# Intermitt Dynamic Compression Conters Anabolic Effects in Articular Cartilage
## 4 Discussion
It is worth noting that different types of mechanical stress are shown to confer different effects in cartilage tissue. While moderate compressive loading has generally been shown to induce anabolic effects, shear stress has been associated with catabolic changes, including upregulation of pro-inflammatory proteases [61]. We used unconfined dynamic compression in this model, which mainly produces compressive stress in the axial direction as well as tensile stress in the radial direction [62]. We decided on unconfined compression, as confinement would not produce tensile stress in the radial direction, thus being less characteristic of in situ contact deformation [63]. The repeated stimulation protocol was 20 min of unconfined compression at 1 Hz frequency with peak cycle load of 1 MPa. We aimed to simulate physiological compressive loading similar to that exerted on cartilage during low-grade everyday exercise, such as when walking. The applied load was chosen based on literature review. Applied loads of 1000-1500 Newton (N) used to simulate walking in intact human tibiofemoral joints cause contact stress of articular cartilage ranging from 1-4 MPa [64]. Another study described that applied loads at 445-1779 N in intact knee joints produce a contact stress of 0.35-2.75 MPa, with increased compressive force corresponding to increased load distribution across the tibiofemoral surface [65]. Thus, the force used in this study is comparable to low-grade physiological load. However, we used bovine cartilage and it is possible that physiological contact stresses in bovine knee joints differ from human joints. Nonetheless, much of the information available on the chondrocyte responses to mechanical stimuli has been achieved in bovine explant cultures, as they provide a more controlled biomechanical environment than in vivo models [26]. In bovine cartilage explants, dynamic compressive stresses at 0.1-5 MPa and 0.1-1 Hz frequency generally produce biosynthetic responses [36; 66; 67], while explants subjected to compressive forces of 6-10 MPa respond with increased apoptosis [21]. While it is unclear which underlying mechanisms resulted in the interaction between compression and IGF-1, it has been suggested that dynamic compression produces a radial fluid flow that aids the distribution of growth factors in the cartilage and, thus, may augment the stimulation of the embedded chondrocytes [68]. Indeed, [PERSON] et al. found that dynamic compression accelerated the transport of iodine-labeled IGF-1 into the cartilage matrix [36]. On a cellular level, mechanotransduction appears to depend on gated ion channels such as PIEZO and TRPV4 [69], cytoskeletal changes such as mechanical activation of the primary cilia [70] as well as direct receptor-stimulation by cytokines and growth factors secreted in response to loading. However, many of the processes involved in the mechanosensitivity of cartilage also depend on the interaction of the chondrocytes and the surrounding matrix, including integrin-mediated signaling. [PERSON] et al. showed that integrin \(\alpha\)1\(\beta\)1 responds to the osmotic pressure that is generated by mechanical loading [71]. Furthermore, mechanical forces may stimulate the release of growth factors bound in the matrix, such as IGF-1, which is stored inactivated in complex with IGF-binding proteins [72]. We did not find any significant contrasts between explants treated with compression and TGF-\(\beta\)1, versus unloaded explants treated only with TGF-\(\beta\)1. Instead, we demonstrated that TGF-\(\beta\) signaling is important in relaying compression-induced cartilage remodeling, as ALK5 inhibition blocked the release of PRO-C2 and FBN-C from compressed explants. Hence, we speculate that the compression-induced PRO-C2- and FBN-C increase may in part be explained by activation of TGF-\(\beta\) stored in the explant ECM. Cartilage ECM contains large stores of TGF-\(\beta\), up to 300 ng/mL [73].