Mediating role of effusion-synovitis in knee pain worsening following quadriceps weakness: data from the osteoarthritis initiative

The cause of increased knee pain related to quadriceps weakness in knee osteoarthritis remains unclear. This study aimed to assess the impact of alterations in the effusion-synovitis, a special kind of nociceptive structure, on changes in knee pain associated with quadriceps weakness. Based on the osteoarthritis initiative cohort, knees with comprehensive records of quadriceps strength, effusion-synovitis, and knee pain assessments at baseline, 12-month and 24-month intervals were included. Quadriceps strength was measured isometrically at baseline, while effusion-synovitis and knee pain were assessed at baseline, 12-month, and 24-month follow-ups. Effusion-synovitis was assessed using the Magnetic Resonance Imaging Osteoarthritis Knee Score while knee pain was evaluated with the Western Ontario and McMaster Universities Osteoarthritis Index. Mediation-effect models were utilized to quantify the extent to which changes in effusion-synovitis, induced by quadriceps weakness, could mediate the impact on the knee pain scale. The analysis involved 1377 knees of 1235 participants with both baseline and 12-month follow-up data (61.1% females, mean age of 61.7 years). Baseline quadriceps strength was significantly associated with knee pain changes over 12 and 24 months, while changes in effusion-synovitis were also directly associated with worsening knee pain at 12- and 24-month follow-ups. More importantly, effusion-synovitis changes mediated the association between baseline quadriceps strength and knee pain worsening over 12 and 24 months, with the mediating proportion of 17.72% and 10.31%, respectively. Additionally, this mediation association remained significant in the population with radiographic osteoarthritis during 12-month follow-up. Effusion-synovitis mediates approximately one-fifth of the association between baseline quadriceps strength and knee pain changes, suggesting that interventions targeting effusion-synovitis could facilitate the treatment of knee pain, especially caused by quadriceps weakness.