Seonghun Park

Cartilage interstitial fluid load support in unconfined compression

Under physiological conditions of loading, articular cartilage is subjected to both compressive strains, normal to the articular surface, and tensile strains, tangential to the articular surface. Previous studies have shown that articular cartilage exhibits a much higher modulus in tension than in compression, and theoretical analyses have suggested that this tension-compression nonlinearity enhances the magnitude of interstitial fluid pressurization during loading in unconfined compression, above a theoretical threshold of 33% of the average applied stress. The first hypothesis of this experimental study is that the peak fluid load support in unconfined compression is significantly greater than the 33% theoretical limit predicted for porous permeable tissues modeled with equal moduli in tension and compression. The second hypothesis is that the peak fluid load support is higher at the articular surface side of the tissue samples than near the deep zone, because the disparity between the tensile and compressive moduli is greater at the surface zone. Ten human cartilage samples from six patellofemoral joints, and 10 bovine cartilage specimens from three calf patellofemoral joints were tested in unconfined compression. The peak fluid load support was measured at 79 +/- 11% and 69 +/- 15% at the articular surface and deep zone of human cartilage, respectively, and at 94 +/- 4% and 71 +/- 8% at the articular surface and deep zone of bovine calf cartilage, respectively. Statistical analyses confirmed both hypotheses of this study. These experimental results suggest that the tension-compression nonlinearity of cartilage is an essential functional property of the tissue which makes interstitial fluid pressurization the dominant mechanism of load support in articular cartilage.

Inhomogeneous Cartilage Properties Enhance Superficial Interstitial Fluid Support and Frictional Properties, But Do not Provide a Homogeneous State of Stress

It has been well established that articular cartilage is compositionally and mechanically inhomogenous through its depth. To what extent this structural inhomogeneity is a prerequisite for appropriate cartilage function and integrity is not well understood. The first hypothesis to be tested in this study was that the depth-dependent inhomogeneity of the cartilage acts to maximize the interstitial fluid load support at the articular surface, to provide efficient frictional and wear properties. The second hypothesis was that the inhomogeneity produces a more homogeneous state of elastic stress in the matrix than would be achieved with uniform properties. We have, for the first time, simultaneously determined depth-dependent tensile and compressive properties of human patellofemoral cartilage from unconfined compression stress relaxation tests. The results show that the tensile modulus increases significantly from 4.1 +/- 1.9 MPa in the deep zone to 8.3 +/- 3.7 MPa at the superficial zone, while the compressive modulus decreases from 0.73 +/- 0.26 MPa to 0.28 +/- 0.16 MPa. The experimental measurements were then implemented with the finite-element method to compute the response of an inhomogeneous and homogeneous cartilage layer to loading. The finite-element models demonstrate that structural inhomogeneity acts to increase the interstitial fluid load support at the articular surface. However, the state of stress, strain, or strain energy density in the solid matrix remained inhomogeneous through the depth of the articular layer, whether or not inhomogeneous material properties were employed. We suggest that increased fluid load support at the articular surface enhances the frictional and wear properties of articular cartilage, but that the tissue is not functionally adapted to produce homogeneous stress, strain, or strain energy density distributions. Interstitial fluid pressurization, but not a homogeneous elastic stress distribution, appears thus to be a prerequisite for the functional and morphological integrity of the cartilage.

Dynamic response of immature bovine articular cartilage in tension and compression, and nonlinear viscoelastic modeling of the tensile response.

