Targeted loss of proteoglycans results in changes of frequency-dependent viscoelastic behavior of the intact articular cartilage

Cartilage is a multi-phasic, viscoelastic material that derives its mechanical behavior of its primary constituents including collagen, proteoglycans, and water. The complex mechanical function of cartilage depends critically on the composition and balance of these constituents. We sought to determine the effects of proteoglycan loss on both the time- and frequency-dependent mechanical behavior of articular cartilage. Using cathepsin d, an enzyme that specifically cleaves proteoglycans, we assessed the in situ mechanical behavior of intact bovine articular cartilage before and after enzymatic digestion using microindentation over loading frequencies ranging between 0.5 hz to 20 hz. The loss of proteoglycans does not affect the elastic components of mechanical behavior (indentation modulus; p=0.67), but have significant consequences on the viscoelastic components (tan δ; p<0.001). Moreover, the changes in the viscoelastic mechanical behavior are more pronounced at higher loading frequencies (p<0.001). Taken together, these results suggest that proteoglycans are critical for providing dynamic stability for the cartilage tissue.