Associations of central obesity with earlier onset of osteoarthritis symptoms

Abstract

Purpose: Obesity is a know risk factor for pain and osteoarthritis (OA), and in most studies, Body Mass Index (BMI) has been used to classify obesity (BMI>=30) in subjects at risk of OA and pain. However, BMI does not take into account the distribution of the fat mass. Waist circumference (WC) and waist to hip ratio (WHR) strongly correlates with abdominal obesity and are commonly used to measure the fat distribution and to determine central obesity. Central obesity is one of the most important components of metabolic syndrome and it has been demonstrated to be a stronger factor associated with knee pain, low back pain, and inflammation more than and, independent of BMI. In our population with knee and/or hip OA, we would like to determine whether central obesity is associated with an earlier presentation of joint pain and which joints are more affected by pain in subjects with OA and central obesity compared with subjects with OA without central obesity. Methods: In this retrospective cohort study, we examined medical records and interviewed 770 patients with a confirmed diagnosis of OA, who attended the orthopedics outpatient clinic-referred from other services with advanced hip and/or knee joint wear and persistent joint pain. We asked the patients the age of onset of OA-pain in years and which joint was the first affected and which joints were currently affected by pain using the Numerical Rating Scale (NRS). Waist and hip circumferences were measured and central obesity was defined as unhealthy when the Waist to Hip Ratio (WHR) was equal or higher than 0.90 in men or equal or higher than 0.85 in women, according to the World Health Organization (WHO). Potential confounders in the relation central obesity and pain, like depression, anxiety, physical activity, education, alcohol intake, and smoking were analyzed. Analysis of variance (ANOVA), linear and logistic Regression Analyses was performed. Betas (β) and Odds Ratio (OR) with 95% confidence interval (CI) are presented. All analyses were adjusted for age, gender, and body mass index. Results: In our population 25% of subjects were affected by hip-OA, 49% with knee-OA, and 26% with both joints affected. Knee was the first joint that reported pain in 56% of the subjects followed by hip (38%), low back (4%), and other joints (2%). From all 770 subjects included in the analysis, 46% were obese (BMI≥30). Most of our population (65%) had central obesity. BMI was slightly associated with earlier onset of OA-pain (B=-0.11, P= 0.07) and central obesity was significantly associated with earlier onset of OA-pain in years (B =-2.77, P = 0.002) and B = -2.53 P = 0.006, after adjustment for age, gender and BMI. Subjects with central obesity started with OA-symptoms 2.5 years earlier than subjects without central obesity (Fig. 1, P=0.006). The odds of starting with OA-pain before the age of 50 years in individuals with central obesity was 1.7 times higher compared with individuals without central obesity (OR: 1.72, CI: 1.07-2.78, P=0.025). In addition, our population with hip or knee OA reported low back pain (35.5%), hand (10%), shoulder (9.2%), foot or ankle pain (9.1%), or elbow pain (3.2%). Central obesity was associated with a higher number of joints affected by pain (Fig. 2). subjects with unhealthy central obesity had approximately 10.4% more joints affected by pain (Fig. 2, P= 0.04). Subjects with central obesity in addition to hip or knee OA-pain had higher odds of foot or ankle pain (OR=2.11, CI: 1.0-4.46) and low back pain (1.51, CI: 1.0-2.28). Conclusions: BMI is a well-established risk factor for pain and OA and regarding joint pain central obesity is perhaps the most important obesity related-trait. Central obesity not only exerts influence on the number of joints with pain but might determine the age of onset of OA-symptoms. Subjects with central obesity and OA, are at higher odds of reporting low back and/or foot or ankle pain. Several potential mechanisms of the relationship between obesity and pain have been proposed. However, today we recognize that mechanical-structural explanations for the association of obesity with OA and pain are not the only ones. Metabolic-inflammatory factors associated with an excess of adipose tissue should be recognized as a more important factor for pain and pain initiation than BMI. According to our Results, central obesity might exert an important role in the development of OA-pain and might influence pain at other joints in subjects already affected with OA. Considering that waist circumference increases in the elderly and that in the last 20 years major increases in WC and WHR have been observed in adults from developed countries, the reduction of abdominal fat should be targeted in programs of joint pain management. Further analysis will try to determine the exact contribution of abdominal fat tissue to joint pain.