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 ankylosing spondylitis.