To assess whether the "spread" of joint pain is related to pain-associated muscle loss in 1 joint leading to increased loading and subsequent pain in other joints.
Associations between persistent knee pain (pain in 1 or 2 knees over 0-3 years vs no persistent pain) and incident shoulder pain at Year 4 were examined in participants from the longitudinal National Institutes of Health Osteoarthritis Initiative. Associations were assessed using log multinomial modeling, adjusted for age, sex, body mass index, depression score, other lower limb pain, and baseline leg weakness (difficulty standing from a sitting position).
In older adults with clinically significant knee osteoarthritis (OA) or at risk of knee OA (n = 3486), the number of painful joints increased yearly, from 2.1 joints (95% CI 2.0-2.2) at baseline increasing by 5.2% (95% CI 2.2-8.3) at Year 4. Shoulders were the next most commonly affected joints after knees (28.5%). Persistent pain in 1 or 2 knees increased risk of bilateral shoulder pain at Year 4 [1 knee: relative risk (RR) 1.59, 95% CI 0.97-2.61; 2 knees: RR 2.02, 95% CI 1.17-3.49] after adjustment for confounders. Further adjustment for leg weakness attenuated effect sizes (1 knee: RR 1.13, 95% CI 0.60-2.11; 2 knees: RR 1.44, 95% CI 0.75-2.77), indicating mediation by functional leg weakness.
Spread of joint pain is not random. Persistently painful knees predict new bilateral shoulder pain, which is likely mediated by leg weakness, suggesting that biomechanical factors influence the spread of pain.