Ultrasonic Methods For Measuring Biomechanical Properties Of Articular Cartilage

Articular cartilage is categorized as an avascular and high specificity tissue. It provides the joint with essential biomechanical functions, such as wear resistance, load bearing, and shock absorption for eight decades or more. At the same time, articular cartilage is one of the most disease-prone parts due to its lack of self-repair ability. This paper aims to develop a novel noncontact ultrasonic technique for assessment of articular cartilage degeneration in the early stage of OA. The other objective of this study is to assess the quality of cartilage that has been repaired using tissue engineering replacement strategies as the indentation tests may cause marked cell death in human cartilage even at low forces and displacements. Based on the architecture of articular cartilage, a layered mechanical model was established. A simulation method for analysis of the acoustic field distribution in multi-layers and the pulse echoes from different interfaces of cartilage layers using the present model. Furthermore, a measurement system using water jet indentation was developed to assessment of viscoelastic parameters in cartilage layer. And a novel nondestructive ultrasonic technique for measuring the sound speed and acoustic impedance of articular cartilage using the pulsed V(z, t) technique was presented. These methods make quantitative, nondestructive ultrasonic measurements possible for the evaluation of the degeneration of articular cartilage.The main research contents include the following parts:1. A layered mechanical model was established based on the architecture of articular cartilage. The superficial layer and radial layer of cartilage were categorized as transversely isotropic continuous media; the intermediate layer was modeled by a nonhomogeneous QLV layer of constant thickness. A simulation method for analysis of the acoustic field distribution in multi-layers and the pulse echoes from different interfaces of cartilage layers based on the present model.2. A novel nondestructive ultrasonic technique using V(z,t) data was developed to measure the sound speed and acoustic impedance of articular cartilage. V(z, t) data include a series of pulsed ultrasonic echoes collected using different distances between the ultrasonic transducer and the specimen. The 2D Fourier transform is applied to the V(z,t) data to reconstruct the 2D reflection spectrum R(θ,ω). To obtain the reflection coefficient of articular cartilage, the V(z, t) data from a reference specimen with a well-known reflection coefficient are obtained to eliminate the dependence on the general system transfer function. The ultrasound-derived aggregate modulus (Ha) is computed based on the measured reflection coefficient and the sound speed. In the experiment,32 cartilage-bone samples were prepared from bovine articular cartilage, and 16 samples were digested using 0.25% trypsin solution. The sound speed and Ha of these cartilage samples were evaluated before and after degeneration. The magnitude of the sound speed decreased with trypsin digestion (from 1663± 5.6 m/s to 1613 ± 5.3 m/s). Moreover, the Young’s modulus in the corresponding degenerative state was measured and was correlated with the ultrasound-derived aggregate modulus. The ultrasound-derived aggregate modulus was determined to be highly correlated with the Young’s modulus (n=16, r> 0.895, p< 0.003, Pearson correlation test for each measurement). The results demonstrate the effectiveness of using the proposed method to assess the changes in sound speed and the ultrasound-derived aggregate modulus of cartilage after degeneration.3. A novel noncontact ultrasonic indentation system, using a water beam as an indenter to stimulate the tissue, was developed to evaluate the viscoelasticity of articular cartilage. The water beam also served as a medium for ultrasound propagation, and the deformation was estimated from the ultrasonic echoes using an improved cross-correlation algorithm. Variable window length of the reference signals from specimen made of silicone rubber (8110, Dow Corning Holding Company, USA) was used in the improved cross-correlation algorithm. The indentation force was calculated from the water pressure measured inside the water pipe. This method does not require a rigid compressor in front of the focused high frequency ultrasound transducer to compress the tissue, so that the additional attenuation caused by the rigid compressor and the strong echoes reflected from its surfaces can be avoided. In order to determine the viscoelastic parameters of cartilage specimen, the curves acquired from water jet indentation tests were performed as the fitting target curves in the finite-element-based inverse solution. A pre-stress and an appropriate ramp speed to improve water pressure were required during the experiments. The results show that the deformation recovery curves acquired by the presented system were sensitive to the changes of the viscoelastic parameters.