(a) The ultrasonic measurement system consists of a transducer, (a) a pulser/receiver, (b) a digital oscilloscope and (c) a personal computer. The system can be used with (d, e) arthroscopy, (f) open surgery and (g) saline bath for experimental purposes. The ultrasound wave output from the transducer travels through saline. The reflected waves return to the transducer and generate electrical signals in the transducer that are proportional to the reflected wave intensity. (b) Typical ultrasound echo (lower) and wavelet map (upper). The wavelet map was calculated from the ultrasound echo using wavelet transform. The first (left) of the two large amplitude groups was the echo (t = 2.0 μs: group N) reflected from the cartilage surface, and the second (right; t = 3.9 μs: group K) was reflected from the subchondral bone. The signal intensity (maximum magnitude, as shown by the scale) of group N represents cartilage stiffness. The time interval between groups N and K represents cartilage thickness. The signal duration of group N represents the surface irregularity of the cartilage.