Mechanical response of bovine articular cartilage under dynamic unconfined compression loading at physiological stress levels

Objective: The objective of this study was to characterize the dynamic modulus and compressive strain magnitudes of bovine articular cartilage at physiological compressive stress levels and loading frequencies. Design: Twelve distal femoral cartilage plugs (3 mm in diameter) were loaded in a custom apparatus under load control, with a load amplitude up to 40 N and loading frequencies of 0.1, 1, 10 and 40 Hz, resulting in peak Cauchy stress amplitudes of 4.8 MPa (engineering stress 5.7 MPa). Results: The equilibrium Young's modulus under a tare load of 0.4 N was 0.49 +/- 0.10 MPa. In the limit of zero applied stress, the incremental dynamic modulus derived from the slope of the stress-strain curve increased from 14.6 +/- 6.9 MPa at 0.1 Hz to 28.7 +/- 7.8 MPa at 40 Hz. At 4 MPa of applied stress, the corresponding increase was from 48.2 +/- 13.5 MPa at 0.1 Hz to 64.8 +/- 13.0 MPa at 40 Hz. Peak compressive strain amplitudes varied from 15.8 +/- 3.4% at 0.1 Hz to 8.7 +/- 1.8% at 40Hz. The phase angle decreased from 28.8degrees +/- 6.7degrees at 0.1 Hz to -0.5degrees +/- 3.8degrees at 40 Hz. Discussion: These results are representative of the functional properties of articular cartilage under physiological load magnitudes and frequencies. The viscoelasticity and nonlinearity of the tissue helps to maintain the compressive strains below 20% under the physiological compressive stresses achieved in this study. These findings have implications for our understanding of cartilage metabolism and chondrocyte viability under various loading regimes. They also help establish guidelines for cartilage functional tissue engineering studies. (C) 2003 OsteoArthritis Research Society International. Published by Elsevier Ltd. All rights reserved.