Quantitative assessment of articular cartilage with morphologic, acoustic and mechanical properties obtained using high-frequency ultrasound.

Osteoarthritis (OA) is one of the most common joint diseases among adults, and its early detection is still not possible. In this study, high-frequency ultrasound and ultrasound-assisted mechanical testing systems were used to quantitatively measure the morphologic, acoustic and mechanical properties of normal and enzymatically degraded bovine articular cartilages in vitro. A total of 40 osteochondral cartilage plugs were prepared from 20 bovine patellae (n=20x2) and divided into two groups for collagenase and trypsin digestions, respectively. A high-frequency ultrasound system (center frequency: 40 MHz) was used to analyze the surface integrity (ultrasound roughness index, URI), thickness and acoustic properties of the articular cartilages before and after enzymatic degradations. Acoustic parameters included the integrated reflection coefficient (IRC) from the cartilage surface, reflection from the cartilage-bone interface (AIB(bone)), integrated attenuation (IA) and integrated backscatter (IBS) of the internal cartilage tissue. A newly developed ultrasound water jet indentation system was used to assess the mechanical properties of the cartilage samples. The results showed that the URI increased significantly (p<0.05) after collagenase digestion while no significant change (p>0.05) was found after trypsin digestion. With regard to acoustic parameters, the IRC decreased significantly (p<0.05) after collagenase digestion while no significant change (p>0.05) was found after trypsin digestion. The AIB(bone) demonstrated an insignificant change after collagenase digestion (p>0.05) but a significant decrease after trypsin digestion (p<0.05). Both enzymatic degradation groups showed insignificant differences (p>0.05) in the IA but a significant increase (p<0.05) in the IBS after both enzymatic degradations. The apparent stiffness measured by ultrasound water jet indentation suggested that articular cartilage from both groups became significantly softer (p<0.05) after the enzymatic degradations. A significant relationship was found to exist between the IRC and URI (p<0.05). This study showed that high-frequency ultrasound can be a comprehensive tool to quantitatively and systematically analyze the morphologic, acoustic and mechanical properties of articular cartilage in association with its degeneration.