MMP9 and arthritic joint disease: To investigate whether oxygen radicals alter the expression of MMPs within the cartilage of an inflamed knee joint, patellar and tibial cartilage layers were isolated at day 3 and day 7 after arthritis induction, and mRNA levels of various MMPs (MMP-2, MMP-3, MMP-9, MMP-12 and MMP-13) and their inhibitors (TIMP-1, TIMP-2, TIMP-3 and TIMP-4) were determined by quantitative RT-PCR.