Deformation patterns of cracked articular cartilage under compression

Abstract

Purpose: It is widely accepted that the mechanical environment surrounding cartilage cells (chondrocytes) has a regulatory role on their metabolism. Very early osteoarthritis (OA) is associated with micro-cracks of the cartilage surface that are thought to alter the mechanical environment of chondrocytes, and hence their metabolism. In this study, we assessed the differences of cartilage deformation patterns between intact and cracked cartilage. Methods: Articular cartilage split line patterns of New Zealand white Rabbits were identified using India-ink. Cracks were made at full thickness of cartilage at 90° to the cartilage surface and oriented perpendicular to the split lines. A controlled load of 2MPa was applied, and stress relaxation observed. Cartilage tissues were then fixed in the compressed state and prepared for histology. Samples were mounted either in plastic and stained with Toluidine blue or mounted in paraffin and stained with Picrosirius red for viewing under a polarized light microscope. Results: The split line patterns of patella, femur, and tibia were found to be consistent. Histological analysis revealed that chondrocyte clusters in the radial zone show compression in the vertical direction and realignment of their vertical long axis to an oblique pattern pointing away from the center of the compression site. Under Linear Polarized Light (LPL), extensive collagen fiber reorientation is apparent as they deform in a combination of crimp and bending (Figure 1). Figure 1, Rabbit tibia mounted in plastic, stained with toluidine blue and viewed under LPL. a, Uncompressed tibia. b, Compressed tibia showing the reoriented collagen fibers (fine blue lines). c, Crack location magnified. Under Circular Polarized Light (CPL), the pattern of collagen fiber reorientation that occurred in the cracked vs. the intact samples is different. There is an upside triangle-shaped zone with the crack representing the center. Collagen fibers at the side of the crack are less reoriented compared to those located farther away, indicating the effect of stress release at the crack edge. The sides of the triangle run at an angle similar to that of the oblique running fibers that are normally present in the radial zone of the cartilage, hence this pattern may emphasize the role of these oblique fibers in supporting compressive loads (Figure 2). Figure 2, Rabbit tibia viewed under CPL. a, Intact compressed tibia cartilage. b, Cracked compressed tibia cartilage highlighting the upside triangle pattern. Conclusions: The combination of crimp and bending of the collagen fibres during deformation may have a protective effect and reduce the amount of deformation transmitted to chondrocytes. The presence of a crack completely changed the deformation patterns in the collagen fibers. This is the first time where the micro-structural architecture and deformation of cracked cartilage under compressive loading has been quantified. The difference in collagen reorientation between intact and cracked samples may provide crucial insight into the load distribution in early OA cartilage that contains cracks, and may help identify how cracked cartilage degenerates. Structural changes in surface zone cartilage are among the earliest signs of very early OA and an understanding of these changes in mechanics and cell signaling may allow for altering the time course of OA by either mechanical or pharmacological intervention.