In osteoarthritis (OA), subchondral sclerosis takes place during cartilage degeneration. High frequency ultrasound (12-55MHz) has been shown to diagnose degenerated articular cartilage, while 0.1-1MHz ultrasound has been applied for clinical characterization of bone and diagnostics of osteoporosis. The aim of the study is to investigate, for the first time, the feasibility of 5MHz ultrasound for simultaneous analysis of articular cartilage and subchondral bone.
Osteochondral samples (n=10) were prepared from fresh and visually normal bovine medial tibial plateaus. Acoustic properties of the cartilage and subchondral bone were measured with a scanning ultrasound system using the pulse-echo geometry and compared with biomechanical, histological and compositional reference data.
The apparent integrated backscatter (AIB) from internal cartilage showed significant partial correlations with hydroxyproline (Hypro) (r=0.58, P=0.000), water content (r=-0.52, P=0.001) and dynamic modulus (r=0.57, P=0.000) of the tissue. Weak but statistically significant correlation was found between the bone AIB and mineral density of the subchondral plate (r=-0.34, P=0.041). Topographical variations in cartilage thickness could be detected using ultrasound. Composition, thickness and mechanical properties of the cartilage varied significantly across the tibial plateau. For the calculated ultrasound parameters, the variation was significant only between a few locations.
Pulse-echo ultrasound geometry at 5MHz was feasible for simultaneous measurement of the acoustic properties of articular cartilage and subchondral bone. However, the relationships between the ultrasound parameters and properties of cartilage and bone were not as strong as reported earlier in studies focusing only either on bone or cartilage. Simultaneous measurement of both tissues compromises, due to natural curvature of articulating surfaces, the perpendicularity of the incidence of the ultrasound pulse. Obviously, this source of uncertainty should be minimized in order to enable effective clinical use of ultrasound in simultaneous measurement of articular cartilage and subchondral bone.