Assessment of Native Human Articular Cartilage: A Biomechanical Protocol

Abstract

Objectives: Recapitulating the mechanical properties of articular cartilage (AC) is vital to facilitate the clinical translation of cartilage tissue engineering. Prior to evaluation of tissue-engineered constructs, it is fundamental to investigate the biomechanical properties of native AC under sudden, prolonged, and cyclic loads in a practical manner. However, previous studies have typically reported only the response of native AC to one or other of these loading regimes. We therefore developed a streamlined testing protocol to characterize the elastic and viscoelastic properties of human knee AC, generating values for several important parameters from the same sample.

Design: Human AC was harvested from macroscopically normal regions of distal femoral condyles of patients (n = 3) undergoing total knee arthroplasty. Indentation and unconfined compression tests were conducted under physiological conditions (temperature 37 °C and pH 7.4) and testing parameters (strain rates and loading frequency) to assess elastic and viscoelastic parameters.

Results: The biomechanical properties obtained were as follows: Poisson ratio (0.4 ± 0.1), instantaneous modulus (52.14 ± 9.47 MPa) at a loading rate of 1 mm/s, Young's modulus (1.03 ± 0.48 MPa), equilibrium modulus (7.48 ± 4.42 MPa), compressive modulus (10.60 ± 3.62 MPa), dynamic modulus (7.71 ± 4.62 MPa) at 1 Hz and loss factor (0.11 ± 0.02).

Conclusions: The measurements fell within the range of reported values for human knee AC biomechanics. To the authors' knowledge this study is the first to report such a range of biomechanical properties for human distal femoral AC. This protocol may facilitate the assessment of tissue-engineered composites for their functionality and biomechanical similarity to native AC prior to clinical trials.

Keywords: articular cartilage; biomechanics; knee; mechanical testing.

Figure 1.

Schematic representation of the biomechanical testing sequence consisting of initial indentation, dynamic compression, cyclic loading, and repeat indentation tests. Four separate sites on the chondral surface (depicted by small arrows diverging from the indenter tip to the cartilage) of each sample were indented for each calculation of the instantaneous or Young’s modulus. The dynamic compression step and cyclic compressive loading steps were repeated sequentially 4 times before moving onto the final indentation step. AC, articular cartilage; SB, subchondral bone.

Figure 2.

Mechanical properties of articular cartilage tested in and equilibrium loading conditions using indentation geometry before (blue) and after (red) 8000 cycles of cyclic compression at a frequency of 4 Hz. Pairwise “before” and “after” comparisons for each sample are shown for (A) instantaneous modulus, (B) Young’s modulus, and (C) equilibrium modulus. The pooled mean values for equilibrium modulus (D), instantaneous modulus (E), and Young’s modulus (F) are illustrated before and after cyclic compression.

Figure 3.

Comparison of dynamic mechanical properties of articular cartilage before (blue) and after (red) cyclic compressive loading for (A) dynamic modulus, (B) storage modulus, (C) loss modulus, (D) compressive modulus, and (E) loss tangent when tested at 1-Hz sinusoidal compression (closest loading frequency of knee joint during walking and running). The “after cyclic compression” data were measured after 4 separate steps of 2,000 cycles of cyclic compression conducted at a frequency of 4 Hz (total of 8,000 cycles).

Figure 4.

Spearman rank correlation matrix to assess the relationship between several mechanical properties of native hyaline articular cartilage before (left) and after (right) cyclic compressive loading. Values of the correlation coefficient rho (ρ) greater than 0.7 were statistically significant positive correlations (P < 0.05).