Cartilage Assessment Requires a Surface Characterization Protocol: Roughness, Friction, and Function

The surface of articular cartilage is integral to smooth, low-friction joint articulation. However, the majority of cartilage literature rarely includes measurements of surface characteristics and function. This may, in part, be due to a shortage of or unfamiliarity with fast, nondestructive, and, preferably, noncontact methods that can be applied to large cartilage surfaces for evaluating cartilage surface characteristics. A comprehensive methodology for characterizing cartilage surfaces is useful in determining changes in tissue function, as for example, in cases where the quality of cartilage grafts needs to be assessed. With cartilage storage conditions being an area of ongoing and active research, this study used interferometry and tribology methods as efficient and nondestructive ways of evaluating changes in cartilage surface topography, roughness, and coefficient of friction (CoF) resulting from various storage temperatures and durations. Standard, destructive testing for bulk mechanical and biochemical properties, as well as immunohistochemistry, were also performed. For the first time, interferometry was used to show cartilage topographical anisotropy through an anterior–posterior striated pattern in the same direction as joint articulation. Another novel observation enabled by tribology was frictional anisotropy, illustrated by a 53% increase in CoF in the medial–lateral direction compared to the anterior–posterior direction. Of the storage conditions examined, 37°C, 4°C, −20°C, and −80°C for 1 day, 1 week, and 1 month, a 49% decrease in CoF was observed at 1 week in −80°C. Interestingly, prolonged storage at 37°C resulted in up to an 83% increase in the compressive aggregate modulus by 1 month, with a corresponding increase in the glycosaminoglycan (GAG) bulk content. This study illustrates the differential effects of storage conditions on cartilage: freezing tends to target surface properties, while nonfreezing storage impacts the tissue bulk. These data show that a bulk-only analysis of cartilage function is not sufficient or representative. The nondestructive surface characterization assays described here enable improvement in cartilage functionality assessment by considering both surface and bulk cartilage properties; this methodology may thus provide a new angle to explore in future cartilage research and tissue engineering endeavors. IMPACT STATEMENT: Cartilage's major functions are load distribution and low-friction articulation. These functions are primarily carried out by the cartilage bulk tissue and surface layer, respectively. Although cartilage bulk properties are frequently assessed, surface characterization is often overlooked despite its importance to proper cartilage function. Toward closing this gap, this study presents cartilage evaluation that uses interferometry, tribology, and lubricin immunohistochemistry for comprehensive surface characterization. In the context of cartilage function preservation after storage, the conditions that alter bulk versus surface characteristics were found to differ, highlighting the importance of assessing surface function for both clinical and tissue engineering applications.