Disease severity and knee extensor force in knee osteoarthritis: data from the Osteoarthritis Initiative.

OBJECTIVE: To determine whether the method of disease severity measurement influences the magnitude of knee extensor force deficits in knee osteoarthritis (OA).
METHODS: Data from the Osteoarthritis Initiative (n = 659) were analyzed. Knee extensor force was assessed with isometric contractions. Clinical severity was measured with the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC). Patients were stratified into tertiles of severity (i.e., moderate, mild, and severe OA) based on the lowest, middle, and highest WOMAC scores, respectively. Kellgren/Lawrence (K/L) grading was used to assess radiographic severity of the tibiofemoral compartment and patients were again stratified into mild (K/L grade <2), moderate (K/L grade 2), and severe (K/L grade >2) knee OA.
RESULTS: When stratifying with the WOMAC, force was significantly lower in the severe group compared to the mild (~18% lower; P < 0.001) and moderate groups (~9% lower; P = 0.03), and in the moderate group compared to the mild group (∼10% lower; P = 0.03). When stratifying with K/L grade, small nonsignificant differences were observed in the severe (~7% lower; P = 0.19) and moderate groups (~8% lower; P = 0.08) compared to the mild group. Large intragroup variability was observed when comparing WOMAC scores across radiographic severity (coefficients of variation were 79.3%, 74.6%, and 61.6% for K/L grade <2, K/L grade 2, and K/L grade >2, respectively).
CONCLUSION: The method of disease severity stratification influences the magnitude of knee extensor force deficits because no difference in force between disease subgroups was observed when stratifying with K/L grade. Furthermore, there was large variability in the WOMAC score within each radiographic subgroup, highlighting the limitations in using radiographic measures to reflect symptom severity.