Tendon entheses can be classed as fibrous or fibrocartilaginous according to the tissue present at the skeletal attachment site. The former can be "bony" or "periosteal", depending on whether the tendon is directly attached to bone or indirectly to it via the periosteum. At fibrocartilaginous entheses, the uncalcified fibrocartilage dissipates collagen fibre bending and tendon narrowing away from the tidemark; calcified fibrocartilage anchors the tendon to the bone and creates a diffusion barrier between the two. Where there are additional fibrocartilaginous specialisations in the tendon and/or bone next to the enthesis, an "enthesis organ" is created that reduces wear and tear. Little attention has been paid to bone at entheses, despite the obvious bearing this has on the mechanical properties of the interface and the clinical importance of avulsion fractures. Disorders at entheses (enthesopathies) are common and occur in conditions such as diffuse idiopathic skeletal hyperostosis and the seronegative spondyloarthropathies. They are also commonly seen as sporting injuries such as tennis elbow and jumper's knee.