Enthesis

Overview
Enthesis (plural: entheses) is the point at which a tendon inserts into bone, where the collagen fibers are mineralized and integrated into bone tissue. These insertion points are commonly called Sharpey's fibers.

Classification
There are two types:
 * Fibrous entheses
 * Fibrocartilaginous entheses

In a fibrous enthesis, the collagenous tendon or ligament directly attaches to the bone, whilst the fibrocartilaginous enthesis displays 4 zones during the transition from tendon/ligament to bone:
 * i) tendinous area displaying longitudinally oriented fibroblasts and a parallel arrangement of collagen fibres


 * ii) a fibrocartilaginous region of variable thickness where the structure of the cells changes to chondrocytes


 * iii) an abrupt transition from cartilaginous to calcified fibrocartilage - the so-called 'tidemark' or 'blue line'


 * iv) bone

Pathology
A disease of the entheses is known as an "enthesopathy" or "enthesitis" and is characteristic of spondyloarthropathy but present in other pathology as well.