Why radiography should no longer be considered a surrogate outcome measure for longitudinal assessment of cartilage in knee osteoarthritis.

Imaging of cartilage has traditionally been achieved indirectly with conventional radiography. Loss of joint space width, or 'joint space narrowing', is considered a surrogate marker for cartilage thinning. However, radiography is severely limited by its inability to visualize cartilage, the difficulty of ascertaining the optimum and reproducible positioning of the joint in serial assessments, and the difficulty of grading joint space narrowing visually. With the availability of advanced magnetic resonance imaging (MRI) scanners, new pulse sequences, and imaging techniques, direct visualization of cartilage has become possible. MRI enables visualization not only of cartilage but also of other important features of osteoarthritis simultaneously. 'Pre-radiographic' cartilage changes depicted by MRI can be measured reliably by a semiquantitative or quantitative approach. MRI enables accurate measurement of longitudinal changes in quantitative cartilage morphology in knee osteoarthritis. Moreover, compositional MRI allows imaging of 'pre-morphologic' changes (that is, visualization of subtle intrasubstance matrix changes before any obvious morphologic alterations occur). Detection of joint space narrowing on radiography seems outdated now that it is possible to directly visualize morphologic and pre-morphologic changes of cartilage by using conventional as well as complex MRI techniques.