Mechanical properties of articular cartilage. (A) Schematic of indentation testing setup. (a) 360° rotating arm. (b) Specimen chamber filled with PBS. (c) 250-g load cell with indenter. (B) Representative four-step stress relaxation curve with 5-μm displacement steps and 200-second relaxation time between each step. (C) Indentation results for control and dominant-negative transforming growth factor β type II receptor (DNIIR) tibiae at ages 12 weeks (n = 3 controls and 3 DNIIR) and 20 weeks (n = 7 controls and 6 DNIIR). Significant changes in stiffness were not observed in DNIIR cartilage at 12 weeks. Instantaneous and equilibrium stiffness were significantly lower in DNIIR cartilage at 20 weeks. In control mice, stiffness increased significantly between ages 12 and 20 weeks. Stiffness was not significantly changed for DNIIR mice over the same time period. (D) Articular cartilage thickness was significantly reduced in DNIIR mutants as compared to age-matched controls at 20 weeks of age, but no difference was observed at 12 weeks. Values are means ± SD. *P < 0.05.