Changes in triphasic mechanical properties of proteoglycan-depleted articular cartilage extracted from osmotic swelling behavior monitored using high-frequency ultrasound

This study aims to obtain osmosis-induced swelling strains of normal and proteoglycan (PG) depleted articular cartilage using an ultrasound system and to investigate the changes in its mechanical properties due to the PG depletion using a layered triphasic model. The swelling strains of 20 cylindrical cartilage-bone samples collected from different bovine patellae were induced by decreasing the concentration of bath saline and monitored by the ultrasound system. The samples were subsequently digested by a trypsin solution for approximately 20 min to deplete proteoglycans, and the swelling behaviors of the digested samples were measured again. The bi-layered triphasic model proposed in our previous study (Wang et al., J Biomech Eng-Trans ASME 2007; 129: 413-422) was used to predict the layered aggregate modulus Hafrom the data of depth-dependent swelling strain, fixed charge density and water content. It was found that the region near the bone, for the normal specimens, had a significantly higher aggregate modulus (Ha1= 20.6 ± 18.2 MPa) in comparison with the middle zone and the surface layer (Ha2= 7.8 ± 14.5 MPa and Ha3= 3.6 ± 3.2 MPa, respectively) (p < 0.001). The normalized thickness of the deep layer h1was 0.68 ± 0.20. After the trypsin digestion, the parametric values decreased to Ha1= 13.6 ± 9.6 MPa, Ha2= 6.7 ± 11.5 MPa, Ha3= 2.7 ± 3.2 MPa, and h1= 0.57 ± 0.28. Other models were also used to analyze data and the results were compared. This study showed that high-frequency ultrasound measurement combined with the triphasic modeling was capable of nondestructively quantifying the alterations in the layered mechanical properties of the proteoglycan-depleted articular cartilage.