the ossific nucleus. The shape change or deformation in the anlagen would occur as a result of alterations in the magnitudes and directions of loading from soft tissue attachments and muscle activity and would necessarily be associated with changes in the stress states within the anlagen and, when present, the ossific nuclei. Given the known role of load and stress history in tissue adaptation, we presume the reduced stress histories influence the enhanced growth rates. These observations contradict some current theories of tissue adaptation since static, rather than dynamic stresses play a crucial role in accelerating the growth and development of anlagen in the infant clubfoot.

Dynamic shear stimulation of bovine cartilage biosynthesis of proteoglycan 4

OBJECTIVE

The boundary lubrication function of articular cartilage is mediated in part by proteoglycan 4 (PRG4) molecules at the articular surface and in synovial fluid. The objective of this study was to determine the effects of dynamic shear stimulation on PRG4 biosynthesis by bovine cartilage explants.

METHODS

Cartilage disks with intact articular surfaces were harvested from immature bovines. Some disks were subjected to 24 hours (day 1) of loading, consisting of a step load of 20% static compression either alone or with superimposed dynamic shear (3% amplitude and 0.1 Hz), while other disks were cultured free-swelling as controls. After the 24-hour loading period, disks were terminated or were further incubated for up to 72 hours (days 2-4) in free-swelling culture to assess chondrocyte responses to, and following, unloading. PRG4 products secreted into culture medium were quantified by enzyme-linked immunosorbent assay and characterized by Western blotting. Chondrocytes expressing PRG4 were localized by immunohistochemistry, and depth-associated variations in chondrocyte PRG4 expression were quantified by image analysis.

RESULTS

Dynamic shear stimulation increased PRG4 secretion to 3-4 times that of unloaded controls and statically compressed samples. Sheared cartilage secreted more PRG4 of 345 kd relative to smaller molecular weight species, as compared with unloaded controls. Immunohistochemistry revealed that shear stimulation also increased the total number of cells expressing PRG4 by inducing expression by cells at a depth of 200-400 microm.

CONCLUSION

The paradigm that certain mechanical stimuli up-regulate biosynthesis in cartilage appears operative not only for load-bearing matrix constituents, but also for PRG4 molecules that mediate lubrication.

Differential effects of hyperosmotic challenge on interleukin-1-activated pathways in bovine articular cartilage

Chondrocytes in situ experience fluctuations in extracellular osmolarity resulting from mechanical loading. The objective of this study was to determine whether hyperosmotic stress causes or exacerbates interleukin-1 (IL-1)-mediated effects in bovine articular cartilage. Disks of cartilage cut from the articular surface of calf radiocarpal joints were incubated for 24h in the presence or absence of IL-1 in Dulbecco's modified Eagle's medium adjusted to various osmolalities with sucrose or NaCl. Cyclooxygenase (COX)-2 levels in the cartilage were examined by Western blot. Culture media were assayed for prostaglandin E(2) (PGE(2)), nitrite as an indicator of nitric oxide (NO) production, and sulfated glycosaminoglycan as an indicator of proteoglycan degradation. We report the osmolality-dependent potentiation of COX-2 and PGE(2) production, and the osmolality-dependent inhibition of NO production and proteoglycan degradation in IL-1-activated cartilage. The data demonstrate that osmotic and cytokine signaling interact to differentially modulate IL-1-stimulated effects in calf articular cartilage.

Depth-varying density and organization of chondrocytes in immature and mature bovine articular cartilage assessed by 3d imaging and analysis

Articular cartilage is a heterogeneous tissue, with cell density and organization varying with depth from the surface. The objectives of the present study were to establish a method for localizing individual cells in three-dimensional (3D) images of cartilage and quantifying depth-associated variation in cellularity and cell organization at different stages of growth. Accuracy of nucleus localization was high, with 99% sensitivity relative to manual localization. Cellularity (million cells per cm3) decreased from 290, 310, and 150 near the articular surface in fetal, calf, and adult samples, respectively, to 120, 110, and 50 at a depth of 1.0 mm. The distance/angle to the nearest neighboring cell was 7.9 microm/31 degrees , 7.1 microm/31 degrees , and 9.1 microm/31 degrees for cells at the articular surface of fetal, calf, and adult samples, respectively, and increased/decreased to 11.6 microm/31 degrees , 12.0 microm/30 degrees , and 19.2 microm/25 degrees at a depth of 0.7 mm. The methodologies described here may be useful for analyzing the 3D cellular organization of cartilage during growth, maturation, aging, degeneration, and regeneration.

Synthesis of proteoglycan 4 by chondrocyte subpopulations in cartilage explants, monolayer cultures, and resurfaced cartilage cultures

OBJECTIVE

To quantify the levels of proteoglycan 4 (PRG4) expression by subpopulations of chondrocytes from superficial, middle, and deep layers of normal bovine calf cartilage in various culture systems.

