Unifying Concepts

ACR Revised Criteria for Early Diagnosis of Knee Osteoarthritis (OA)

Osteoarthritis (OA) is cartilage failure resulting in joint pain and loss of joint functions. Knee OA is the OA of knee that mechanical forces have major effect on initiation and progression of it. Knee OA is the most common disease of knee especially in the middle to old ages. The most common findings in the history and physical examination of the patients with knee OA are mechanical knee pain, gelling knee pain, crepitus on knee motion, bony tenderness and bony enlargement in the joint line. During the flare up of Osteoarthritis, knee can show swelling due to joint effusion called “Hydrarthrosis” that is a mechanical type of synovial fluid.

ACR revised Criteria for early diagnosis of knee OAa

Entry Criteria:

  • Knee pain and/or knee bony tenderness
  • Absence of exclusion criteria b
Domain I:

  • Mechanical knee pain c
  • Knee bony tenderness
  • Crepitus on knee motion
  • Compatible synovial fluid d


Domain II:

  • Age at onset ≤50 years old
  • Age at onset > 50 years old
  • Knee bony enlargement e
  • Osteophyte in knee X-Ray or compatible knee MRI


a In the presence of 3 points out of 10 with at least 1 point from Domain II along with all entry criteria, the diagnosis of knee OA can be established
b Exclusion criteria are including: 1) moderate to significant knee synovitis 2) Hot or red knee 3) history and/or physical examination
findings compatible with the internal derangement of knee
c Knee pain that is initiated or increased with knee activity/exercise and finished or decreased with knee resting
d Clear fluid with normal viscosity accompanied by WBC count less than 2000/mm3 with less than 25% PMN
e It must be ignored in the presence of osteophyte in knee X-Ray


Practical guideline approaching towards the diagnosis of knee OA
Step I : History and physical examination by expert physician
Step II : Knee X-Ray

  • II A : AP and lateral X-Ray
  • II B: Axial view and interchondilar view

Step III: Synovial fluid analysis if it is presented
Step IV: Knee MRI



  1. Sharma L. Osteoarthritis of the Knee. N Engl J Med. 2021 Jan 7;384(1):51-59. [Medline]
  2. Block JA, Cherny D. Management of Knee Osteoarthritis: What Internists Need to Know. Med Clin North Am. 2021 Mar;105(2):367-385. [Medline]


Created Mar 03, 2021.

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