KL and rheumatoid arthritis: However, there were several shortcomings in their study, for example, Yue et al.'s study excluded patients with KL 4 from the database, which is an important classification for KOA, the lack of such an important variable may have an impact on the final accuracy of the results; furthermore, the study did not include patients with a history of rheumatoid arthritis in the database as part of their exclusion criteria, having known that rheumatoid arthritis is a significant risk factor for KOA; Rheumatoid arthritis can therefore be considered as a significant confounder.