Alignment, body mass and their interaction on dynamic knee joint load in patients with knee osteoarthritis

Objective: To examine the interaction and relative contributions of frontal plane alignment and body mass on dynamic knee joint loading in patients with knee osteoarthritis (OA). Methods: We completed three-dimensional gait analyses and hip-to-ankle standing anteroposterior radiographs on 487 patients with knee OA referred to a tertiary care center specializing in orthopaedics. Results: Using sequential (hierarchical) linear regression, the interaction term (mechanical axis angle×mass) contributed significantly (P<0.001) to a model (total adjusted R²=0.70) predicting the external knee adduction moment, that included mechanical axis angle (R²=0.37) and mass (R²=0.06) while controlling for age, sex, height, Kellgren and Lawrence grade, pain score during walking, gait speed, toe out angle and trunk lean (R²=0.25). When the sample was split into tertiles for mass, mechanical axis angle accounted for 32-54% of explained variance in knee adduction moment. In the tertile with greatest mass, results suggest a 3.2Nm increase in knee load for every 1° increase in varus alignment. When split into tertiles for mechanical axis angle, mass accounted for 6-10% of explained variance in the knee adduction moment. In the tertile with the most varus alignment, results suggest a 0.4Nm increase in knee load for every 1kg increase in mass. Conclusion: Our findings describe the interaction between alignment and body mass on dynamic knee joint loading, with the association between alignment and load highest in patients with the highest mass. Our findings also emphasize the role of malalignment on knee load at all levels of mass, and have implications for better understanding risk factors and intervention strategies for knee OA.