METHODS

Bovine calf articular cartilage discs or isolated cells were used in 1 of 3 systems of chondrocyte culture: explant, monolayer, or transplant, for 1-9 days. PRG4 expression was quantified by enzyme-linked immunosorbent assay of spent medium and localized by immunohistochemistry at the articular surface and within chondrocytes in explants and cultured cells.

RESULTS

Superficial chondrocytes secreted much more PRG4 than did middle and deep chondrocytes in all cultures. The pattern of PRG4 secretion into superficial culture medium varied with the duration of culture, decreasing with time in explant culture (from approximately 25 microg/cm(2)/day on days 0-1 to approximately 3 microg/cm(2)/day on days 5-9), while increasing in monolayer culture (from approximately 1 pg/cell/day on days 0-1 to approximately 7 pg/cell/day on days 7-9) and tending to increase in transplant culture (reaching approximately 2 microg/cm(2)/day by days 7-9). In all of the culture systems, inclusion of ascorbic acid stimulated PRG4 secretion, and the source of PRG4 was immunolocalized to superficial cells.

CONCLUSION

The results described here indicate that the phenotype of PRG4 secretion by chondrocytes in culture is generally maintained, in that PRG4 is expressed to a much greater degree by chondrocytes from the superficial zone than by those from the middle and deep zones. The marked up-regulation of PRG4 synthesis by ascorbic acid may have implications for cartilage homeostasis and prevention of osteoarthritic disease. Transplanting specialized cells that secrete PRG4 to a surface may impart functional lubrication and be generally applicable to many tissues in the body.

A biomechanical approach to musculoskeletal disease

The importance of biomechanical factors in musculoskeletal disease may be appreciated in the clinical characteristics and pathophysiology of joint degeneration in osteoarthritis. The biomechanical characteristics of the neuromuscular system are integral in determining the function and stability of the synovial joint and in mediating the biochemical structure of articular cartilage. Alterations in the neuromuscular system including abnormal gait and dynamic joint loading patterns as well as muscle strength and proprioception deficits have been independently studied and associated with osteoarthritis. These factors have close functional and physiological interactions. Nevertheless, specific relations between muscle strength and proprioception and their independent contributions to dynamic joint loading are not yet clear. Targeted interventions to moderate these factors may, in the future, be a therapeutic option for the management of osteoarthritis.

Influence of tissue maturation and antioxidants on the apoptotic response of articular cartilage after injurious compression

OBJECTIVE

To study the influence of tissue maturation and antioxidants on apoptosis in bovine articular cartilage induced by injurious compression.

METHODS

Bovine articular cartilage disks were obtained from the femoropatellar groove of animals ages 0.5-23 months and placed in culture. Cartilage disks were preincubated overnight with the cell-permeable superoxide dismutase (SOD) mimetic Mn(III) porphyrin (0-12.5 microM) or alpha-tocopherol (0-50 microM) and then injured by a single unconfined compression to a final strain of 50% at a velocity of 1 mm/second. After 4 days of additional incubation, the disks were fixed and embedded for light and electron microscopy. Apoptotic cells were quantified morphologically by the appearance of nuclear blebbing on light microscopy. Biosynthetic activity was demonstrated by incorporation of radiolabeled proline. The antioxidative action of the SOD mimetic was confirmed by histologic examination of cartilage after incubation with nitroblue tetrazolium.

RESULTS

Injurious compression induced significantly more apoptosis in cartilage disks from newborn calves (22% of cells) than in cartilage from more mature cows (2-6%). In cartilage from 22-month-old animals, the SOD mimetic reduced the percentage of apoptotic cells induced by injury in a dose-dependent manner (complete inhibition with 2.5 microM), while alpha-tocopherol had no effect. Neither antioxidant altered protein biosynthesis or cellular ultrastructure.

CONCLUSION

Our data suggest that the apoptotic response of articular cartilage to mechanical injury is affected by maturation and is mediated in part by reactive oxygen species. The antioxidative status of the tissue might be important for the prevention of mechanically induced cell death in articular cartilage.

Tensile mechanical properties of bovine articular cartilage: variations with growth and relationships to collagen network components

One approach to repairing articular defects is to regenerate cartilage by recapitulating the changes that occur during fetal and postnatal growth into adulthood, and to thereby restore functional biomechanical properties, especially those of the normally strong superficial region. The objectives of this study were (1) to characterize and compare tensile biomechanical properties of the superficial region of articular cartilage of the patellofemoral groove (PFG) and femoral condyle (FC) from bovine animals over a range of growth stages (third-trimester fetal, 1-3 week-old calf, and adult), and (2) to determine if these properties were correlated with collagen network components. With growth from the fetus to the adult, the equilibrium and dynamic tensile moduli and strength of cartilage samples increased by an average of 391-1060%, while the strain at the failure decreased by 43%. The collagen concentration (per wet weight) increased by 98%, and the pyridinoline cross-link concentration increased by 730%, while the glycosaminoglycan concentration remained unchanged or decreased slightly. Some growth-associated changes were location-specific, with tensile moduli and strength attaining higher values in the PFG than the FC. The growth-associated variation in tensile moduli and strength were associated strongly with variation in the contents of collagen and pyridinoline cross-link, but not sulfated glycosaminoglycan. The marked changes in the tensile properties and collagen network components of articular cartilage with growth suggest that such parameters may be used to evaluate the degrees to which regenerated cartilage recapitulates normal development and growth.

