Ultrasonography has advantages for assessing psoriatic arthritis (PsA) due to its ability to evaluate several targets, including joints, entheses, and tendons, but also skin and nails. Although ultrasound is widely used in PsA, nail ultrasound, despite its potential as a non-invasive method for the early detection of inflammation in the nail apparatus, has low applicability in medical practice, as probes with a higher frequency are needed compared with the frequency of probes usually used. In the present article, we have narratively evaluated the studies published in the last 5 years (19 February 2018–18 February 2023) on nail ultrasound value in the diagnosis and monitoring of PsA. The studies published during this period share common measurement parameters, such as the OMERACT definition of enthesis published in 2018. We included original articles published in PubMed and Web of Science using the following search terms in all possible combinations: psoriatic arthritis, psoriasis, ultrasound, and nail. Articles were declared relevant if they presented data on nail morphology, power Doppler evaluation and nearby structure evaluation, while focusing on digitorum extensor enthesitis. In most of the studies, common morphological parameters were generally increased in patients with psoriasis or PsA and were demonstrated to change with medication. The thickness of the extensor tendon was greater in patients with PsA and psoriasis versus controls and it was correlated with nail clinical changes, especially the presence of onycholysis. The presence of PD showed large variability in healthy individuals. The link between enthesitis and nail involvement is still a subject of controversy, taking into account the latest histological findings. The use of ultrasound in the evaluation of nail and DIP enthesitis remains a challenge and an area of research in the coming years.

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.