Very limited information is currently available on the constitutive modeling of the tensile response of articular cartilage and its dynamic modulus at various loading frequencies. The objectives of this study were to (1) formulate and experimentally validate a constitutive model for the intrinsic viscoelasticity of cartilage in tension, (2) confirm the hypothesis that energy dissipation in tension is less than in compression at various loading frequencies, and (3) test the hypothesis that the dynamic modulus of cartilage in unconfined compression is dependent upon the dynamic tensile modulus. Experiment 1: Immature bovine articular cartilage samples were tested in tensile stress relaxation and cyclical loading. A proposed reduced relaxation function was fitted to the stress-relaxation response and the resulting material coefficients were used to predict the response to cyclical loading. Adjoining tissue samples were tested in unconfined compression stress relaxation and cyclical loading. Experiment 2: Tensile stress relaxation experiments were performed at varying strains to explore the strain-dependence of the viscoelastic response. The proposed relaxation function successfully fit the experimental tensile stress-relaxation response, with R2 = 0.970+/-0.019 at 1% strain and R2 = 0.992+/-0.007 at 2% strain. The predicted cyclical response agreed well with experimental measurements, particularly for the dynamic modulus at various frequencies. The relaxation function, measured from 2% to 10% strain, was found to be strain dependent, indicating that cartilage is nonlinearly viscoelastic in tension. Under dynamic loading, the tensile modulus at 10 Hz was approximately 2.3 times the value of the equilibrium modulus. In contrast, the dynamic stiffening ratio in unconfined compression was approximately 24. The energy dissipation in tension was found to be significantly smaller than in compression (dynamic phase angle of 16.7+/-7.4 deg versus 53.5+/-12.8 deg at 10(-3) Hz). A very strong linear correlation was observed between the dynamic tensile and dynamic compressive moduli at various frequencies (R2 = 0.908+/-0.100). The tensile response of cartilage is nonlinearly viscoelastic, with the relaxation response varying with strain. A proposed constitutive relation for the tensile response was successfully validated. The frequency response of the tensile modulus of cartilage was reported for the first time. Results emphasize that fluid-flow dependent viscoelasticity dominates the compressive response of cartilage, whereas intrinsic solid matrix viscoelasticity dominates the tensile response. Yet the dynamic compressive modulus of cartilage is critically dependent upon elevated values of the dynamic tensile modulus.

Mechanical load inhibits IL-1 induced matrix degradation in articular cartilage

OBJECTIVE Osteoarthritis is a disease process of cellular degradation of articular cartilage caused by mechanical loads and inflammatory cytokines. We studied the cellular response in native cartilage subjected to a mechanical load administered simultaneously with an inflammatory cytokine interleukin-1 (IL-1), hypothesizing that the combination of load and cytokine would result in accelerated extracellular matrix (ECM) degradation. METHODS Mature bovine articular cartilage was loaded for 3 days (stimulation) with 0.2 and 0.5 MPa stresses, with and without IL-1 (IL-1alpha, 10 ng/ml), followed by 3 days of no stimulation (recovery). Aggrecan and collagen loss were measured as well as aggrecan cleavage using monoclonal antibodies AF-28 and BC-3 for cleavage by aggrecanases (ADAMTS) and matrix metalloproteinases (MMPs), respectively. RESULTS Incubation with IL-1 caused aggrecan cleavage by aggrecanases and MMPs during the 3 days of stimulation. A load of 0.5 MPa inhibited the IL-1-induced aggrecan loss while no inhibition was found for the 0.2 MPa stress. There was no collagen loss during the treatments but upon load and IL-1 removal proteoglycan and collagen loss increased. Load itself under these conditions was found to have no effect when compared to the unloaded controls. CONCLUSIONS A mechanical load of sufficient magnitude can inhibit ECM degradation by chondrocytes when stimulated by IL-1. The molecular mechanisms involved in this process are not clear but probably involve altered mechanochemical signal transduction between the ECM and chondrocyte.

Isoflurane Protects Human Kidney Proximal Tubule Cells against Necrosis via Sphingosine Kinase and Sphingosine-1-Phosphate Generation

