Extraction of mechanical properties of articular cartilage from osmotic swelling behavior monitored using high frequency ultrasound

Articular cartilage is a biological weight-bearing tissue covering the bony ends of articulating joints. Negatively charged proteoglycan (PG) in articular cartilage is one of the main factors that govern its compressive mechanical behavior and swelling phenomenon. PG is nonuniformly distributed throughout the depth direction, and its amount or distribution may change in the degenerated articular cartilage such as osteoarthritis. In this paper, we used a 50 MHz ultrasound system to study the depth-dependent strain of articular cartilage under the osmotic loading induced by the decrease of the bathing saline concentration. The swelling-induced strains under the osmotic loading were used to determine the layered material properties of articular cartilage based on a triphasic model of the free-swelling. Fourteen cylindrical cartilage-bone samples prepared from fresh normal bovine patellae were tested in situ in this study. A layered triphasic model was proposed to describe the depth distribution of the swelling strain for the cartilage and to determine its aggregate modulus [H.sub.a] at two different layers, within which [H.sub.a] was assumed to be linearly dependent on the depth. The results showed that [H.sub.a] was 3.0 [+ or -] 3.2, 7.0 [+ or -] 7.4, 24.5 [+ or -] 11.1 MPa at the cartilage surface, layer interface, and deep region, respectively. They are significantly different (p Keywords: articular cartilage, ultrasound, elastography, biomechanics, triphasic theory, proteoglycan, fixed charge density, osmotic loading, osteoarthritis