"Slow walk" mimetic tensile loading maintains human meniscus tissue resident progenitor cells homeostasis in photocrosslinked gelatin hydrogel

Meniscus, the cushion in knee joint, is a load-bearing tissue that transfers mechanical forces to extracellular matrix (ECM) and tissue resident cells. The mechanoresponse of human tissue resident stem/progenitor cells in meniscus (hMeSPCs) is significant to tissue homeostasis and regeneration but is not well understood. This study reports that a mild cyclic tensile loading regimen of ∼1800 loads/day on hMeSPCs seeded in 3-dimensional (3D) photocrosslinked gelatin methacryloyl (GelMA) hydrogel is critical in maintaining cellular homeostasis. Experimentally, a "slow walk" biomimetic cyclic loading regimen (10% tensile strain, 0.5 Hz, 1 h/day, up to 15 days) is applied to hMeSPCs encapsulated in GelMA hydrogel with a magnetic force-controlled loading actuator. The loading significantly increases cell differentiation and fibrocartilage-like ECM deposition without affecting cell viability. Transcriptomic analysis reveals 332 mechanoresponsive genes, clustered into cell senescence, mechanical sensitivity, and ECM dynamics, associated with interleukins, integrins, and collagens/matrix metalloproteinase pathways. The cell-GelMA constructs show active ECM remodeling, traced using a green fluorescence tagged (GFT)-GelMA hydrogel. Loading enhances nascent pericellular matrix production by the encapsulated hMeSPCs, which gradually compensates for the hydrogel loss in the cultures. These findings demonstrate the strong tissue-forming ability of hMeSPCs, and the importance of mechanical factors in maintaining meniscus homeostasis.

The effect of 4 mm bicortical drill hole defect on bone strength in a pig femur model

INTRODUCTION

In orthopaedic surgery, small bicortical circular bone defects are often produced as a result of internal fixation of fractures. The aim of this study was to determine the amount of torsional strength reduction in animal bone with a bicortical bone defect and how much residual strength remains if the bicortical bone defect was occluded.

METHOD

Forty pig femurs were divided into four groups. Group 1 femurs were left intact. Group 2 femurs were given a 4 mm bicortical bone defect. Group 3 were prepared as in Group 2, but occluded with a 4.5 mm cortical screw. Group 4 were prepared as in Group 2, but occluded with plaster of paris. Measurements including the length of the bone, working length of the bone, mid-diaphyseal diameter and cortical thickness were recorded. All specimens were tested until failure under torsional loading. Peak torque at failure and angular deformation were recorded. One-way analysis of variance was used to test the sample groups, with a value of P < 0.05 considered to be statistically significant.

RESULTS

When compared with Group 1, all of the other groups showed a reduction in peak torque at failure point. Only the difference in peak torque between Groups 1 and 2 was statistically significant (P = 0.007). Group 2 showed the most reduction with 23.11% reduction in peak torque and 38.19% reduction in total energy absorption. No significant difference was found comparing the bone length, bone diameter and the cortical thickness.

CONCLUSION

The presence of the defect remains the major contributing factor in long bone strength reduction. It has been shown that a 10% bicortical defect was sufficient to produce a reduction in peak torque and energy absorption under torsional loading. By occluding this defect using a screw or plaster of paris, an improvement in bone strength was achieved. These results may translate clinically to an increased vulnerability to functional loads immediately following screw removal and prior to the residual screw holes healing.