The entheseal stress hypothesis for the relationship between inflammation and ankylosis in spondyloarthritis. BMP, bone morphogenetic protein; DISH, diffuse idiopathic skeletal hyperostosis; FOP, fibrodysplasia ossificans progressiva; IBD, inflammatory bowel disease; ERAP, endoplasmic reticulum aminopeptidase. Reproduced with permission from Lories RJ, Luyten FP, de Vlam K: Progress in spondylarthritis. Mechanisms of new bone formation in spondyloarthritis. Arthritis Res Ther 2009, 11:221 [19].