Aim: The aim of this study is to visualize the structural changes of both the matrix collagen meshwork and the chondrocyte cytoskeleton of articular cartilage when it is subjected to tensile strain.

Materials and Methods: Dumbbell-shape specimens were harvested from the articular surface of the femur. Specimens were placed with the articular surface uppermost each in individual mini tension device and subjected to a graded series of tensile strains, whilst being observed with phase contrast light microscopy. Thereafter each specimen was fixed in its particular position of strain, and stained with fluorochrome conjugated primary antibodies specific for actin and vimentin and with DAPI for nuclear staining for observation by confocal laser scanning microscopy (CLSM).

Results: Phase contrast microscopy visualized the reorganization of the matrix which became aligned parallel to the direction of strain, resulting in the deformation of the chondrocyte and their nuclei into an elliptical shape. CLSM demonstrated the reorganization of the matrix and chondrocyte cytoskeleton; at no strain, the vimentin meshwork spanned the cytoplasm from plasma membrane to nuclear membrane. At 20% strain, the vimentin meshwork became aligned parallel to the direction of strain and the nucleus was deformed into elliptical shape.

Discussions: There are two possibilities to explain the structural changes in the chondrocyte under tensile strain. 1.The collagen meshwork becomes aligned parallel to the direction of tensile strain, squeezing the chondrocyte into the observed elliptical shape subsequently with the cytoskeleton reorganizing in response to it. 2.The collagen meshwork transfers the tensile strain through the plasma membrane to the vimentin meshwork which reorganizes and subsequently results in the changes in chondrocyte morphology. Further explanation is required to test the above two hypotheses.

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.