Comparison of an anteroposterior radiograph and a coronal magnetic resonance image of the knee. (a) Anteroposterior radiograph of the left knee demonstrates marginal osteophytes of the medial and lateral femur and tibia (arrows). Joint space width appears normal in the lateral tibiofemoral compartment, but there is mild to moderate medial tibiofemoral joint space narrowing. No other obvious bony abnormalities are seen. (b) Coronal fat-suppressed proton density-weighted magnetic resonance imaging performed on the same day reveals a subchondral bone marrow lesion (thin white arrow) at the medial tibial plateau subjacent to a focal full-thickness cartilage defect. Multiple partial-thickness defects of the medial femoral condyle cartilage (white arrowheads) are also noted. Notably, focal full-thickness cartilage defects (gray arrowhead) are more extensive at the lateral femoral condyle and subchondral bone (black arrowheads) is almost completely denuded at the lateral tibial condyle, despite radiographically normal appearance of the lateral tibiofemoral joint space width. Most of the joint space narrowing of the medial tibiofemoral joint is secondary to a partially macerated and extruded medial meniscus (thick white arrow). Additionally, there is attrition of the medial and lateral tibial plateaus and marginal osteophytosis. This example demonstrates why radiography should no longer be considered a surrogate outcome measure for longitudinal assessment of cartilage in knee osteoarthritis.