Fig. 1

a A pen-like ultrasonic transducer was used (DUB micro®, Rev. 2.14a, Taberna pro medicum, Lüneburg, Germany). b The ultrasonic signal fades while traveling through cartilage. At the cartilage–bone border, the signal is partially reflected (dashed lines). Cartilage thickness is calculated based on the time the reflected signal needs to travel through cartilage. Therefore, thickness directly relates to speed of sound in cartilage. c A-mode ultrasonic image: upper cartilage border (arrow); “leading interface” (LI), an interference pattern which occurs due to the great difference in acoustic impedance of cartilage and gel; cartilage–bone border (asterisk)