Figure 3.

Compositions and structures of the natural enthesis. Structures of natural enthesis can be divided into five parts: pure dense fibrous tissues (I) including fibroblast (17–20 nm, 20–30 nm in other studies, long axis) and collagen type I (45–120, 20–200 or 50–200 nm in other studies, usually gathering into bundles). Uncalcified fibrocartilage (II) includes fibrochondrocytes (10–15 nm, long axis) and thinner collagen type II. Tidemark (2–5 μm). Calcified fibrocartilage (III) includes hypertrophic fibrochondrocyte (20 nm, long axis). Bone (IV) includes osteoblast, osteoclast and osteocyte. The content of calcium rises from 0% (unmineralized region) to 60% (gradient region), then it declines from 70% to 60% in the mineralized region. The strain is about 7% in the compliant region under 50 GPa [13].