Basic concepts of enthesis biology and immunology.

The article highlights key features of entheses relevant to understanding psoriatic arthritis (PsA). It is emphasized that entheses are regions of stress concentration and that stress levels are reduced by anatomical adaptations at the insertion site and its adjoining tissues. These adaptations for stress dissipation include fascial expansions, the flaring out of soft tissue as it approaches the enthesis, the reduction of insertional angle changes by pulleys or retinacula, and fibrocartilage buffers near the bony interface. Despite such adaptations, however, microdamage is common at entheses and can be associated with the presence of microscopic cellular infiltrates, including macrophages and lymphocytes that can be seen as a normal age-related finding. Observations pertaining to the close functional interdependence between the enthesis and adjacent synovium have led to the concept of a synovio-entheseal complex, which is important for understanding joint physiology and pathophysiologic mechanisms of synovitis in PsA.