Ultrasonic characterization of transient and inhomogeneous swelling behavior and progressive degeneration of articular cartilage

Abstract

Articular cartilage is the soft connective tissue that covers the bony ends in diarthrodial joints providing good distribution of the load on the bone and good lubrication for joint movement. However, many factors may result in the degeneration of articular cartilage and then lead to osteoarthritis (OA). Swelling is an electrochemical mechanical property of articular cartilage and plays an important role in weight bearing and joint lubrication. The overall aim of this study is to use high-frequency ultrasound to characterize the transient and inhomogeneous swelling properties and progressive degeneration of articular cartilage in situ. In this study, an ultrasound swelling measurement system (USMS) was developed. 95 full-thickness cartilage-bone specimens were prepared from fresh mature bovine patellae for different tests. The osmotic free shrinkage and swelling behavior was induced by changing the concentration of bathing saline solution from 0.15 M to 2 M and then back to 0.15 M. The hydration behavior in 0.15 M saline was monitored by the USMS after the specimen was dehydrated in the air for 45 minutes. The progressive degeneration of articular cartilage was induced by trypsin digestion. Swelling tests, histological assessments and acoustic measurements were performed for both normal and trypsin-digested cartilage specimens. Moreover, the aggregate moduli at different layers were extracted from the swelling strains detected by the USMS using a newly-developed bi-layered triphasic model. A good repeatability of the present ultrasonic measurement for the shrinkage-swelling behavior was obtained (n = 10, ICC > 0.8, CV% < 10%). The USMS successfully detected the progressive penetration of trypsin through the cartilage tissue. The digestion speed slowed down from the initial rate of 0.62 +- 0.16 um/s to 0.04 +- 0.02 um/s when the digestion front reaching the deep region (approximately 70% of the full thickness). It was demonstrated that the cartilage specimens might experience an "overshoot-relaxation" behavior during the swelling-shrinkage test. The absolute peak shrinkage strain (0.010 +- 0.005, n = 50) was significantly (p < 0.001) larger than the peak swelling strain (0.003 +- 0.003). The non-uniformity of the swelling strains at different depths was observed. The degenerated cartilage specimens (n = 30) showed weaker shrinkage, swelling and hydration behaviors. Using the triphasic model, it was demonstrated that the region near the bone, for the normal specimens, had a significantly higher aggregate modulus (Ha1 = 18.5 +- 15.5 MPa, p < 0.001) in comparison with the middle zone and the surface layer (Ha2= 7.6 +- 11.6 MPa and Ha3= 3.6 +- 3.3 MPa, respectively). The predicted normalized thickness of the deep layer h1 was 0.67 +- 0.22. For the specimens with 30-minute trypsin digestion, the values decreased to Ha1 = 11.1 +- 10.1 MPa, Ha2 = 6.7 +- 16.9 MPa, Ha3 = 1.9 +- 2.8 MPa, h1 = 0.55 +- 0.24. It was found that the acoustic parameters of the degenerated specimens had insignificant changes in comparison with the normal specimens. Moreover, the correlations among swelling, acoustic and biochemical parameters were obtained. In summary, the USMS proposed in this study has been successfully used to measure the transient and inhomogeneous swelling behavior and to track the trypsin penetration, which simulated the progressive degeneration of the OA-like articular cartilage in situ.