Treadmill exercise promotes bone tissue recovery in rats subjected to high + Gz loads.

INTRODUCTION: High + Gz loads, the gravitational forces experienced by the body in hypergravity environments, can lead to bone loss in pilots and astronauts, posing significant health risks. MATERIALS AND METHODS: To explore the effect of treadmill exercise on bone tissue recovery, a study was conducted on 72 male Wistar rats. These rats were subjected to four weeks of varying levels of periodic high + Gz loads (1G, 8G, 20G) experiments, and were subsequently divided into the treadmill group and the control group. The treadmill group underwent a continuous two-week treadmill experiment, while the control group rested during this period. The mechanical properties, microstructure, and molecular markers of their tibial bone tissue were measured using three-point bending, micro-CT, and PCR. RESULTS: The results showed that treadmill exercise improved the elastic modulus, ultimate deflection, and ultimate load of rat bone tissue. It also increased the number, density, and volume fraction of bone trabeculae, and decreased their separation. Moreover, treadmill exercise enhanced osteogenesis and inhibited osteoclastogenesis. CONCLUSION: This study demonstrates that treadmill exercise can promote the recovery of bone tissue in rats subjected to high + Gz loads, providing a potential countermeasure for bone loss in pilots and astronauts.

[Effects of removing superficial layer of cartilage on the surface morphology and mechanical behavior of cartilage].

Superficial cartilage defect is an important factor that causes osteoarthritis. Therefore, it is very important to investigate the influence of superficial cartilage defects on its surface morphology and mechanical properties. In this study, the knee joint cartilage samples of adult pig were prepared, which were treated by enzymolysis with chymotrypsin and physical removal with electric friction pen, respectively. Normal cartilage and surface treated cartilage were divided into five groups: control group (normal cartilage group), chymotrypsin immersion group, chymotrypsin wiping group, removal 10% group with electric friction pen, and removal 20% group with electric friction pen. The surface morphology and structure of five groups of samples were characterized by laser spectrum confocal microscopy and environmental field scanning electron microscopy, and the mechanical properties of each group of samples were evaluated by tensile tests. The results show that the surface arithmetic mean height and fracture strength of the control group were the smallest, and the fracture strain was the largest. The surface arithmetic mean height and fracture strength of the removal 20% group with electric friction pen were the largest, and the fracture strain was the smallest. The surface arithmetic mean height, fracture strength and fracture strain values of the other three groups were all between the above two groups, but the surface arithmetic mean height and fracture strength of the removal 10% group with electric friction pen, the chymotrypsin wiping group and the chymotrypsin soaking group decreased successively, and the fracture strain increased successively. In addition, we carried out a study on the elastic modulus of different groups, and the results showed that the elastic modulus of the control group was the smallest, and the elastic modulus of the removal 20% group with electric friction pen was the largest. The above study revealed that the defect of the superficial area of cartilage changed its surface morphology and structure, and reduced its mechanical properties. The research results are of great significance for the prevention and repair of cartilage injury.

Hypergravity stimulates mechanical behavior and micro-architecture of tibia in rats.

INTRODUCTION: The bone tissue is susceptible to hypergravity (+ G) environment. It is necessary to discuss the extent to which specific + G values are beneficial or detrimental to bone tissue. The objective of this study was to characterize the effects of high + G values on mechanical properties, microstructures, and cellular metabolism of bone. MATERIALS AND METHODS: 30 male Wistar rats aged 12 weeks were randomly divided into 5 groups, and bore different + G (namely + 1G, + 4G, + 8G, + 10G and + 12G) environments respectively for 4 weeks, 5 days each week, and 3 minutes each day. The macro-mechanical parameters, microstructure parameters, and mRNA transcription levels of the tibia were determined through the three-point bending method, micro-CT detection, and q-PCR analysis, respectively. RESULTS: As the + G value increases, hypergravity becomes increasingly detrimental to the macro-mechanical performance of rat tibia. Concerning the microstructure of cancellous bone, there appears to be a favorable trend at + 4G, followed by a progressively detrimental trend at higher G values. In addition, the mRNA transcription levels of OPG and RANKL show an initial tendency of enhanced bone absorption at +4G, followed by an increase in bone remodeling capacity as G value increases. CONCLUSION: The higher G values correspond to poorer macro-mechanical properties of the tibia, and a + 4G environment benefits the microstructure of the tibia. At the cellular level, bone resorption is enhanced in the + 4G group, but the bone remodeling capability gradually increases with further increments in G values.

Irrigating degradation properties of silk fibroin-collagen type II composite cartilage scaffold in vitro and in vivo.

