# Intermitt Dynamic Compression Conters Anabolic Effects in Articular Cartilage
## 4 Discussion  
This study aimed to characterize the matrix turnover effects of long-term intermittent dynamic compression in a novel ex vivo cartilage model. We showed that daily low-grade cyclic compression of cartilage explants leads to increased type II collagen formation and pericellular remodeling, which can be distinguished by the biomarker release of PRO-C2 and FBN-C to the conditioned media. We demonstrated that PRO-C2 release was greater when dynamic compression was repeated five versus three times per week, while FBN-C release was similar at three or five operations. Thus, we showed that type II collagen formation was augmented with repeated loading operations, in agreement with previous studies that studied the effect of repeated loading on type II collagen mRNA [18, 24]. We further found that PRO-C2 increased synergistically as a result of combining IGF-1 and intermittent dynamic compression. Our finding is similar to a previous study in which IGF-1 and dynamic compression increased collagen formation in an additive manner during 0.1 Hz stimulation for 48 h [36]. TGF-\(\beta\)1 increased both PRO-C2 and FBN-C release; however, either increase was not affected by intermittent dynamic compression. Instead, we found that compression-induced cartilage formation was dependent on TGF-\(\beta\) signaling,  
Figure 5: Inhibition of ALK5 blocks cartilage remodeling induced by compressive loading. Cartilage ECM turnover biomarkers released to the explant conditioned media over a three-week culture period. Bovine cartilage explants (\(n=12\) for each condition) were treated with SB-525334 without loading (SB) or compressed 5 times per week in the absence- (w/o + compression) or presence of SB-525334 (SB + compression). Explants cultured in TGF\(\beta\)-1 [50 mg/mL] in the absence- (TGF\(\beta\)-1) or presence of SB-525334 (SB + TGF\(\beta\)-1) were co-cultured as a control of the SB-525334 ability to block signaling from the ALK5 receptor. (**A**) alamarBlue metabolic activity assessment at day 0, 7, 14 and 21 of culture. (**B**) Metabolic activity at day 21 endpoint. (**C**) PRO-C2 release measured at day 0, 7, 14 and 21 of culture. (**D**) PRO-C2 release at day 21. (**E**) FBN-C release measured at day 0, 7, 14 and 21 of culture. (**F**) FBN-C measured at day 21. Data points and bars represent geometric least-squares (LS) mean values and error bars represent 95% confidence intervals. (**A**,**C**,**E**) are presented on a logarithmic scale and (**B**,**D**,**F**) are presented on a linear scale. Asterisks (* or #) indicate Sidak-adjusted \(p\)-values of the shown contrasts: */#: \(p<0.05\), **/##: \(p<0.01\), ***/##: \(p<0.001\), ***/###: \(p<0.0001\). as PRO-C2 and FBN-C release from dynamically compressed explants was blocked by the ALK5 inhibitor SB-525334. The SB-525334 inhibitor was previously shown to inhibit aggrecan and type II collagen mRNA levels in TGF-\(\beta\)1-stimulated rat chondrocytes [45]. Our focus was on intermittent dynamic compression, in which each stimulation procedure lasted for only 20 min. This is a relatively short time duration, in contrast to most other studies investigating the effects of cyclic cartilage compression. The time of the loading procedure was generally longer in other studies, spanning from 4 to 48 h, or even weeks of continuous loading [18; 19; 20; 24; 36]. However, we wanted to simulate in vivo conditions, in which periods of cyclic loading rarely last more than 1 h and are interspersed with periods of partial recovery but also of full recovery that last at least 16 h per day [25]. We only applied a single stimulation protocol per day. It is possible that a greater effect of intermittent compression would be achieved by having multiple 20 min stimulations per day, allowing time for recovery in between, or by prolonging the daily stimulation protocol past 20 min. These will be considerations for future studies. Normal joint motion in vivo produces a superposition of compressive, tensile and shear deformations of cartilage.