The difference in synovitis (synovial membrane) and angiogenesis between young obese, young lean and old lean people with primary knee osteoarthritis

Abstract

Purpose: OA is a complex disorder with multiple risk factors including obesity. The excess weight and lipids inherent in obesity have been implicated in the development of OA through mechanical and metabolic effects and low-grade inflammation. Furthermore, synovitis and inflammation have gained an enhanced significance in research as a cause of OA. The aim of this laboratory research: 1: To determine the relationship between BMI and synovial vascularity and inflammation among different age groups with advanced OA. 2: To examine the effect of increased lipid stores as indicated by the fat cell size on the vascularity and inflammation. 3: To assess the difference in synovitis among young obese, young lean and old lean people with advanced OA. Methods: A total of 17 patients (matched for gender, the advanced stage of OA requiring total knee arthroplasty, Kellgren-Lawrence score, comorbidities, ASA grade, medications and functional outcome score) were recruited into three groups based on age (young <60, old >70) and BMI (lean <30, obese >30). Group-1: young-obese (N=6) with age and BMI mean (SD) 53.3 (2.33) and 38.53 (1.16) respectively. Group-2: young-lean (N=5) with age and BMI 55.60(4.5) and 27.01(1.59) respectively. Group-3: old-lean (N=6) with age and BMI 75.33 (3.2) and 25.64 (2.5) respectively. Synovial tissues obtained during surgery were examined using H&E staining and Immunohistochemistry (IHC) on 5μm sections. The monoclonal antibodies included vWF and CD34 for angiogenesis, CD68 for macrophage, CD3 for T-lymphocytes, CD20 for B-lymphocytes and vimentin for fibroblast (all Dako Ltd.). Visualization employed DAB. Quantitative assessment of each marker used image J software. Each slide was divided into synovial and fat sectors based on the content. The software measured each marker in the synovial and fat sectors separately and combined. Further analysis followed regrouping patients into two groups based on fat cell diameter, measured as small (< 68 μm) and large (>68 μm) fat cells. The IHC markers were reanalyzed based on the fat cell size. Additional manual counts, vessel diameter and distance to the synovial surface, were undertaken on vessels in the synovial and fat sectors. A synovitis score was determined using the H&E stained sections (Krenn et al., 2006). Results: Using the initial grouping, overall vascularity did not show any significant differences (p=0.468); vimentin positivity was significantly reduced in the old lean group (p=0.006), and inflammatory markers showed a trend of increasing with increase BMI. Regrouping patients based on fat cell size revealed multiple significant differences. Macrophages, T cells, B cells and fibroblasts positivity was significantly higher in the presence of big fat cells (p= 0.035, 0.016, 0.046, 0.014 respectively). The vascular density was significantly higher in the fatty sections with big fat cells (p= 0.028). Inflammatory markers showed positive correlation with BMI and negative correlation with age. An increasing trend of vascular density, the distance to the synovial surface and vascular wall thickness was observed in the young obese group. There was no clear difference between the groups in relation to synovitis but majority of patients showed mild grade synovitis. Conclusions: The synovium of the obese patients showed more inflammatory response but the vascularity was not significantly different. Increased fat cell size attracted significantly more inflammatory cells. Moreover, vascular hyperplasia, which was noticed mainly in the obese people, could be an indication of atherosclerosis in the synovial vessels. Low grade synovitis is common in primary knee OA. This image showed H&E stain of the synovium of an obese person. The vessels has increased wall thickness very close to the intimal surface.