The structure and histopathology of the "enthesis organ" at the navicular insertion of the tendon of tibialis posterior.

Objective: To investigate the structure, histopathology, and molecular composition of tissue specializations of the tibialis posterior enthesis. They collectively reduce stress concentration at the insertion site and are part of an "enthesis organ." This has implications for understanding the basis of enthesopathy.

Methods: Fifty-two specimens of tibialis posterior and the associated superomedial part of the calcaneonavicular ligament taken from cadavers were sectioned longitudinally and examined by routine histology (42 samples) or immunohistochemistry (10 samples). Serial sections of formalin fixed material were stained with Masson's trichrome, toluidine blue, or hematoxylin, eosin and alcian blue. A panel of antibodies against collagens, glycosaminoglycans, and proteoglycans was used to immunolabel methanol fixed material.

Results: The enthesis organ consists of the enthesis itself, the superomedial part of the calcaneonavicular ligament (which may fuse with the tendon), the tendon sheath, and associated accessory bones. The accessory bones lay in a region of fibrocartilage that was present even in specimens where the bones themselves were absent. Degenerative changes were seen at the enthesis, around the accessory bones, and in the walls of the tendon sheath. The navicular and accessory bone entheses, together with the calcaneonavicular ligament, were all rich in fibrocartilage. This immunolabeled for aggrecan, link protein, type II collagen, and versican.

Conclusion: The complexity of the enthesis organ, and the diversity of sites showing histopathological changes, suggest that enthesopathy may not be located precisely at the osteotendinous junction. It could target a number of adjacent locations, in accord with what happens at other entheses; e.g., in patients with spondyloarthropathy. The prominence of fibrocartilage in the enthesis organ, and the degenerative changes to which it is subject, support the view that spondyloarthropathy has an underlying biomechanical basis.