[Space flight/bedrest immobilization and bone. Mechanical stress : a double-edged sword to osteoarthritis]

Osteoarthritis (OA) is a disease caused by the degeneration and destruction of joint cartilage followed by peri-articular or subchondral bone formation and concomitant degeneration of other components of joints including synovium. Several animal OA models that adopt surgically induced joint instability have been developed. Analysis of those OA models indicates that increased mechanical stress exacerbates OA. In contrast, the effectiveness of physical therapy to alleviate OA symptoms suggests that optimal mechanical stimulation can impede the progression of OA. We propose that there are two facets of mechanical stress in light of the influence on OA ; one is detrimental, whilst the other is beneficial for articular cartilage and other joint compositions. From the malalignment and incongruity that underlie hip OA succeeding to congenital hip dislocation (acetabular hypoplasia) and the varus (or valgus) deformity observed in knee OA, shear force appears to prone to be a villain mechanical stress. On the other hand, compressive force may facilitate the maintenance or turnover of the articular cartilage unless it becomes excessive. Although essential differences between detrimental and beneficial mechanical stresses remain elusive, we speculate that the degree or magnitude of the deformation of mechano-sensor(s) may be crucial. Uncovering the mechano-sensing machinery in joints may demarcate the beneficial mechanical stress, and bring about a paradigm shift in the therapeutic strategy for OA.