Maturation-dependent change and regional variations in acoustic stiffness of rabbit articular cartilage: an examination of the superficial collagen-rich zone of cartilage

SummaryObjectiveThe purpose of the study was to investigate maturation-dependent changes of acoustic (ultrasound) stiffness and other ultrasound features of articular cartilage in healthy rabbit knees.MethodsFive groups of rabbits of various ages (3 weeks, 8 weeks, 6 months, 1 year, 2.5 years) consisting of five rabbits per group were examined. Signal intensity (index of stiffness), signal duration (index of surface irregularity) and interval between signals (index of thickness) of the ultrasound reflection from articular cartilage were examined at four sites: posterior lateral femoral condyle, posterior medial femoral condyle, lateral tibial plateau, and medial tibial plateau. The sites were observed macroscopically and microscopically with a light microscope and a polarized light microscope.ResultsAt the lateral and medial condyles and the lateral tibial plateau, signal intensity was least in 3-week-old specimens. The intensity increased until 6 months or 1 year of age. At these sites, the signal durations and intervals between signals were least at the ages of 6 months or 1 year. At the medial tibial plateau, the intensity was the least at 2.5 years of age and the interval between signals was least at 3 weeks of age; there was no effect of age on signal duration. Cartilage surfaces of all specimens were smooth and no degenerative changes were macroscopically or microscopically evident. The surface brightness of cartilage under the polarized light microscope was consistent with signal intensity values.ConclusionsThe response of articular cartilage to ultrasound was maturation-dependent. Acoustic properties differed from mechanical stiffness properties, which were determined using indentation. Ultrasound may detect properties of the surface collagen of the articular cartilage.