Topographical variations in articular cartilage and subchondral bone of the normal rat knee are age-related

In osteoarthritis animal models the rat knee is one of the most frequently investigated joint. However, it is unknown whether topographical variations in articular cartilage and subchondral bone of the normal rat knee exist and how they are linked or influenced by growth and maturation. Detailed knowledge is needed in order to allow interpretation and facilitate comparability of published osteoarthritis studies. For the first time, the present study maps topographical variations in cartilage thickness, cartilage compressive properties and subchondral bone microarchitecture between the medial and lateral tibial compartment of normal growing rat knees (7 vs. 13 weeks). Thickness and compressive properties (aggregate modulus) of cartilage were determined and the subchondral bone was analyzed by micro-computed tomography. We found that articular cartilage thickness is initially homogenous in both compartments, but then differentiates during growth and maturation resulting in greater cartilage thickness in the medial compartment in the 13-week-old animals. Cartilage compressive properties did not vary between the two sites independently of age. In both age-groups, subchondral plate thickness as well as trabecular bone volume ratio and trabecular thickness were greater in the medial compartment. While a high porosity of subchondral bone plate with a high topographical variation (medial/lateral) could be observed in the 7-week-old animals, the porosity was reduced and was accompanied by a reversion in topographical variation when reaching maturity. Our findings highlight that there is a considerable topographical variation in articular cartilage and subchondral bone within the normal rat knee in relation to the developmental status.