Although injuries to articular cartilage may lead to radiographic osteoarthritis, pain, and decreased joint function, the actual effects of such injury and of its treatment on joint function are not completely understood. The mechanisms of repair after impact loading are different from those after frank disruption by fracture of the articular cartilage, but basic and clinical research both indicate that the resultant articular surface is prone to degeneration. The sensitivity of a joint to resultant incongruity varies considerably, depending on the thickness and modulus of the articular cartilage and the geometry of the joint. Also, factors other than articular congruity play a substantial role in determining outcomes after treatment. For these reasons, defining a single threshold for articular displacement that correlates with outcomes in all joints is not practical. Some articular fractures injure cartilage so severely that the joint will degenerate even with an accurate articular reduction. Also, radiographic evidence of osteoarthritis does not necessarily correlate with poor function. More reliable measurement techniques are needed to accurately assess how treatment affects arthritis, and factors other than articular congruity are needed to predict posttraumatic arthritis.