To study the age dependence of the uniaxial tensile behavior of bovine articular cartilage, 216 specimens of articular cartilage from the patellofemoral joint were grouped into two age categories:​ those from joints with growth plates present (open physes) and those from joints without growth plates (no physes). We prepared standard, dumbbell-shaped test specimens from the three ultrastructural zones of the tissue:​ the superficial tangential zone, the middle zone, and the deep zone. As was done in the early works of Kempson et al. and of Woo et all., we prepared specimens whose long axes were either parallel or perpendicular to the axis of the local split line on the joint surface. In these tensile tests we observed a profound difference between the two age groups in terms of the variations in the tensile properties related to the depth below the surface. With increasing distance from the articular surface, the tensile strength and stiffness increased in the open-physis group and decreased in the no-physis group. Directional dependence of the tensile response was manifested by increased stiffness and strength of the samples oriented parallel to the split-line axis when compared with the samples oriented perpendicular to it. Distortion of the initially flat, straight-sided portion of the gauge section was observed in most of the 216 specimens immediately after the application of tensile strains. This effect casts serious doubts on all previous measurements of cross-sectional dimensional changes. At present, we believe that this distortion occurs because articular cartilage is an inhomogeneous, layered material. We also observed that cartilage exudes substantial amounts of its interstitial fluid when the applied tensile strain becomes large. These two effects create serious problems in assessing the true volumetric changes and the material properties of cartilage specimens during uniaxial tension experiments.

CLINICAL RELEVANCE:​ The results of this biomechanical study strongly suggest that the collagen of immature articular cartilage (open-physis group) may have unique biophysical properties and a unique architecture. The superior mechanical response in tension of immature as compared with mature tissue is a manifestation of these biophysical properties and of the ultrastructure. After maturation, there is a decrease in the tensile-strength properties of the middle and deep zones but not in those of the superficial tangential zone of the tissue. Therefore, after maturation, the role of the surface layer of articular cartilage may be of the utmost importance in protecting the tissue from un- favorable mechanical stresses during joint function. Any disruptions of the integrity of this surface layer, as for example those caused by the fibrillation of early osteoarthrosis, will expose the biomechanically inferior portions of the tissue to large tensile stresses and strains, which in turn could lead to ultimate loss of normal joint function.