Myeloperoxidase (MPO) immunohistochemistry and image analysis showing increased expression of MPO in macrophages and fibroblasts within the fibrotic regions of the arthrofibrotic tissue. A - C show representative images for (A) control tissue and (B) low bone volume (BV) and (C) high BV arthrofibrotic tissue. Note the large increase in the presence of MPO positive macrophages (round cells) and fibroblasts (elongated cells) in the fibrotic regions of the 'high' bone arthrofibrotic periarticular tissue. (D) A representative image of MPO expression in tissue from the non-arthrofibrotic stiff knee cohort, showing that MPO was not expressed by control tissue fibroblasts. Image analysis of the patient cohorts was based on the average percentage positive per total cell number. There was a correlative increase in MPO expression with tissue calcification. The fold increases for the high BV group were statistically significant as compared to control and to the low BV group (*; P < 0.05). Magnification 100× for (A -C), micron bar equals 100 μm.