Silk fibroin-collagen type II scaffolds are promising in cartilage tissue engineering due to their suitable biological functionality to promote proliferation of chondrocytes in vitro. However, their degradation properties, which are of crucial importance as scaffold degradation should consistent with the new tissue formation process, are still unknown. In this study, degradability of silk fibroin-collagen type II cartilage scaffolds was probed both in vitro and in vivo. In vitro degradation experiments show that the scaffolds decreased 32.25 % ± 0.62 %, 34.27 % ± 0.96 %, 36.27 % ± 2.39 % in weight after 8 weeks of degradation at the irrigation velocity of 0 mL/min, 7.89 mL/min and 15.79 mL/min. The degradation ratio, which increases with time and increasing irrigation velocity, is described by combining the built mathematic model and finite element modeling method. The scaffolds after 8 weeks of degradation in vitro keep their mechanical structural integrity to support new tissues. In vivo degradation experiments conducted in rabbits further show that the scaffolds degrade gradually, be absorbed with time and finally collapse in structure. The degradation process is accompanied by the growth of fibrous tissues and the scaffold is filled by fibrous tissues after 12 weeks of implantation. Immunohistology analysis shows that the inflammation caused by scaffolds is controllable and gradually alleviates with time. To sum up, silk fibroin-collagen type II cartilage scaffolds, which show suitable mechanical properties and biocompatibility during degradation in vitro and in vivo, have great potential in cartilage repair. The novelty of the study is that it not only introduces a mathematical model to predict the irrigation degradation ratio, but also provides experimental degradation data support for clinical application of silk fibroin-collagen type II cartilage scaffolds.

[Study on transport of small molecule rhodamine B within different layers of cartilage].

The small molecule nutrients and cell growth factors required for the normal metabolism of chondrocyte mainly transport into the cartilage through free diffusion. However, the specific mass transfer law in the cartilage remains to be studied. In this study, using small molecule rhodamine B as tracer, the mass transfer models of cartilage were built under different pathways including surface pathway, lateral pathway and composite pathway. Sections of cartilage at different mass transfer times were observed by using laser confocal microscopy and the transport law of small molecules within different layers of cartilage was studied. The results showed that rhodamine B diffused into the whole cartilage layer through surface pathway within 2 h. The fluorescence intensity in the whole cartilage layer increased with the increase of mass transfer time. Compared to mass transfer of 2 h, the mean fluorescence intensity in the superficial, middle, and deep layers of cartilage increased by 1.83, 1.95, and 3.64 times, respectively, after 24 h of mass transfer. Under lateral path condition, rhodamine B was transported along the cartilage width, and the molecular transport distance increased with increasing mass transfer time. It is noted that rhodamine B could be transported to 2 mm away from cartilage side after 24 h of mass transfer. The effect of mass transfer under the composite path was better than those under the surface path and the lateral path, and especially the mass transfer in the deep layer of cartilage was improved. This study may provide a reference for the treatment and repair of cartilage injury.

Mechanical properties of cracked articular cartilage under uniaxial creep and cyclic tensile loading.

Cracks may change the mechanical properties of articular cartilage, and further lead to early osteoarthritis. The study aimed to probe the mechanical properties of cracked cartilage under uniaxial tensile loading. The fracture process of cracked cartilage can be divided into two stages, namely crack-tip blunting stage and crack growth stage. The creep strain of cracked cartilage increases rapidly and then slowly with time, and it is well predicted by the nonlinear viscoelastic creep model. Compared with intact cartilage, cracked cartilage shows larger creep strain. During cyclic loading, the mean strain, degree of necking and crack-tip blunting of cracked cartilage increase with the increase of peak stress, while they decrease with the increase of loading frequency. The crack-tip morphology shows that cyclic loading has induced irreversible deformation in cartilage with a large number of collagen fibrils yielding, and further damaged the collagen fibril network of cartilage. However, no obvious crack growth is observed under the testing conditions.

Creep-recovery behaviors of articular cartilage under uniaxial and biaxial tensile loadings.

Creep deformation in cartilage can be observed under physiological loads in daily activities such as standing, single-leg lunge, the stance phase of gait. If not fully recovered in time, it may induce irreversible damage in cartilage and further lead to early osteoarthritis. In this study, 36 cruciform-shape samples in total from 18 bulls were employed to conduct the uniaxial and biaxial creep-recovery tests by using a biaxial cyclic testing system. Effects of stress level (σ = .5, 1.0, 1.5 MPa) and biaxial stress ratio (B = 0, .3, .5, 1.0) on creep-recovery behaviors of cartilage were characterized. And then, a viscoelastic constitutive model was employed to predict its creep-recovery behaviors. The results showed that the creep strain and its three components, namely instantaneous elastic strain, delayed elastic strain and viscous flow strain, increase with the increasing stress level or with the decreasing biaxial stress ratio. Compared with uniaxial creep-recovery, biaxial creep-recovery exhibits a smaller creep strain, a faster recovery rate of creep strain and a smaller residual strain. Besides, the built viscoelastic model can be used to describe the uniaxial creep-recovery behaviors of cartilage as a good correlation between the fitted results and test results is achieved. The findings are expected to provide new insights into understanding normal joint function and cartilage pathology.