Role of external forces in the mechanobiology of stem and differentiated chondrogenic cells embedded in a tissue-engineered construct for cartilage repair.

Articular cartilage, as a mechanosensitive tissue, supports and distributes various mechanical forces-including compression, shear, hydrostatic pressure, and tensile strain-during joint loading and motion. These external forces deform not only the chondrocytes but also their pericellular matrix and the surrounding extracellular matrix (ECM). Those mechanical cues are detected by mechanosensors on the plasma membrane (e.g., integrins) and transmitted through the cytoskeleton, ultimately being converted into biochemical signals. These signals activate key mechanoresponsive intracellular pathways-including TGF-β-induced SMAD, Rho-GTPase, MAPKs (ERK, JNK, p38), PI3K/AKT/mTOR, MRTF-SRF, and YAP/TAZ-that regulate chondrogenic differentiation and cartilage-specific matrix synthesis. This field of study is known as mechanobiology. Over the past decades, it has gained increasing recognition, particularly with the emergence of tissue-engineering constructs as a novel strategy for cartilage repair. However, progress in chondrogenic mechanobiology has primarily centred on intrinsic substrate- or matrix-derived cues, while overlooking the role of extrinsic mechanical forces. This review therefore provides an updated perspective on chondrogenic mechanobiology, with a particular focus on the cellular responses to external mechanical stimuli. It also emphasizes the therapeutic potential of incorporating mechanical stimulation into tissue-engineering strategies for cartilage repair, an emerging filed referred to as Regenerative Rehabilitation (RR). Since this concept has so far been investigated mainly in vitro, we highlight only those studies and refer to it as In vitro Regenerative Rehabilitation. Moreover, this review also addresses post-traumatic osteoarthritis (PTOA), a common joint disorder that frequently results from traumatic cartilage damage. It explores the mechanobiological mechanisms underlying OA and discusses in vitro regenerative rehabilitation studies, highlighting how external forces could serve as an alternative to conventional biochemical treatments for preventing OA progression.