Serum vitamin D and knee osteoarthritis: is its role over-emphasized?

Abstract

Purpose: Vitamin D deficiency (VDD) is a global health problem that impacts the musculoskeletal health. Approximately 70-90% of apparently healthy population is vitamin D deficient in India. Similarly, VDD has been reported to be prevalent globally. It is widespread irrespective of age, sex, profession, rural/urban settings or regional distribution. Low levels of serum 25(OH)D has been reported to be associated with many diseases including knee Osteoarthritis (KOA). Recently we have reported that vitamin D supplementation in KOA with hypovitaminosis D improves clinico-radiological severity; however we did not study the comparison of clinico- radiological severity in KOA cases of vitamin D sufficient (VDS) levels and vitamin D deficient (VDD) levels. This cross-sectional case control study was conducted to correlate the serum vitamin D levels with clinical scores (VAS and WOMAC) and radiological severity (KL grade and articular cartilage volume) in KOA subjects with VDD (<30 ng/ml) as cases and with VDS levels as controls. Methods: The present study was conducted in the Dept of Orthopaedic Surgery, King George’s Medical University, Lucknow. 180 subjects of either gender with primary KOA were recruited using American College of Rheumatology criteria for diagnosis. Biochemical assessment of subjects was done for serum 25(OH) D. Subjects were divided into two groups: Group 1 with sufficient serum vitamin D level (VDS) and Group 2 with hypovitaminosis D or VDD. Clinical severity was assessed by self- reported Western Ontario Mac Master University Index Scores for knee pain, stiffness, physical function and total scores of the subjects. Visual Analogue Scale Score was also used for knee pain on 0-10 point scale. Serum 25(OH) D level was correlated with all the clinical parameters. Radiological severity on X-ray was assessed by using Kellgren and Lawrence (KL) grading system in all the subjects. Articular cartilage volume (ACV) measurement was done using MRI in 100 subjects (50 in each group). The study was approved by Institutional Ethics Committee, KGMU. Informed consent was obtained from all the participants. Data was analyzed using statistical software SPSS 16.0. Student t test was used to analyze continuous data. Pearson Correlation Coefficient and one way ANOVA were used to correlate continuous and categorical variables respectively. Results: Mean serum vitamin D levels in Group 1 and Group 2 were 34.96±4.2 and 16.03±7.01 ng/ml respectively and the difference was significant (p<0.0001). Continuous variables viz; clinical parameters and ACV were compared between both the groups and a statistically significant difference was observed (WOAMC pain: p=0.012, stiffness: p=0.022, physical function: p=0.043, total WOMAC: p=0.016, VAS: p= 0.034 and ACV: p=0.037). Group 1: Mean scores for WOMAC pain, stiffness, physical function, total WOMAC and VAS in Group 1 were 9.83±2.16, 2.18±0.24, 21.47±6.26, 33.85±8.71 and 5.98±1.01 respectively. An insignificant negative correlation of serum 25(OH) D was observed with all the clinical variables (WOAMC pain: p=0.218, stiffness: p=0.094, physical function: p=0.139, total WOMAC: p=0.073 and VAS: p=0.104). Mean serum vitamin D levels in KL grades 2, 3 and 4 were 36.14±5.26, 34.19±4.69 and 33.28±4.02 ng/ml respectively. An insignificant association was observed between serum vitamin D and KL grade (p=0.381). Mean ACV in KOA subjects was 4.927±0.86cm3. An insignificant association was observed between serum 25(OH) D and ACV (p=0.422). Group 2: Mean scores for WOMAC pain, stiffness, physical function, total WOMAC and VAS in Group 2 were 10.94±3.63, 2.52±1.38, 23.41±6.51, 37.06±9.06 and 6.27±0.8 respectively. A significant negative correlation of serum 25(OH)D was observed with all the clinical variables (WOAMC pain: p<0.001, stiffness: p=0.047, physical function: p<0.001, total WOMAC: p<0.001 and VAS: p= 0.041). Mean serum vitamin D levels in KL grades 2, 3 and 4 were 18.07±7.95, 16.95±7.56 and 15.03±8.35 ng/ml respectively. A statistically insignificant association was observed between serum vitamin D and KL grade (p=0.478). Mean ACV in KOA subjects was 4.415±1.49cm3. An insignificant association was observed between serum 25(OH) D and ACV (p=0.759). Conclusions: This study found that in KOA subjects having VDD levels, significant association existed with all the clinical features studied and insignificant association with those having VDS levels. This validates the contention that VDD plays a significant role in pain and other symptoms experienced by KOA subjects. Contrary to this were our findings with radiology. In both the groups of KOA with VDD and VDS levels, insignificant association/ correlation was found with the radiological features studied viz. KL grades and ACV. Whereas the clinical features are more subjective, radiological features are not only objective, they also closely reflect the morphological changes undergone in any given disease. The lack of association between the levels of vitamin D and the radiological features in KOA subjects needs to be examined before the hype of vitamin D in controlling KOA, as they are truer representation of the disease process and the severity.