Image Interpretation: Volar first CMC joint demonstrates cortical irregularity with marginal osteophyte formation, most pronounced at the base of the trapezium, consistent with osteoarthritis. Anechoic distension of the joint capsule is identified volarly, consistent with a joint effusion. Synovial hypertrophy present. Adjacent flexor tendons of the thumb are unremarkable in echotexture without tenosynovitis.

Teaching Pearl: The first CMC joint is a common site of osteoarthritis, typically affecting women in their 7th decade of life. It is characterized by pain with pinching, gripping, and turning motions. Ultrasound has emerged as a bedside diagnostic modality for correlating exam findings with intra-articular pathology. Ultrasound of the CMC joint is more sensitive than other modalities for detecting osteophytes, which are more strongly associated with hand pain than inflammatory features. Ultrasound can also be used to detect effusions and synovial thickening. Treatment for first CMC OA is commonly injection-based, making ultrasound a powerful tool, given its versatility as a diagnostic imaging modality for guided joint injections, without requiring the patient to undergo costly fluoroscopy and multiple appointments. References: 1. Iagnocco A, Coari G. Usefulness of high resolution US in the evaluation of effusion in osteoarthritic first carpometacarpal joint. Scand J Rheumatol. 2000;29(3):170-173 2. Kroon FPB, van Beest S, Ermurat S, et al. In thumb base osteoarthritis structural damage is more strongly associated with pain than synovitis. Osteoarthritis Cartilage. 2018;26(9):1196-1202. 3. Sivakumaran P, Hussain S, Ciurtin C. Comparison between several ultrasound hand joint scores and conventional radiography in diagnosing hand osteoarthritis. Ultrasound Med Biol. 2018;44(7):1402-1412. Funding Disclosures/Conflicts of Interest: The views expressed in this manuscript are those of the authors and do not necessarily reflect the official policy or position of the Department of the Army, Department of War, or the U.S. Government. We are military service members. This work was prepared as part of my official duties. Title 17 U.S.C. 105 provides that "Copyright protection under this title is not available for any work of the United States Government." Title 17 U.S.C. 101 defines United States Government work as a work prepared by a military service member or employee of the United States Government as part of that person's official duties.