Pivotal role of the synovioentheseal complex in the imaging of arthritis and rheumatic diseases

Imaging plays a key role in the diagnosis and management of rheumatic diseases. Although joints and periarticular tissue are commonly involved in rheumatic diseases, entheses further away from joints, such as in the Achilles tendon or plantar fascia insertion onto the calcaneus, as well as skin and subcutaneous tissue, are among other -sometimes overlooked- targets. The link of enthesitis, which describes inflammation at the insertions of ligaments, tendons, or joint capsules, with spondyloarthritis (SpA) was established just before the turn of the century as a characteristic feature based on imaging studies with histopathological correspondence. To highlight the association between enthesitis and synovitis in SpA, the anatomical unit of the “synovioentheseal complex” (SEC) and the concepts of “functional enthesis” and “articular enthesis,” apart from the better known “insertional enthesis,” were introduced to encompass other inflammatory lesions associated with SpA. Studies from the last two decades revealed the involvement of the SEC in rheumatic and non-rheumatic disorders with different pathogeneses. Although such involvement is sometimes distinctive, it does not necessarily point to a specific diagnosis at other times. Nevertheless, the potential of SEC inflammation in the differentiation of SpA from other forms of arthritis remains important. The purpose of this review was to provide essential information concerning the involvement of the SEC in the diagnosis of rheumatic diseases and arthritis, focusing on imaging characteristics.