The enthesis (plural: entheses) is the connective tissue between tendon or ligament and bone.
There are two types of entheses: Fibrous entheses and fibrocartilaginous entheses.
In a fibrous enthesis, the collagenous tendon or ligament directly attaches to the bone, whereas the fibrocartilaginous interface encompasses four transition zones:
- Tendinous area displaying longitudinally oriented fibroblasts and a parallel arrangement of collagen fibres
- Fibrocartilaginous region of variable thickness where the structure of the cells changes to chondrocytes
- Abrupt transition from cartilaginous to calcified fibrocartilage—the so-called 'tidemark' or 'blue line'
A disease of the entheses is known as an enthesopathy or enthesitis. Enthetic degeneration is characteristic of spondyloarthropathy and other pathologies.
The enthesis is the primary site of disease in ankylosing spondylitis.