Enthesitis—inflammation where tendons, ligaments, or joint capsules insert into bone—is a hallmark feature of spondyloarthritides (SpA) and psoriatic arthritis (PsA).1,2 Enthesitis plays a central role in disease pathogenesis, symptoms, and long-term outcomes.
Despite its importance, entheseal assessment remains a major challenge in rheumatology.3 Traditional clinical exams, long considered the diagnostic gold standard, have limitations: subjectivity, variable sensitivity, and poor specificity, particularly in differentiating inflammatory enthesitis from mechanical or degenerative causes.4,5
Over the past 2 decades, ultrasound (US) has transformed entheseal assessment by offering a noninvasive, dynamic, and highly sensitive method to visualize entheseal anatomy and pathology.6,7
The Outcome Measures in Rheumatology (OMERACT) US task force has played a key role in defining the elementary lesions of enthesitis, such as entheseal thickening, hypoechoic changes, power Doppler (PD) signal, enthesophytes, calcifications, and bone erosions, and in standardizing definitions to ensure consistency in research and clinical practice.8-10 Yet, despite these advances, important questions remain regarding the reproducibility, clinical relevance, and interpretation of these ultrasound findings.11,12
In this context, the Defining Enthesitis on Ultrasound in Spondyloarthritis (DEUS) initiative stands as a pivotal international, multicenter effort to rigorously evaluate the performance and utility of OMERACT US definitions in SpA. Comprising 5 key studies, DEUS examines US interpretation reliability, lesion discriminative value, the correlation between US and clinical examination, development of a combined clinical-US enthesitis index (ie, DEUS Enthesitis Index [DEI]), and characterization of structural damage phenotypes.13-17
This editorial aims to synthesize the main findings of the DEUS studies, highlight their clinical and research implications, and propose future directions for optimizing entheseal assessment in SpA.
Reliability of OMERACT US definitions: Strengths and limitations
Reliability is fundamental to the clinical utility of any imaging definition. The DEUS initiative’s landmark web-based study involved 47 rheumatologists from 37 centers in 15 countries who assessed 101 images and 39 video clips of lower limb entheses in patients with SpA. The study evaluated inter- and intraobserver reliability of key OMERACT-defined lesions: entheseal thickening, hypoechoic areas, PD signal, enthesophytes/calcifications, and bone erosions.13
These findings highlight a crucial insight: whereas some US lesions, especially bone erosions and PD signals, are reliably identified, others like entheseal thickening and hypoechoic changes remain subjective and variable. This variability has significant clinical implications. Highly reliable features like PD signal and bone erosions may serve as stronger markers for diagnosis, monitoring, and trials. Conversely, less reliable signs should be interpreted cautiously or given less weight in composite scores and established diagnoses. These results suggest that the current OMERACT definitions may benefit from refinement, emphasizing the need for improved imaging protocols, clearer morphological criteria, and standardized training. Incorporating detailed lesions descriptions and dedicated image atlases or databases of representative examples could further enhance diagnostic consistency and interreader agreement.
The discriminative value of US lesions in SpA
Building on the reliability findings, the DEUS studies aimed to identify which US features best differentiate SpA-related enthesitis from other causes of entheseal involvement such as mechanical stress or fibromyalgia.14 This distinction is crucial, as entheseal pain and tenderness have diverse causes, and accurate diagnosis is essential for proper management.
In a multicenter cohort of over 400 patients with SpA and nearly 300 controls with osteoarthritis or fibromyalgia, US assessments of large lower limb entheses showed that most OMERACT-defined lesions were significantly linked to SpA in univariate analyses—except enthesophytes and calcifications, which can be commonly detected in noninflammatory conditions as well as in healthy subjects.18
After adjusting for confounders, only PD signal and bone erosions remained strongly and independently associated with SpA, with odds ratios of 8.77 and 4.75, respectively.14 This highlights that PD signal, reflecting active inflammation through increased blood flow, and bone erosions, indicative of structural damage, are the most discriminative US markers for SpA-related enthesitis. Interestingly, among entheses examined, the Achilles tendon stood out as the site most strongly linked to SpA. This aligns with clinical experience, where the Achilles insertion is frequently involved and is often symptomatic.19
In our recent study,14 we also proposed a modified OMERACT definition of “active” enthesitis.8 Unlike the original OMERACT definition, which requires the presence of entheseal thickening or hypoechoic areas alongside a positive PD signal, our definition includes these features but also considers moderate or higher PD scores alone sufficient to define activity, even in the absence of thickening or hypoechoic changes. Applying this modified definition increased the number of patients with SpA meeting the criteria for active enthesitis by 10% compared to the original definition without losing specificity.
In conclusion, the study14 showed that US can differentiate SpA-related enthesitis from mechanical enthesopathies, improving diagnostic accuracy beyond clinical examination alone. These findings aid early diagnosis, guides targeted therapy, and helps avoid potentially unnecessary treatment in noninflammatory cases.
US vs physical examination: Complementary tools
A common challenge in rheumatology is aligning physical exam findings with imaging results. The DEUS multicenter study evaluated over 4000 entheses in 413 patients with SpA using US and clinical assessment.15 Agreement between US and physical exam varied by lesion type and site. PD signal and DEUS active enthesitis on US showed almost perfect agreement with clinical tenderness, whereas enthesophytes had moderate agreement. Patellar tendon entheses had the highest concordance, whereas the Achilles tendon enthesis—a key area in patients with SpA—showed the lowest agreement between physical exam and US.
