Quantitative analysis of shrinkage and swelling behavior of articular cartilage using high-frequency ultrasound

Articular cartilage provides joints with excellent lubrication, wearing characteristics and a smooth efficient weight-bearing system. Swelling of articular cartilage serves as an important biomechanical property and one of the early signs of osteoarthritis (OA). The aims of this study were to build a high-frequency ultrasound system and to investigate the transient osmosis-induced free shrinkage-swelling behavior of articular cartilage in a non-destructive and non-contact way. Cylindrical cartilage-bone specimens were prepared from the normal mature bovine patellae. The transient shrinkage (de-swelling) and swelling behaviors of cartilage were induced by changing the concentration of the bathing saline from 0.15 M to 2 M and then back to 0.15 M. The shrinkage and swelling phases were monitored and quantified using ultrasound. It was found that both shrinkage and swelling responses showed 'overshoot-relaxation' phenomena. The absolute value of the peak shrinkage strain (0.75% ± 0.57%) was significantly larger than the peak swelling strain (0.28% ± 0.28%) (p < 0.05). The finding that the change rate of ultrasound speed in cartilage was fitted as an exponential function of time agreed to the diffusion of ions between cartilage and the bathing saline. It was demonstrated that the ultrasound system had a good reliability (ICC > 0.98) and a good reproducibility (relative error < 5%). This method could be potential to quantify the progressive degeneration of cartilage for the early diagnosis of OA.