From Invisible to Visible: Cutting-Edge Ultrasound Insights into Entheses of the Distal Extremities in Rheumatology.

Musculoskeletal ultrasound (MSUS) is a well-established and reliable tool for the evaluation of entheses and enthesitis, particularly at larger and accessible sites. Recent technological advances, including high- and ultra-high-frequency transducers, have expanded its potential, enabling detailed assessment of distal extremity entheses. This narrative review provides a focused and updated perspective on this evolving field, highlighting three key advances. First, the identification and characterization of previously underrecognized entheseal sites in the distal extremities, such as pulley systems, retinacula, novel tendon insertions, and collateral ligaments, broadening the morphological spectrum of entheseal imaging. This is complemented by improved evaluation of vascularization through microvascular imaging and contrast-enhanced US (CEUS). Second, the emergence of interventional approaches, particularly US-guided entheseal biopsy, offers a novel means to investigate entheseal tissue in vivo and may establish a link between imaging and histopathology. Third, the integration of advanced functional imaging modalities, including elastography and multispectral optoacoustic tomography (MSOT), provides preliminary additional insights into the biomechanical and molecular properties of the enthesis beyond conventional structural assessment. Collectively, these developments support new investigational perspectives, positioning MSUS as a dynamic and integrative modality capable of exploring new anatomical territories and biological dimensions, with the potential to reshape the understanding and evaluation of entheseal involvement in rheumatology.