Glycosaminoglycans Contribution to the Mechanical and Electrokinetic Properties of the Articular Cartilage

Articular cartilage is present in joints to provide load distribution and nearly frictionless surfaces. Its extracellular matrix is mainly composed of water, type II collagen and proteoglycans. The glycosaminoglycans (GAGs) are a negatively charged component of the proteoglycan’s molecules and contribute to 1) the elastic properties through electrostatic forces, 2) the low permeability due to its tight structure, and 3) the electrokinetic properties where streaming potentials are compression-induced by the fluid flow through the extracellular matrix. Therefore, the objective of this study is to validate a testing methodology that can evaluate the mechanical and electrokinetic properties of articular cartilage and study their relationship to GAG concentration; as an effort to contribute to a better understanding of the role of the extracellular matrix in tissue maintenance and function.

Cartilage disks were extracted from the medial tibial plateaus of equine stifle joints. Then, the mechanical properties were measured in unconfined compression for 2 saline concentrations baths (0.15 M and 0.60 M). Streaming potentials were assessed simultaneously with a 1 mm in diameter Ag/AgCl electrode mounted against the articular surface of the samples. A digital video recorder tracked the radial edge of the disk which was used to extract the volumetric change. Five consecutive 5% strain stress-relaxation ramps were performed, and GAG content was measured using a DMMB assay. Stress-relaxation curves were curve-fit to a fibril reinforced poroelastic model to extract the mechanical properties. Fluid pressure inside articular cartilage was estimated to be equal to the transitional stress (recorded stress minus the equilibrium stress) and then the electrokinetic constant ke was computed as the slope of the streaming potential versus the fluid pressure. The GAG content at the beginning of each ramp was estimated from the biochemistry testing and the volumetric strain.

The relationship between the streaming potential and the fluid pressure is close to linearity throughout individual ramps. After each stress-relaxation ramp the elastic moduli (equilibrium and fibrils) increased while the volume of the disks, the hydraulic permeability and the electrokinetic constant ke diminished (from -5.3 to -1.5 mV/MPa). Increasing the saline concentration resulted in an increase in the hydraulic permeability but a decrease in both elastic moduli and the electrokinetic constant ke (from -5.3 to -1.3 mV/MPa for 0.15M and 0.60M, respectively). A positive correlation was observed between the equilibrium modulus and the GAG content while the hydraulic permeability and the electrokinetic constant ke showed a negative correlation with the GAG.

The results related to the mechanical properties follow the same trends documented in the existing literature. The observed decrease of the electrokinetic constant ke when the saline concentration is increased is also in agreement with previous publications. To the best of our knowledge, this is the first study to compare the electrokinetic constant ke and the GAG content. The inverse relationship between ke and the GAG content could be explained from the existing theory about the electrokinetic constants where ke is known to be inversely proportional to the electrical conductivity which is known to increase with GAG concentration.