Achilles Enthesitis in Psoriatic Arthritis: Inter-Observer Reliability of Ultrasound Findings

Background/Objectives: Enthesitis is a hallmark feature across the spondylarthritis spectrum, including psoriatic arthritis (PsA). In recent years, advanced imaging techniques, particularly musculoskeletal ultrasound (MSUS), have demonstrated higher sensitivity than clinical examination in detecting enthesitis. This study aimed to evaluate the inter-observer agreement for the diagnosis of Achilles enthesitis in a cohort of PsA patients. A secondary objective was to explore specific ultrasound diagnostic criteria for identifying active, inflammatory enthesitis in this population. Methods: Adult patients with PsA, all fulfilling CASPAR classification criteria, were recruited and underwent both clinical and ultrasonographic assessment of the bilateral Achilles tendons. Each patient was scanned by 4 rheumatologists in a direct study, followed by a blinded evaluation of static images of the same patients. The examiners assessed the presence of enthesitis components according to the OMERACT criteria. In addition, the images were subsequently evaluated by 10 MSUS-experienced rheumatologists who were asked to classify the enthesitis as inflammatory by selecting one of the following responses: "yes", "no," or "possible". Results: Ten PsA patients, with a median age of 60 and a median DAPSA score of 21, were included. Both direct and image-based inter-observer studies showed high agreement values for enthesophytes (κ > 0.6), erosions (κ > 0.5), and entheseal thickness (κ > 0.5). In both, low agreement was observed for hypoechogenicity (κ between 0.1 and 0.4). Erosions and power Doppler (PD) signal in erosions showed statistically significant differences between the "possible" and definite ("yes") inflammatory enthesitis groups. A PD signal of grade 2 or 3 within the enthesis or erosions was observed exclusively in cases classified as definite ("yes") inflammatory enthesitis. Similarly, a grade 3 PD signal in the bursa was found only in patients with definite inflammatory enthesitis. This study proposes a novel ultrasound scoring system for defining inflammatory enthesitis. The score demonstrated overall good diagnostic performance, with a sensitivity of 67% and a specificity of 100%. Conclusions: The relatively low inter-observer agreement regarding hypoechogenicity and the presence of PD highlights the need for targeted educational interventions to improve interpretation in MSUS. Erosions and PD signal within erosions appear to be significant discriminatory features for identifying inflammatory enthesitis.