Quadriceps Muscle Strength Correlates With Serum Vitamin D and Knee Pain in Knee Osteoarthritis

Background: Quadriceps muscle weakness and vitamin D deficiency are associated with knee osteoarthritis (KOA). This study aimed to investigate the relationship between quadriceps muscle strength (QMS) and vitamin D in KOA. Methods: Patients with KOA aged 40 years and above were studied. QMS was assessed by the dynanometry method and serum 25-hydroxyvitamin D (25-OHD) by the ELISA method. Serum 25-OHD < 20 ng/mL was considered as a deficiency. The intensity of knee pain was determined by the Western Ontario and McMaster Universities Osteoarthritis Index Pain Scale. The Pearson test was used for correlation analysis between QMS and serum 25-OHD as well as knee pain. Results: A total of 92 patients (female, 80%) with a mean age of 49.6 +/- 11.7 years were studied. QMS was correlated positively with serum 25-OHD (r=0.304, r(2)=9.24%, P=0.005) and negatively with knee pain (r=-0.232, r(2)=5.3%, P=0.034). After adjustment for age, sex, and body mass index, the positive correlation increased to a higher level (r=0.496, r(2)=24.9%, P=0.01). For each 1 ng/mL increase in serum 25-OHD, the value of QMS increased by 14.2% +/- 3.5% (P=0.014). There was no significant correlation between serum 25-OHD and knee pain (P=0.13). Conclusions: These findings demonstrated a significant correlation between QMS with both serum vitamin D and knee pain, indicating a confounding role for quadriceps muscle in the association between serum vitamin D and osteoarthritis knee pain. On the basis of the findings of this study, vitamin D supplementation may affect pain by strengthening quadriceps muscle in KOA.