In the 1950s, Kellgren and Lawrence carried out the first large-scale epidemiological studies of osteoarthritis. Obtaining x-rays on multiple subjects from the populations of various communities in England, they defined a grading scale for osteoarthritis that is still widely used and called by their names, the Kellgren and Lawrence (K&L) scale.1 This osteophyte-based scale is used in most large and small-scale clinical and epidemiological studies to define whether osteoarthritis is present or absent on the radiograph.

Unfortunately, neither Kellgren and Lawrence nor subsequent users of their scale were consistent in how they described each of their grades. Importantly, for grade 2, which usually defines whether osteoarthritis is present, it has been labelled as ‘definite osteophyte’ to ‘definite osteophyte with possible narrowing of the joint space’. As noted by Schiphof and colleagues,2 3 as investigators have come up with various descriptions of K&L grade 2, each of these studies may have identified a somewhat different group of persons with radiographic knee osteoarthritis.

With the emergence of large longitudinal studies of osteoarthritis have come questions about how to define new-onset disease and progressive disease, questions that the K&L scale was never designed to address. Even so, many studies, especially large epidemiological investigations, have used the K&L scale to identify knees with new-onset disease or progressive disease. In the Framingham and Rotterdam Studies,4 5 for example, investigators have defined new-onset disease as the emergence of K&L grade 2 disease in knees that had previously been graded as either K&L grades 0 or 1. Rotterdam Study investigators have alternatively used as disease incidence the new onset of K&L grade 1 in knees previously graded as grade 0.5 Progression of extant disease has also been characterised using K&L grades with progression by one grade or more often listed as one criterion …