ULTRASTRUCTURE OF CARTILAGE UNDER TENSILE STRAIN

Abstract

Introduction: Cartilage is an anisotropic material whose structure and tensile properties vary with the depth from the articular surface. Further, ultrastructural changes of articular cartilage under strain are poorly understood. The aim of this study therefore was to visualize the zonal variations in ultrastructural changes of cartilage when subjected to a range of tensile strains to failure.

Materials and Methods: 3 osteochondral plugs were harvested from the femur of a 3 years old bovine with a cylindrical reamer. Cartilage was cut parallel to the articular surface into the superficial, middle and deep layers, 300μm thick each and then each was cut normal to the surface into dumbbell shaped specimen 10 mm long. Each specimen (9 in total) was clamped in an individual mini tension device and subjected to a specific strain, then fixed and processed whilst still under strain within its tension device for observation with SEM.

Results: When specimens were observed in en face view under no strain, a fibrillar meshwork was seen to run parallel to the articular surface in the superficial layer, randomly in the middle layer and perpendicular to the articular surface in the deep layer. Under strain the fibrillar meshwork began to reorient parallel to strain (tangential to the surface) in each layer. At 20% strain the whole fibrillar meshwork was reoriented and formed bundles in the superficial layer. In the middle layer almost whole of the fibrillar meshwork was reoriented at 40% strain. In the deep layer the fibrillar mesh-work was reoriented parallel to the strain in some areas, while in the other areas it was still seen perpendicular to the surface even at 70% strain.

Conclusions: The collagen meshwork of cartilage was reorganised under strain and this appears to play an important role in cartilage extension. Thus the more rapid reorientation in the superficial layer may result in its reduced extensibility compared to that of the deeper layers.