Background/Aims: We previously showed that the inhalational anesthetic isoflurane protects against renal ischemia reperfusion injury in part via sphingosine kinase (SK)-mediated synthesis of sphingosine-1-phosphate (S1P). In this study, we tested the hypothesis that isoflurane directly targets renal proximal tubule cells via SK activation, S1P synthesis and activation of S1P receptors to initiate cytoprotective signaling. Methods and Results: Isoflurane-mediated phosphorylation of extracellular signal-regulated kinase (ERK) and Akt and induction of HSP70 in human kidney proximal tubule (HK-2) cells were inhibited by dimethylsphingosine (DMS), an SK inhibitor, and VPC23019, an S1P1/3 receptor selective antagonist, in HK-2 cells. A selective S1P1 receptor agonist, SEW2781, mimicked isoflurane-induced phosphorylation of ERK and Akt and induction of HSP70. Moreover, isoflurane-mediated protection against H2O2-induced necrosis of HK-2 cells was significantly attenuated by an S1P1/3 receptor antagonist, VPC23019, and by SK inhibitors DMS or 4-[[4- (4-chlorophenyl)-2-thiazolyl]amino]phenol. Finally, overexpression of the SK1 enzyme in HK-2 cells protected against H2O2-induced necrosis. Conclusions: Collectively, our study demonstrates that S1P released via isoflurane-mediated SK1 stimulation produces direct anti-necrotic effects probably via S1P1 receptor-mediated cytoprotective signaling (ERK/Akt phosphorylation and HSP70 induction) in HK-2 cells. Our findings may help to unravel the cellular signaling pathways of volatile anesthetic-mediated renal protection and lead to new therapeutic applications of volatile anesthetics during the perioperative period.

The effect of TNFα secreted from macrophages activated by titanium particles on osteogenic activity regulated by WNT/BMP signaling in osteoprogenitor cells

Wear particles are the major cause of osteolysis associated with failure of implant following total joint replacement. During this pathologic process, activated macrophages mediate inflammatory responses to increase osteoclastogenesis, leading to enhanced bone resorption. In osteolysis caused by wear particles, osteoprogenitors present along with macrophages at the implant interface may play significant roles in bone regeneration and implant osteointegration. Although the direct effects of wear particles on osteoblasts have been addressed recently, the role of activated macrophages in regulation of osteogenic activity of osteoblasts has scarcely been studied. In the present study, we examined the molecular communication between macrophages and osteoprogenitor cells that may explain the effect of wear particles on impaired bone forming activity in inflammatory bone diseases. It has been demonstrated that conditioned medium of macrophages challenged with titanium particles (Ti CM) suppresses early and late differentiation markers of osteoprogenitors, including alkaline phosphatase (ALP) activity, collagen synthesis, matrix mineralization and expression of osteocalcin and Runx2. Moreover, bone forming signals such as WNT and BMP signaling pathways were inhibited by Ti CM. Interestingly, TNFα was identified as a predominant factor in Ti CM to suppress osteogenic activity as well as WNT and BMP signaling activity. Furthermore, Ti CM or TNFα induces the expression of sclerostin (SOST) which is able to inhibit WNT and BMP signaling pathways. It was determined that over-expression of SOST suppressed ALP activity, whereas the inhibition of SOST by siRNA partially restored the effect of Ti CM on ALP activity. This study highlights the role of activated macrophages in regulation of impaired osteogenic activity seen in inflammatory conditions and provides a potential mechanism for autocrine regulation of WNT and BMP signaling mediated by TNFα via induction of SOST in osteprogenitor cells.

Preparation of polyurea microcapsules with different composition ratios: structures and thermal properties

Abstract Polyurea microcapsules were prepared by emulsion polymerization after adding an aqueous solution of poly(vinyl alcohol) as protective colloid to an organic solution of migrin oil as the core substance with aliphatic isophorone diisocyanate (IPDI) and aromatic 2,4-toluene diisocyanate (TDI) as wall-forming materials. Diisocyanate types were investigated for their effects on mean particle size, size distributions and thermal properties. Mean particle size decreased and the size distribution narrowed with the concentration of aliphatic IPDI in the wall. This was deemed to be due to the inferior reactivity of aliphatic IPDI with active hydrogen in water compared to aromatic TDI. Microcapsules from TDI alone in the wall had a broader and larger melting transition at 300–350°C whereas those from IPDI alone showed higher thermal stability.

Mechanical properties of bovine articular cartilage under microscale indentation loading from atomic force microscopy.

