According to the ACR criteria, how many findings out of 9 are needed to diagnose knee OA?

• A. 3 findings out of 9
• B. 5 findings out of 9
• C. 7 findings out of 9
• D. 9 findings out of 9

Answer: (B)

The question tests how the ACR criteria classify knee OA on radiographs. According to these criteria, you diagnose knee OA when at least five of nine specific radiographic findings are present. This threshold helps ensure that the diagnosis reflects a pattern of degenerative change rather than a single feature that could occur with aging or other conditions.

Think of the typical radiographic signs you’d expect in OA: osteophyte formation, joint-space narrowing, subchondral sclerosis, subchondral cysts, and deformity or malalignment of the knee, among other related changes. When five or more of these features are found, the radiographic pattern is considered consistent with OA. If fewer than five are present, OA is less certain by these criteria.

So, five findings out of nine is the point at which knee OA is diagnosed according to the ACR criteria.