Abstract
We present a case of hypertrophic osteoarthropathy (PHO), with painful synovium hyperplasia involving both knees that was refractory to corticosteroid treatment. His rheumatoid factor and anti-CCP antibody was negative, and his serum ESR and CRP level was within normal range. Histological examination of the synovium obtained from his right knee revealed endothelial hyperplasia and vascular thickening without inflammation that was in association with aberrant expression of CD200/CD200R1, which highlighted the importance of aberrant CD200/CD200R1 in the regulation of the endothelial activation that contributed to the development of synovium hyperplasia in this PHO patient.