A Comparison between the Mechanical Behaviors of Different Equine Articular Cartilage Surfaces

Abstract

In order to determine the stiffness and thickness of the equine articular cartilage, an indentation test and needle probe test were performed on fresh equine articular cartilage surfaces from the fetlock, carpal, and stifle joints. The results demonstrated that the stiffness and thickness vary on different joints showing different mechanical behavior. 120 and 87 samples were used in the stiffness and thickness measurement. They were cultivated from fetlock, carpal, and stifle joints of 24 deceased, half of which were measured within 20 hours of death. After approximately three minutes of exposure to air during dissecting, all cartilage samples were preserved in a saline solution to keep the articular cartilage hydrated for testing. In the indentation test, a flat-ended cylindrical indenter is lowered at a constant rate for 20 seconds until the indentation depth reaches 0.2mm (velocity of 10μm/s). The test was performed on three different locations on the same sample. The stiffness of the cartilage was determined by fitting cubic equations to the force versus indentation depth curves obtained from the tests. Afterwards, the stiffness values of the fetlock, carpal, and stifle joints were compared in magnitude throughout the test. Four different surfaces in the fetlock and four in the carpal joint were also compared. It was shown that the articular cartilage of the fetlock is stiffer than the carpal and stifle joint. The average stiffness of the fetlock, carpal, and stifle joint are 46.1N/mm, 20.5N/mm, and 2.73N/mm, respectively. The coefficients of the cubic equations for the joints were statistically compared as well using the student t-test. The differences of some coefficients between the fetlock, carpus, and stifle were “very highly significant” (p<0.001). Four different surfaces in the fetlock and four in carpal joint were compared as well. The front lateral, front medial, rear lateral, and rear medial cartilage surfaces in the fetlock were not significantly different in stiffness. In the carpus, the distal radius and proximal radial carpal bone articular cartilage surfaces showed visually different stiffness from the others, while the distal radial carpal bone and proximal third carpal bone articular cartilage surfaces possessed similar stiffness values. The cartilage surfaces from the radiocarpal joint were stiffer than the midcarpal joint. Clear trends in the correlations between stiffness and weight as well as stiffness and age of the horse were not observed. The needle probe test and analysis of the test data were implemented based on the established methodology from existing literature. The thickness of the articular cartilages from the fetlock, carpal, and stifle joints were measured. The needle is lowered at a velocity of 20μm/s until the measured force exceeds 2.5N. The thickness was measured on five different spots on the same sample. In the same manner as the stiffness measurement, the thicknesses of the fetlock, carpus, and stifle were compared. The articular cartilage of the stifle was thicker than the fetlock and carpus, while the fetlock and the carpus had similar thickness values. The average thickness of the fetlock, carpal, and stifle joint are 0.86mm, 0.87mm, and 2.1mm, respectively. They were statistically compared using the student t-test as well. The differences on the articular cartilage thicknesses between the fetlock and stifle, and carpus and stifle were “very highly significant” (p<0.001). This indicates that the articular cartilage thickness of the stifle is significantly different from that of the fetlock and carpus. Four different surfaces in the fetlock and four in carpal joint were also compared. The front lateral, front medial, rear lateral, and rear medial fetlock cartilage surfaces possessed thicknesses on 0.87mm, 0.81mm, 0.89mm, and 0.87mm, respectively. Significant difference between the four surfaces was not observed. In the carpus, the difference in thickness between the distal radius and proximal third carpal bone articular cartilage surfaces as well as the proximal radial carpal bone and distal radial carpal bone articular cartilage surfaces were statistically significant. The difference between the proximal radial carpal bone (the thickest in the carpus) and proximal third carpal bone articular cartilage surfaces (the thinnest in the carpus) was highly significant. The cartilage thicknesses of the distal radius, proximal radial carpal bone, distal radial carpal bone, and proximal third carpal bone articular cartilage surfaces were 0.92mm, 0.99mm, 0.80mm, and 0.76mm, respectively. It is demonstrated that cartilage surfaces from the radiocarpal joint were thicker than the midcarpal joint. It is believed that the different mechanisms between various joints cause the different mechanical properties. Various factors were considered as the possible reasons for the different mechanism in different joints, such as: joint type, pressure distribution in the forelimb and hindlimb, composition of the articular cartilage, proximity to the body, and different geometries such as size and shape of the surface.