New perspectives in enthesis ultrasound: validation of enthesis fibrocartilage

This article aims to evaluate the study by Aydin et al., who introduce new methods to assess and understand the Achilles enthesis using ultrasound [1]. This original paper focuses on peripheral extra-articular enthesitis, at the Achilles level. Inflammation at the entheses, the sites of attachment of the tendon, ligament, fascia or joint capsule to the bone, is a characteristic finding of ankylosing spondylitis and spondyloarthritis (SpA). Historically, enthesis evaluation has been difficult and, as a consequence, enthesitis, the hallmark of SpA, is underestimated in the diagnosis and clinical assessment of SpA. This is an important reason to improve the knowledge in this field. Enthesitis has been viewed as a focal, insertional problem of soft tissue; however, in the last decade we have come to understand that inflammation also affects the bone, because we have seen osteitis adjacent to fibrocartilaginous enthesis – an important finding on MRI [2,3] – and this supports the concept of a sinovio-entheseal complex proposed by McGonagle et al. over the past few years [2–7]. The essential importance of the enthesis in understanding the pathophysiology of inflammation and structural damage in SpA has reemerged in front of the only mechanical overload concept of the pathogenesis of enthesopathy. The link between enthesitis and osteitis in SpA has been clarified in recent studies, which demonstrate a close functional integration of the enthesis with the neighboring bone, with the entheses fibrocartilare playing a central role. Entheseal microdamage and inflammatory cell infiltration has been described; the enthesis architecture was abnormal in the SpA group, with increased vascularity and immune cell infiltration compared with normal subjects [3]. The predominant infiltrating cell at the entheses fibrocartilage was the macrophage, but there was a lack of lymphocytes contributing to inflammation at the insertion point [3,4]. Owing to this, previous concepts that mechanical stress is a central cause of enthesis damage in SpA are being questioned as histology, MRI and ultrasound imaging at the enthesis level demonstrate relevant changes in SpA diseases similar to that of an autoimmune disease. Until recently, the radiographic features of enthesitis have played an important role in defining enthesitis lesion images in SpA. This includes bone insertion osteopenia, bone cortex irregularity at insertion, erosion, entheseal soft tissue calcification and new bone formation. However, entheseal bone changes appear late in the disease and are also common in mechanical disorders and in crystal-related pathology. Therefore, over the past few years, different image techniques such as MRI and in particular ultrasound have become a valuable source for the understanding and applicability on future diagnosis and clinical management of SpA. Ultrasound has proven to be a highly sensitive and noninvasive tool, especially interesting when assessing enthesis involvement. The importance of ultrasound in SpA occurs because clinical examination lacks sensitivity and specificity, which has been demonstrated by the poor sensitivity to change and by several studies comparing the clinical evaluation with the new imaging techniques [8,9]. Ultrasound, having demonstrated its usefulness in the detection and assessment of synovitis in Evaluation of: Aydin SZ, Bas E, Basci O et al.: Validation of ultrasound imaging for Achilles entheseal fibrocartilage in bovines and description of changes in humans with spondyloarthritis. Ann. Rheum. Dis. 69(12), 2165–2168 (2010). Enthesitis, the hallmark of spondyloarthritis is underestimated in clinical practice and improving the knowledge of structural damage and inflammation in enthesis is challenging. The results of a study by Aydin et al. are put into the context of the recent advances and definitions in the field of enthesis ultrasound and the new contributions of this original paper in the visualization of the enthesis fibrocartilage and its relationship with the histology are discussed.