Correlation of high frequency ultrasound backscatter with cartilage matrix constituents

We have shown in a previous work that high frequency quantitative backscatter ultrasound (US) was sensitive to progressive changes related to various stages of osteoarthritis (OA) and to normal development of rat patellar cartilage. The goal of the study was to identify the cartilage matrix constituents that are responsible for the changes in the echographic signal. To this end, echographic signals from the cartilage were explored after proteoglycan (PG) and collagen degradation by hyaluronidase and papain enzymes. The patellar cartilages of immature rats were degraded and examined in vitro using a three-dimensional (3-D) 55 MHz backscatter ultrasound scanner. B-scan images were correlated with histologic images. For all the specimens US images of control cartilages showed a hyperechoic internal structure involving mainly the first two-thirds of the tissue and consisting of the transitional and deep layers of the cartilage. The backscattered signal from degraded cartilage did not change when hyaluronidase has selectively extracted an important part of the PGs during 6 hours. Whereas the echographic signal from the cartilage surface was significantly affected after 10-minute-papain degradation which induced a first stage of collagen network alteration. These preliminary results demonstrate that high frequency ultrasound does not provide information about variations in PG content but seems to be very sensitive to changes in collagen fibrillar network.