Abstract
Summary
The mechanical properties of porcine tibial plateau (TP) cartilage are shown to vary topographically. Low Elastic moduli (Em) were found in the positions where unicompartimental knee osteoarthritis (OA) lesions are typically expected to develop. These results suggest that there is a different response to load in these areas.
Introduction
OA is one of the ten most disabling diseases in developed countries. OA of the knee, in particular, is a major cause of mobility impairment; up to 40% of the population over the age of 70 suffers from OA of the knee. It has been observed that unicompartmental knee OA occurs with very distinct and repeatable lesion patterns. It is hypothesised that these patterns are the result of differences in the material properties throughout articular cartilage. The aim of this study was to measure the mechanical properties of porcine cartilage in a whole undamaged TP.
Materials and methods
A Whole Articular Surface Indentation Machine (WASIM) was used to measure material properties in whole intact articular surface. WASIM has five degrees of freedom (DOF). The vertical axis (Z) holds an indenter tied to a load cell and a high resolution laser. Five porcine TP were scanned using a high resolution laser to obtain the topography. Using a custom program, a grid of equally spaced points (6 mm) was defined. In vivo loading for daily activities occurs normal to the surface, therefore indentation was carried out on the same orientation. The normal vector for each indentation point was calculated by averaging the normal vectors of the points within the contact area at full load. The resulting vector allowed the calculation of angles, rotations and translation to obtain normal indentation of each point. Using a novel whole articular surface indentation machine (WASIM) in combination with a custom program, the TP was rotated to obtain normal indentation. Displacement controlled indentation was performed at 10 percent per second (pps) to 15% of the total cartilage thickness. Em was calculated at each indentation point by using Hertz contact theory and the Field and Swain Method. It was assumed that the initial portion of the unloading was purely elastic.
Results and Conclusions
Em of 45 to 50 points throughout the TP were obtained for each knee. Results show low Em values in the anterior medial area. Additionally, it was possible to find an area in the posterior lateral section of the TP delimited with low Em values. These areas correspond to the unicompartimental knee OA lesions. These results suggest a correlation exists between the material properties of the TP and the locations of early lesions in knee OA. This correlation can partly be explained by the relationship between articular cartilage stiffiness and matrix integrity.
