Tribological and Mechanical Characterization of Bovine Articular Cartilage after Treatment in EDTA

Articular cartilage is a connective tissue present in human and animal joints. Its main functions include the transmission of load across the surface to allow smooth articulation, and shock absorption. In order to pursue this aim, lubrication and low friction are provided by the tissue thanks to its complex structure and components. Its mechanical properties include high tensile and shear strength, in addition to elasticity and a fluid pressurization mechanism that provides a good resistance to compressive loads. However, joint damage is very frequent and subsequent cartilage deterioration plays a big role in this phenomenon. The mechanisms that leads to failures in cartilage are still poorly understood, especially regarding the initiation, and cartilage studies are complicated by the anisotropic properties, which are depth and strainrate dependent. Articular cartilage, in fact, owes its properties to its complex structure that can be treated as a biphasic tissue with a solid phase (containing chondrocytes, collagen, preoteolgycans and other non collagenous proteins) and a fluid phase (water). In addition, ions have been identified as a third phase and investigated. The importance of ions, in fact, stands in their ability to move across the selective membrane, neutralizing the negative charges of proteoglycans in order to reach the Donnan equilibrium. To this aim, many studies have been conducted on the relationship between cartilage properties and ions and one previous finding motivated this work. The experiment performed was included in a study on divalent ions, using EDTA as a ion-chelator, and a custom made bioreactor to apply shear stress on cartilage. After 3 hours of test the tissue showed loss of integrity and a high level of deterioration compared to a control group treated in CaCl2, suggesting the necessity of conducting further investigation. Therefore, the present study investigated cartilage properties in tissue treated in EDTA, which should chelate divalent ions. Subsequently, the tissue was characterized at different levels of depth to relate some results to the anisotropy of the tissue. This work suggests a major role of divalent ions on the mechanical properties of articular cartilage. Also, it became evident that depth is an important variable to take into consideration for mechanical analysis of the tissue. Wear tests were performed to conduct a tribological analysis on samples treated in EDTA. A custom made bioreactor has been used to this aim to run shear test on cartilage, showing the loss of integrity of the tissue when divalent ions were chelated. Tensile tests were performed on 80μm thick dogbone shaped cartilage samples after EDTA treatment. The depth ranged from 30 μm to 900 μm, exhibiting a stiffer behavior in the superficial (30-110μm) and deeper(300-900μm) zones compared to the middle one (120-300μm). This work raises attention to the particular role of divalent cations in cartilage, which is still poorly understood, but appears crucial for the preservation of the tribological and mechanical properties of the tissue. In addition, this study supports the depth-dependent analysis of cartilage, which is necessary in order to understand the mechanisms that occur in the tissue.