Axial Spondyloarthritis and New Bone Formation

Axial spondyloarthritis is an inflammatory disease of the axial skeleton. Its pathogenesis is only partly understood. At the beginning, there are inflammatory changes in the sacroiliac joints which are followed by inflammation in vertebral bodies and in facet joints. Low back pain occurring in the morning hours is the dominant clinical symptom. In the early phase, inflammatory changes are detectably by MRI. Inflammation promotes a process of joint remodelling in the sacroiliac joints which leads to erosions, sclerosis and bony bridging, i.e. ankylosis, which are detectable by X-ray. In the spine, vertical osteophytes developing at sites of previous inflammation connect vertebral bodies as syndesmophytes. Additional ossification of longitudinal ligaments contributes to the so-called bamboo spine. Ossification of the spine promotes fixation of a severe kyphosis of the thoracic spine which strongly impairs spine mobility and quality of life. High disease activity seems a prominent risk factor for development of structural damage. However, although NSAIDs improve clinical symptoms, they do not reduce new bone formation. In contrast, TNF alpha and IL-17 inhibitors seem to retard new bone formation apart from their clinical efficacy. Research work of the last years identified immunological pathways of inflammation. However, the trigger and cellular components of the immune reaction in the bone marrow are still poorly defined. Osteoclasts are involved in the destruction of the subchondral bone, while osteoblasts facilitate new bone formation and cartilage ossification. This review gives an overview about diagnostics and therapy of axSpA and about risk factors for the development of structural damage. Concepts about the immune pathogenesis and joint remodeling in AS are given under recognition of genetic and histopathological studies.