Ultrasound speed varies in articular cartilage under indentation loading

In ultrasound elastography, tissue strains are determined by localizing changes in ultrasound echoes during mechanical loading. The technique has been proposed for arthroscopic quantification of the mechanical properties of cartilage. The accuracy of ultrasound elastography depends on the invariability of sound speed in loaded tissue. In unconfined geometry, mechanical compression has been shown to induce variation in sound speed, leading to errors in the determined mechanical properties. This phenomenon has not been confirmed in indentation geometry, the only loading geometry applicable in situ or in vivo. In the present study, ultrasound speed during indentation of articular cartilage was characterized and the effect of variable sound speed on the strain measurements was investigated. Osteochondral samples (n = 7, diameter = 25.4 mm), prepared from visually intact bovine patellae (n = 7), were indented with a plane-ended ultrasound transducer (diameter = 5.6 mm, peak frequency: 8.1 MHz). A sequence of three compression tests (strain-rate = 10%/s, 2700-s relaxation) was applied using the mean strains of 2.2%, 4.5%, and 6.4%. Then, ultrasound speed during the ramp and stress-relaxation phases was determined using the time-offlight technique. To investigate the role of cartilage structure and composition for sound speed in loaded articular cartilage, a sample-specific fibril-reinforced poroviscoelastic (FRPVE) finite element model was constructed and fitted to experimental mechanical data. Ultrasound speed in articular cartilage decreased significantly during dynamic indentation (p < 0.05). The magnitude of the decrease in speed during indentation was related to the applied strain. However, the relative error in acoustically determined tissue strain was inversely related to the magnitude of true strain. The modeling results suggested that the compression-related variation in sound speed is controlled by changes in the collagen architecture during dynamic indentation. To conclude, variation in sound speed during dynamic indentation of articular cartilage may lead to significant errors in the values of measured mechanical parameters. Because the relative errors are inversely proportional to applied strain, higher strains should be used to minimize the errors in, e.g., in vivo measurements

Strain-Dependent Modulation of Ultrasound Speed in Articular Cartilage Under Dynamic Compression

Mechanical properties of articular cartilage may be determined by means of mechano-acoustic indentation, a clinically feasible technique for cartilage diagnostics. Unfortunately, ultrasound speed varies in articular cartilage during mechanical compression. This can cause significant errors to the measured mechanical parameters. In this study, the strain-dependent variation in ultrasound speed was investigated during dynamic compression. In addition, we estimated errors that were induced by the variation in ultrasound speed on the mechano-acoustically measured elastic properties of the tissue. Further, we validated a computational method to correct these errors. Bovine patellar cartilage samples (n = 7) were tested under unconfined compression. Strain-dependence of ultrasound speed was determined under different compressive strains using an identical strain-rate. In addition, the modulation of ultrasound speed was simulated using the transient compositional and structural changes derived from fibril-reinforced poroviscoelastic (FRPVE) model. Experimentally, instantaneous compressive strain modulated the ultrasound speed (p < 0.05) significantly. The decrease of ultrasound speed was found to change nonlinearly as a function of strain. Immediately after the ramp loading ultrasound speed was found to be changed -0.94%, -1.49%, -1.84%, -1.87%, -1.89% and -2.15% at the strains of 2.4%, 4.9%, 7.3%, 9.7%, 12.1% and 14.4%, respectively. The numerical simulation revealed that the compression-related decrease in ultrasound speed induces significant errors in the mechano-acoustically determined strain (39.7%) and dynamic modulus (72.1%) at small strains, e.g., at 2.4%. However, at higher strains, e.g., at 14.4%, the errors were smaller, i.e., 12.6% for strain and 14.5% for modulus. After the proposed computational correction, errors related to ultrasound speed were decreased. By using the correction, with e.g., 2.4% strain, errors in strain and modulus were decreased from 39.7% to 7.2% and from 72.1% to 35.3%, respectively. The FRPVE model, addressing the changes in fibril orientation and void ratio during compression, showed discrepancy of less than 1% between the predicted and measured ultrasound speed during the ramp compression. (E-mail: [email protected])