Restoring the lost heat of the joint: Thermomechanical stimulation as a candidate treatment for early-stage osteoarthritis.

Two of the oldest osteoarthritis treatments, local heat and therapeutic exercise, have evolved along separate paths: heat as a symptomatic adjunct and exercise as a functional intervention. Neither intervention alone modifies the disease. This separation reflects clinical tradition, not joint physiology. Articular cartilage dissipates some of the energy absorbed during deformation. At rest, knee cartilage is 32-33 °C and warms toward 37 °C under loading, which enhances matrix production. Physical activity therefore provides two coupled cues: mechanical deformation and the resulting heat. These cues are successive stages of one physical event, not independent therapeutic levers. Critically, this coupling is impaired early in osteoarthritis. Under load, normal-appearing cartilage from an osteoarthritic joint dissipates roughly half as much energy as healthy cartilage. This deficit, which is undetectable by stiffness-based measures, prevents the tissue from heating itself to the optimal temperature range. In vitro, combined thermal and mechanical stimulation elicits stronger chondroinductive and chondroprotective responses than either cue alone, through pathways involving transient receptor potential vanilloid 4 (TRPV4) and TWIK-related potassium 1 (TREK1) channels, calcium signaling, and heat shock protein 70 (HSP70). We propose that early-stage osteoarthritis, when residual matrix and viable chondrocytes remain, offers a therapeutic window in which exogenous heat could restore the lost cue, potentially making exercise a structure-modifying intervention.