Knee Pain Predicts Subsequent Shoulder Pain and the Association Is Mediated by Leg Weakness: Longitudinal Observational Data from the Osteoarthritis Initiative

Objective: To assess whether the ‘spread’ of joint pain is related to pain-associated muscle loss in one joint leading to increased loading and subsequent pain in other joints. Methods: Associations between persistent knee pain (pain in one or two knees over years 0-3 versus no persistent pain) and incident shoulder pain at year 4 were examined in participants from the longitudinal NIH Osteoarthritis Initiative (OAI). Associations were assessed using log multinomial modelling, adjusted for age, sex, BMI, depression score, other lower limb pain and baseline leg weakness (difficulty standing from a sitting position). Results: In older adults with clinically significant knee 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 joint after knees (28.5%). Persistent pain in 1 or 2 knees increased risk of bilateral shoulder pain at year 4 (1 knee RR 1.59 (95% CI 0.97, 2.61); 2 knees RR 2.02 (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 (0.75, 2.77)), indicating mediation by functional leg weakness. Conclusions: 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.