The "enthesis organ" concept - Why enthesopathies may not present as focal insertional disorders

Objective. The Achilles tendon insertion is associated with a complex of adjacent fibrocartilages, a bursa, and a fat-pad, and is functionally much more than a focal insertion. This has important implications for a better understanding of the spondylarthropathies (SpA). However, the degree to which other insertions form comparable "enthesis organs" has not been established. The aim of this study was to demonstrate the applicability of the enthesis organ concept to other insertion sites. Methods. Both joint-related (articular) and extraarticular entheses were removed from 28 sites in the limbs of formalin-fixed cadavers (age at death 70-101 years) that had been donated for anatomic study. The samples were prepared for paraffin histologic analysis and sectioned longitudinally. The presence and extent of enthesis organs was evaluated at each site in serial sections stained with Masson's trichrome and toluidine blue. Results. Articular enthesis organs were found at 14 entheses, including the attachments of the digital extensor tendons and collateral ligaments, the cruciate ligaments, tibialis anterior, the lateral collateral ligament of the knee, and the popliteal tendon. Extraarticular enthesis organs were seen at 2 sites, the biceps brachii and patellar tendon insertions. In all enthesis organs, sesamoid and/or periosteal fibrocartilage was present in close association with synovium. Conclusion. The concept of an enthesis organ is of general significance in understanding attachment sites and may explain the diverse pathologic changes, including synovitis, bursitis, and extracapsular changes, seen adjacent to tendon/ligament entheses in SpA. These findings may provide insight into the reason the target tissues in SpA are apparently so diverse.