Ultrasound in inflammatory conditions of subcutaneous and articular adipose tissue in the extremities.

Adipose tissue of the extremities has been largely neglected in musculoskeletal ultrasound (US), beyond the assessment of superficial nodules or the exclusion of deep complications in superficial infections. Three developments have brought it to the fore: a more refined understanding of its anatomy and function - recognizing adipose tissue not merely as a passive fat store but as a metabolic and endocrine organ and a regulator of inflammation; advances in US technique; and the wider adoption of US across medical specialties, particularly dermatology and plastic surgery. Panniculitis refers to inflammation of the subcutaneous tissue and encompasses numerous disorders with considerable clinical overlap. Patients typically present with painful nodules in the extremities and often require histopathological confirmation. Histologically, panniculitides are classified according to the predominant site of involvement - septal, lobular, or mixed. US correlates strongly with these patterns. This review describes the principal sonographic appearances of the most frequent panniculitides and highlights key clinical and pathological features that help narrow the differential diagnosis. Entities that may mimic panniculitis on US are also discussed. Beyond diagnosis, ultrasonography supports disease assessment and monitoring, and it reliably guides targeted biopsy. Finally, the US features of inflammation in articular and periarticular adipose tissue are summarized, with emphasis on findings that should be considered during routine joint US examinations, as such involvement often accompanies internal derangement and may also represent the primary source of patient-reported symptoms.