Rehabilitation After Oswestry Autologous-Chondrocyte Implantation: The OsCell Protocol

Objective:

To develop a postoperative rehabilitation protocol for patients receiving autologous-chondrocyte implantation (ACI) to repair articular-cartilage defects of the knee.

Data Sources:

careful review of both basic science and clinical literature, personal communication with colleagues dealing with similar cases, and the authors’ experience and expertise in rehabilitating numerous patients with knee pathologies, injuries, and trauma.

Data Synthesis:

Postoperative rehabilitation of the ACI patient plays a critical role in the outcome of the procedure. The goals are to improve function and reduce discomfort by focusing on 3 key elements: weight bearing, range of motion, and strengthening.

Conclusions:

The authors present 2 flexible postoperative protocols to rehabilitate patients after an ACI procedure to the knee.

Signal transduction by mechanical strain in chondrocytes

PURPOSE OF REVIEW

Exercise and passive motion exert reparative effects on inflamed joints, whereas excessive mechanical forces initiate cartilage destruction as observed in osteoarthritis. However, the intracellular mechanisms that convert mechanical signals into biochemical events responsible for cartilage destruction and repair remain paradoxical. This review summarizes how signals generated by mechanical stress may initiate repair or destruction of cartilage.

RECENT FINDINGS

Mechanical strain of low magnitude inhibits inflammation by suppressing IL-1beta and TNF-alpha-induced transcription of multiple proinflammatory mediators involved in cartilage degradation. This also results in the upregulation of proteoglycan and collagen synthesis that is drastically inhibited in inflamed joints. On the contrary, mechanical strain of high magnitude is proinflammatory and initiates cartilage destruction while inhibiting matrix synthesis. Investigations reveal that mechanical signals exploit nuclear factor-kappa B as a common pathway for transcriptional inhibition/activation of proinflammatory genes to control catabolic processes in chondrocytes. Mechanical strain of low magnitude prevents nuclear translocation of nuclear factor kappa B, resulting in the suppression of proinflammatory gene expression, whereas mechanical strain of high magnitude induces transactivation of nuclear factor kappa B, and thus proinflammatory gene induction.

SUMMARY

The beneficial effects of physiological levels of mechanical signals or exercise may be explained by their ability to suppress the signal transduction pathways of proinflammatory/catabolic mediators, while stimulating anabolic pathways. Whether these anabolic signals are a consequence of the inhibition of nuclear factor kappa B or are mediated via distinct anabolic pathways is yet to be elucidated.

Microenvironment regulation of extracellular signal-regulated kinase activity in chondrocytes: effects of culture configuration, interleukin-1, and compressive stress

OBJECTIVE

To compare extracellular signal-regulated kinase (ERK) activity in response to interleukin-1 (IL-1) in chondrocytes under various culture configurations designed for the study of cartilage biology and repair, and also in response to dynamic load for chondrocytes in cartilage.

METHODS

Isolated bovine articular chondrocytes were maintained in serum-supplemented medium under 4 culture configurations: high-density monolayer, attached to a cut surface of cartilage, within tissue-engineered constructs, or within intact cartilage explants. Samples were subjected to a change of medium with or without IL-1. Cartilage explants were also subjected to dynamic compression.

RESULTS

In chondrocyte monolayers, both basal and IL-1-stimulated ERK activities were similarly elevated at 0.5 hours after medium change, diminishing by 74% after 16 hours. In contrast, chondrocytes in other culture configurations exhibited lower basal levels of ERK activity and a moderate activation of ERK in response to IL-1 that was sustained over the 16-hour treatment time. The dynamic component of loading of cartilage explants led to a 5-fold activation of ERK, compared with free-swelling controls, that was indistinguishable from the effects of IL-1.

CONCLUSION

ERK signaling in response to IL-1 in chondrocyte monolayers exhibited a pattern that was distinct from that in other culture systems, suggesting that the extracellular matrix plays an important regulatory role in modulating the response to extracellular stimuli. Since IL-1 and dynamic loading have distinct effects on chondrocyte biosynthesis, signaling pathways other than ERK participate in the chondrocyte responses to these stimuli.