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The paper presents a theoretical model for the normal contact of a rigid sphere with an elastic-perfectly plastic half-space or an elastic-perfectly plastic sphere with a rigid wall. F ormulae describing the force-displacement relationship for static contact problems and the coef cient of restitution for dynamic impact problems are derived. The present model can be considered as a modi cation of Johnson's model by using a more detailed pressure distribution function which is based on nite element analysis (F EA) results and considering the variation in the curvature of the contact surface during the contact interaction. In order to verify the theoretical model, nite element analyses are also conducted, and results are compared with those predicted by the model for both contact force-displacement relations and restitution coef cients. G ood agreements between the model predictions and the F EA results are found.

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BackgroundThe characteristics and therapeutic potential of subtypes of mesenchymal stem cells (MSCs) are largely unknown. In this study, CD146+ and CD146– MSCs were separated from human umbilical cords, and their effects on regulatory T cells (Tregs), Th17 cells, chondrogenesis, and osteogenesis were investigated.MethodsFlow cytometry was used to quantify IL-6 and TGF-β1 expressed on CD146+ and CD146– MSCs. The therapeutic potential of both subpopulations was determined by measuring the clinical score and joint histology after intra-articular (IA) transfer of the cells into mice with collagen-induced arthritis (CIA).ResultsCompared with CD146– MSCs, CD146+ MSCs expressed less IL-6 and had a significantly greater effect on chondrogenesis. After T lymphocyte activation, Th17 cells were activated when exposed to CD146– cells but not when exposed to CD146+ cells both in vitro and in vivo. IA injection of CD146+ MSCs attenuated the progression of CIA. Immunohistochemistry showed that only HLA-A+ CD146+ cells were detected in the cartilage of CIA mice. These cells may help preserve proteoglycan expression.ConclusionsThis study suggests that CD146+ cells have greater potency than CD146– cells for cartilage protection and can suppress Th17 cell activation. These data suggest a potential therapeutic application for CD146+ cells in treating inflammatory arthritis.

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