Regional variations of indentation stiffness and thickness of normal rabbit knee articular cartilage

The rabbit knee is frequently used as an experimental model for cartilage repair, but the choice of different joint regions for such studies makes comparisons between methods difficult. Furthermore, there is only limited information available about the regional variations of the biomechanical properties in normal rabbit knee articular cartilage. In the present study in situ indentation tests were used to map the short-term stiffness and thickness of articular cartilage at seven locations (anterior and posterior areas of the medial and lateral femoral condyles, the patellar groove, and the central areas of the medial and lateral tibial plateaus) in nine normal rabbit knee joints. Short-term cartilage stiffness was described by elastic moduli under ramp loading and 15-s creep conditions. The moduli were calculated according to Hayes' single-phase elastic model. A moderate positive correlation (r = 0.54) between cartilage stiffness and thickness was found for rabbit femoral cartilage, but was not confirmed for tibial cartilage, which had the thickest, but also the softest cartilage of all areas. The cartilage in the patellar groove and the medial compartments of both femoral condyles and tibial plateaus was stiffer and thicker than that in the lateral components, similar to previous findings in dogs. However, the dog femoral cartilage was found to be stiffer at the anterior than at the posterior regions, but we found the opposite in the rabbit. These dissimilarities between animal models may be caused by different joint loading characteristics. Accordingly, in rabbits repair processes in the more anterior femoral areas with less stiff cartilage may not be comparable to repairs in more posterior areas where cartilage is stiffer.