Background/Purpose:  Accelerated knee osteoarthritis (AKOA) may be a unique subset of knee osteoarthritis (KOA) that is associated with a knee injury and greater age, body mass index (BMI), and knee pain. There are several definitions of AKOA, making it challenging to understand this phenotype.  It is important to compare the performance of these AKOA definitions. Therefore, we evaluated agreement among several definitions of AKOA and construct validity by comparing their individual associations with injury, age, obesity, and knee pain.

Methods: We selected knees from the Osteoarthritis Initiative (OAI) that had no radiographic KOA (Kellgren-Lawrence [KL]=0 or 1) at baseline and had quantitative medial joint space width (JSW) measures on high-quality radiographs at >2 consecutive visits (n=1,061 knees, 738 participants). Quantitative medial JSW was based on a semi-automated method and location specific (x=0.25). We compared a KL-based definition of AKOA, which accounts for changes throughout a knee, to 4 JSW-based definitions that are derived from previous latent class analyses: 1) stringent JSW (averaged): average JSW loss >1.05 mm/year over 4 years, 2) stringent JSW (consistent): JSW loss >1.05 mm/year for >2 years, 3) lenient JSW (averaged): average JSW loss >0.25 mm/year over 4 years, and 4) lenient JSW (consistent): JSW loss >0.25 mm/year for >2 years. We also defined AKOA as progression from no radiographic KOA to end-stage KOA (KL=3 or 4) within 4 years. We calculated kappa statistics between the JSW-based definitions and KL-based definition of AKOA. We then examined how the different definitions influenced the effect size of group differences (AKOA or no AKOA) of an injury during the first four years of the OAI, baseline age, baseline BMI, and average WOMAC pain score from baseline and the first four annual visits as a form of construct validity.

Results: Over 4 years the incidence rate of AKOA was 0.7%, 1.2%, 18.1%, 24.7%, and 11.2% based on the stringent JSW (averaged and consistent), lenient JSW (averaged and consistent), and KL-based definitions. Everyone meeting the stringent JSW definition also met the KL-based definition. There was fair-moderate agreement between the lenient JSW (averaged) and KL-based definitions. KL-based definition led to larger effect sizes for injury, age, BMI, and average pain.

Conclusion: A KL-based definition of AKOA may be ideal because it offers greater effect estimates for known risk factors for the condition.  The performance of the KL-based definition may be related to the broader definition of joint deterioration compared with those focused on just joint space alone.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/best-performing-definition-of-accelerated-knee-osteoarthritis/