A technique for measuring the compressive modulus of articular cartilage under physiological loading rates with preliminary results

Site dependence of thickness and speed of sound in articular cartilage of bovine patella

Researchers have made efforts to quantify thickness of articular cartilage as well as its acoustic and mechanical properties using various ultrasound (US) techniques during the last decades, because they are important indicators of articular cartilage degeneration. However, the variation of the thickness and speed of sound of articular cartilage at different anatomical sites would result in the uncertainty of US assessment of degeneration. In this paper, the site dependences of speed of sound and thickness of bovine patellar articular cartilage (n = 10) were investigated using a custom-made US measurement system. The thickness and speed of sound of articular cartilage at different locations of the bovine patella were measured on excised specimens ex situ using a noncontact US approach. A total of 10 patellae were tested. The results showed the overall mean value of the speed of sound in the articular cartilage at the 25 measured sites was 1626 +/- 86 m/s (range, 1507 to 1834 m/s). No statistically significant difference in the speed of sound was observed among the 25 locations or among the four quadrants of the patella. The highest speed of sound (1834 +/- 74 m/s) was obtained at the medial-upper quadrant and the lowest value (1507 +/- 74 m/s) at the medial-lower quadrant. Further grouping of the data revealed that the speed of sound in the central region (1633 +/- 21 m/s) was significantly (p < 0.01) larger than that for the surrounding region (1621 +/- 22 m/s). The overall mean thickness of the patellar articular cartilage was 1.34 +/- 0.34 mm. No significant difference was obtained in the thickness among the 25 locations and also among the four quadrants. However, when the thickness values were divided diagonally, a significant difference (p < 0.01) was observed between the upper region (1.27 +/- 0.11 mm) and the lower region (1.31 +/- 0.41 mm) of the patellae. Although no significant differences in the thickness and speed of sound among the tested sites were observed, it was demonstrated in this study that they did show some degree of site dependence.

Measurement of depth-dependence and anisotropy of ultrasound speed of bovine articular cartilage in vitro

The inhomogeneous and anisotropic mechanical properties and structural components of articular cartilage (artC) may cause complex acoustic properties in this important tissue. In this study, we used 50-MHz ultrasound (US) to measure in vitro the depth-dependence and anisotropy of the US speed of artC collected from the bovine patellae. The US speeds of 18 disk artC specimens sampled from 18 different patellae were measured in two orthogonal directions. One full-thickness layer (n=18) and three horizontal slices (n=18 x 3) with approximately equal thickness were prepared from each artC disk and measured with the US beam perpendicular to the artC surface. One full-thickness vertical slice (n=10) was measured at different depths with the US beam parallel to the artC surface. The measured US speeds of artC in the two orthogonal directions were significantly different (p < 0.001). The US speeds also significantly increased with the increase of tissue depth for both measurement directions (p < 0.001). The US speeds of artC from the superficial to deep regions were 1518 +/- 17 (mean + SD), 1532 +/- 26 and 1554 +/- 42 m/s for the US beam parallel to the artC surface, and 1574 +/- 29, 1621 +/- 34 and 1701 +/- 36 m/s for the beam perpendicular to the artC surface. The sound speed of the full-thickness layer was 1636 +/- 25 m/s. The results suggested that the depth-dependence and anisotropy of the US speed in artC should be taken into account when US is used for the artC measurement. It was also demonstrated in this study, using additional specimens (5 x 2), that the swelling of artC after detaching from its subchondral bone could cause the change of its sound speed.

An ultrasonic measurement for in vitro depth-dependent equilibrium strains of articular cartilage in compression

The equilibrium depth-dependent biomechanical properties of articular cartilage were measured using an ultrasound-compression method. Ten cylindrical bovine patella cartilage-bone specimens were tested in compression followed by a period of force-relaxation. A 50 MHz focused ultrasound beam was transmitted into the cartilage specimen through a remaining bone layer and a small hole at the centre of a specimen platform. The ultrasound echoes reflected or scattered within the articularcartilage were collected using the same transducer. The displacements of the tissues at different depths of the articular cartilage were derived from the ultrasound echo signals recorded during the compression and the subsequent force-relaxation. For two steps of 0.1 mm compression, the average strain at the superficial 0.2 mm thick layer (0.35 +/- 0.09) was significantly (p < 0.05) larger than that at the subsequent 0.2 mm thick layer (0.05 +/- 0.07) and that at deeper layers (0.01 +/- 0.02). It was demonstrated that the compressive biomechanical properties of cartilage were highly depth-dependent. The results suggested that the ultrasound-compression method could be a useful tool for the study of the depth-dependent biomechanical properties of articular cartilage.

Measurement of the layered compressive properties of trypsin-treated articular cartilage: an ultrasound investigation

An ultrasound-compression system has been developed for the study of the layered biomechanical properties of articular cartilage. Cartilage specimens harvested from the bovine patella groove, with and without trypsin digestion, were tested using this system. It was noted that a large ultrasound reflection can be detected in the interface of the trypsin digestion front. This ultrasound reflection signal was used to differentiate the deformations of different portions of the cartilage throughout its depth when a load was applied. The equilibrium compression moduli of the digested, undigested and entire portions of articular cartilage were measured. The modulus of the cartilage without any digestion was 660+/-230 kPa. After 1h digestion with 1 mg ml(-1) trypsin solution, the thickness of the digested portion was 0.50+/-0.06m, and the modulus of the entire cartilage layer changed to 125+/-42kPa. The moduli of the digested and undigested portions were 58+/-24 kPa and 470+/-31 kPa, respectively. Similar results were obtained for the cartilage with trypsin digestion for 2 h.