What constitutes the fat signal detected by MRI in the spine of patients with ankylosing spondylitis? A prospective study based on biopsies obtained during planned spinal osteotomy to correct hyperkyphosis or spinal stenosis

Objective Study the MRI signal of fatty lesions (FL) by immunohistological analysis of vertebral body biopsies of patients with ankylosing spondylitis (AS) compared with degenerative disc disease (DDD). Methods Biopsies obtained during planned surgery from vertebral edges where MRI signals of FL was detected were stained with H&E. Immunofluorescence (IF) staining was performed to quantify osteoblasts and osteoclasts. Bone marrow (BM) composition, grade of cellularity and quantification of cells were analysed on six randomly chosen high-power fields (HPF; 0.125 mm(2)) at 200-fold magnification per patient by two experienced researchers in a blinded manner. Results Biopsies of 21 patients with AS and 18 with DDD were analysed. Adipocytes were found in the BM of 19 patients with AS (90.5%) versus 5 with DDD (27.8%) (p<0.001), while inflammatory infiltrates were found in in the BM of 8 patients with AS (38.1%) versus 14 with DDD (77.8%) (p=0.035) and fibrosis in 6 patients with AS (28.6%) versus 4 with DDD (22.2%) (p=n.s.). The most frequently detected cells were adipocytes in AS (43.3%) versus DDD (16.1%, p=0.002) and inflammatory mononuclear cells in DDD (55%) versus AS (11.0%, p=0.001). Using IF staining, there was more osteoblastic than osteoclastic activity (6.9 vs 0.17 cells/HPF) in FL as compared with inflammatory BM (1.3 vs 7.4 cells/HPF), respectively. Conclusion MRI FL correspond to presence of adipocytes, resulting to change of cellular homeostasis towards diminution of osteoclasts in the BM of patients with AS. The cross-talk between the different cell types and osteitis, fat and new bone formation needs further study.