Abstract Atomic force microscopy (AFM) techniques have been increasingly used for investigating the mechanical properties of articular cartilage. According to the previous studies reporting the microscale Youngs modulus under AFM indentation tests, the Hertz contact model has been employed with a sharp conical tip indenter. However, the non-linear microscale behaviour of articular cartilage could not be resolved by the standardized Hertz analysis using small and sharp atomic force microscope tips. Therefore, the objective of this study was to evaluate the microscale Youngs modulus of articular cartilage more accurately through a non-Hertzian approach with a spherical tip of 5 μm diameter, and to characterize its microscale mechanical behaviour. This methodology adopted in the present study was proved by the consistent values between the microscale (2 per cent, about 9.3 kPa; 3 per cent, about 17.5 kPa) and macroscale (2 per cent, about 8.3 kPa; 3 per cent, about 18.3 kPa) Youngs moduli for 2 per cent and 3 per cent agarose gel (n = 100). Therefore, the microscale Youngs modulus evaluated in this study is representative of more accurate measurements of cartilage stiffness at the 600 nm deformation level and corresponds to approximately 30.9 kPa (n = 100). Furthermore, on this level of the microscale deformation, articular cartilage showed depth-dependent and frequency-independent behaviour under AFM indentation loading. These findings reveal the microscale mechanical behaviour of articular cartilage more accurately and can be employed further to design microscale structures of chondrocyte-seeded scaffolds and tissue-engineered cartilage by evaluating their microscale properties.

Recurrent Varicoceles: Causes and Treatment Using Angiography and Magnification Assisted Subinguinal Varicocelectomy

Purpose To investigate the causes of varicocele recurrence and assess the use of embolization and subinguinal varicocelectomy in its treatment in patients with angiography and subinguinal varicocelectomy. Materials and Methods The present study involved 15 patients with recurrent varicoceles. The mean patient age was 21.2 years (range: 12-42 years). Preoperative angiography was performed in 11 patients. Embolization was used in patients with patent internal spermatic veins (ISVs). Patients without patent ISVs or preoperative angiography underwent magnification-assisted subinguinal varicocelectomy which included testicular retrieval and ligation of all collateral veins except arteries and deferential veins. Results Seven among 11 patients (64%) which had preoperative angiography had patent ISVs and underwent embolization and 8 patients underwent subinguinal varicocelectomy. Of those 8 patients, 6 had dilated ISVs and external spermatic veins (ESVs), one had dilated ISVs and gubernacular veins, and one had dilated ISVs, ESVs and gubernacular veins. No patient experienced recurrence or testis atrophy. Conclusion Patent ISVs or collateral veins may be the cause of recurrence after varicocelectomy. Angiographic embolization was successful in 64% of recurrent varicoceles patients with patent ISVs. However, microscope-assisted subinguinal varicocelectomy may be the best overall treatment for patients with recurrent varicoceles.

Variability analysis of lower extremity joint kinematics during walking in healthy young adults

The first objective of this study was to determine the kinematic variability of the lower extremity joints using methods from the mathematical chaos theory in a normal walking environment in conjunction with a large population of healthy young adults. The second objective was to test the hypothesis that variability characteristics are different between joints and to further investigate differences between male and female and right and left subgroups. A total of forty young healthy subjects (20 males: 24.1+/-3.1 years; 20 females: 22.5+/-3.2 years) volunteered, and their joint motions were captured while walking on a treadmill for 90 s in order to estimate Lyapunov Exponent (LE) values. Means and standard deviations of the LEs ranged from 0.035+/-0.016 (right ankle) to 0.073+/-0.023 (left knee) for the male subjects and from 0.028+/-0.014 (left ankle) to 0.065+/-0.028 (right hip) for the female subjects. Between the males and females, differences in LEs were observed to be statistically significant only for the left knee. There were no statistically significant differences between the right and left sides of the joints. However, differences between joints were statistically significant except between the hip and knee. These results are the first such comparison of the variability in the lower extremity without the confounding effect of walking speed on the variability of joint motions, and can serve as a normative database.