Structure and histopathology of the insertional region of the human Achilles tendon.

The Achilles tendon inserts onto the calcaneus, and the retrocalcaneal bursa intervenes between it and the bone immediately proximal to the enthesis. The enthesis, the bursa, and the bursal walls form a complex insertional region protecting against wear and tear. We examined the structure and histopathology of the insertional region in 50 tendons from cadavers (age at time of death, 57-96 years). The enthesis contained fibrocartilage typical of attachment sites. In specimens with a prominent superior tuberosity (the majority), the walls of the bursa also were fibrocartilaginous. On the anterior wall, fibrocartilage replaced the calcaneal periosteum; on the posterior wall, there was a sesamoid fibrocartilage in the deep part of the tendon. When the tuberosity was not prominent, the bursal fibrocartilages were absent. Histopathological features were observed in 31 entheses. Bone spurs extended from the calcaneus into the tendon and probably formed by endochondral ossification of enthesial fibrocartilage. Longitudinal fissures were splits in the fibrocartilage along the lines of the endotenon, and small transverse tears occurred at the tendon-bone junction. Longitudinal fissures showed evidence of repair; they were filled with amorphous material and surrounded by clusters of cells. In the bursal walls, calcification or degradation, or both, were observed in 37 specimens and usually involved both sesamoid and periosteal fibrocartilages. These fibrocartilages could therefore be implicated in retrocalcaneal bursitis.