In knee osteoarthritis, arthroscopy provides valuable information that suggests an original approach to the disease. In combination with recent imaging study data, this information can be used to develop a novel concept of the evaluation of knee osteoarthritis. The main mechanisms responsible for flares of the disease seem to be synovitis, degenerative meniscal disease, and subchondral bone lesions. Treatments targeted at each of these mechanisms can be used as appropriate. Determination of the exact nature of the lesions can be difficult and relies at present on sophisticated investigations not readily available in everyday practice. Further work is needed to determine how a finer analysis of symptoms and signs in combination with simple investigations can differentiate between the various pathological patterns of knee osteoarthritis.