A striking finding was the discordance between clinical and ultrasound assessments at patient level: nearly 70% of SpA patients with clinical enthesitis showed no active enthesitis on ultrasound, suggesting tenderness may reflect factors like mechanical pain or central sensitization.15 Conversely, about 15% without clinical enthesitis had subclinical active enthesitis on US. Subclinical enthesitis was linked to local structural damage but not to demographic or disease activity differences, raising questions about whether it represents early or smoldering disease that might predict progression, and whether these patients need closer monitoring or more aggressive treatment. These questions remain unresolved.
Overall, the study15 highlights that neither US nor physical examination alone is sufficient to fully characterize entheseal involvement. Instead, a comprehensive approach integrating both modalities promises a more accurate assessment, capturing both symptomatic inflammation and silent tissue pathology.
The DEI: Toward an integrated assessment tool
Recognizing the complementary value of US and clinical exams, the DEUS initiative developed the DEI, a composite score combining both for a comprehensive evaluation of enthesitis in SpA.16
The DEI included bilateral assessments of 5 key lower limb entheses (quadriceps tendon, proximal and distal patellar tendons, Achilles tendon, and plantar fascia). Each enthesis was scored 0 or 1 for clinical signs and 0 or 1 for US-detected enthesitis, yielding total scores from 0 to 20.
The DEI demonstrated strong associations with established indexes of disease activity and severity, including inflammatory markers and US-detected structural damage. Compared to clinical examination alone, US more effectively accounted for C-reactive protein levels and structural lesions, such as enthesophytes and bone erosions, highlighting its greater sensitivity to both systemic inflammation and chronic structural changes at entheses, the latter potentially reflecting past inflammatory activity. Interestingly, clinical examination alone correlated more strongly with patient-reported outcomes and disease activity scores, suggesting that symptoms and tenderness capture aspects of disease not fully reflected by imaging.
This dual nature underscores the DEI’s value as a balanced tool that bridges subjective and objective measures, providing rheumatologists with a more nuanced view of entheseal involvement. Incorporating the DEI into clinical trials and routine practice could enhance patient stratification, treatment decisions, and monitoring of therapeutic responses.
US-detected structural damage and distinct SpA phenotypes
Beyond active inflammation, the DEUS studies also examined structural damage at the enthesis, revealing key phenotypic differences between axial SpA and PsA.17 Among 413 patients, structural lesions, including enthesophytes, calcifications, and bone erosions, were highly prevalent. Enthesophytes were the most common, particularly in patients with PsA (86.8% vs 71.9% in axial SpA).17 Calcifications and bone erosions were also frequent, with erosions strongly associated with inflammatory markers, HLA-B27 positivity, clinical enthesitis, and longer disease duration. Enthesophytes correlated with psoriasis and clinical enthesitis. Notably, calcifications were associated with metabolic comorbidities, such as hypertension, metabolic syndrome, and obesity, indicating that entheseal pathology may be linked to systemic factors in addition to immune-mediated inflammation, although other unmeasured covariates may also contribute. All structural damage lesions were associated with the use of biologic disease-modifying antirheumatic drugs.
These findings underscore distinct US phenotypes within SpA, reflecting underlying pathophysiological heterogeneity. Recognizing these phenotypes could support personalized medicine by guiding targeted therapies and improving prognostication.
Future directions
The DEUS initiative13-17 represents a landmark achievement in refining entheseal assessment in SpA, addressing longstanding challenges with a rigorous, collaborative, and data-driven approach. These advances pave the way for more accurate diagnosis, monitoring, and personalized care.
Key lessons emerge:
• US is an invaluable adjunct to physical examination, enhancing sensitivity and specificity for active enthesitis and structural damage.
• Not all US lesions are equally reliable or clinically meaningful; PD signal and bone erosions stand out as robust markers.
• The Achilles tendon enthesis is confirmed to be a key site in patients with SpA.
• Subclinical enthesitis on US is common and linked to structural damage, warranting further longitudinal study to clarify its prognostic significance.
• Composite indexes like the DEI hold promise for integrating multiple facets of entheseal disease, balancing objective imaging with clinical relevance.
Nonetheless, challenges persist. The variable reliability of certain US features calls for ongoing refinement of definitions and training standards. The clinical implications of subclinical enthesitis need elucidation to inform management algorithms. Further, longitudinal studies are essential to establish how US findings predict disease course, response to therapy, and outcomes.
Looking ahead, integrating advanced imaging with artificial intelligence and machine learning could improve the assessment of entheses by making evaluations more accurate, consistent, and personalized. These technologies may help bridge the gap between clinical findings, imaging results, and patient-reported outcomes. In addition, international US groups, including OMERACT, have highlighted the importance of assessing small entheses in the hands such as pulleys, peritendinitis, and ligament insertions, expanding entheseal evaluation beyond the traditional focus on larger sites.20 Incorporating these areas could further improve the sensitivity and comprehensiveness of entheseal assessment in SpA.
Footnotes
CONTRIBUTIONS
ADM: writing – original draft, conceptualization, writing – review & editing. EF: writing – review & editing, supervision. SDD: writing – review & editing.
FUNDING
The authors declare no funding or support for this work.
COMPETING INTERESTS
ADM has received speaking fees from AstraZeneca and Janssen, and research grant support from Alfasigma. EF reports receiving speaking fees from AbbVie, IBSA, J&J, Novartis, and UCB. SDD declares no conflicts of interest relevant to this article.
- Copyright © 2026 by the Journal of Rheumatology
