Enthesitis is inflammation of the entheses, the sites where tendons or ligaments insert into the bone.It is also called enthesopathy, or any pathologic condition involving the entheses.a) Short tau inversion recovery (STIR) image, showing high signal intensity at the Achilles tendon insertion (enthesitis, thick arrow) and in the synovium of the ankle joint (synovitis, long thin arrow). Bone marrow oedema is seen at the tendon insertion (short thin arrow).