Image Interpretation: Findings: About 5cm proximal to its insertion, the anterior tibialis tendon was thickened with a moderate-sized underlying anechoic fluid collection and marked synovial thickening in the tendon sheath with significant hyperemia. Sonopalpation only mildly reproduced the patient's pain. The Tibiotalar joint was normal. The subtalar joint and talonavicular joints were normal. Impression: Synovial thickening and effusion of the left anterior tibialis tendon sheath without significant tendinosis

Teaching Pearl: The tibialis anterior muscle is the largest dorsiflexor of the ankle. Pathology involving the tibialis anterior tendon is rare but can arise from trauma, direct injury, or be linked to conditions like arthritis or diabetes. Repeated movements like running or wearing certain shoes, such as boots, can lead to pain and swelling. The recurrence of trauma and persistent inflammation induces heightened vascularization within the tendons and myxoid degeneration of the tendon interstitium. The most common site is on the upper edge of the superior retinaculum. Most cases improve with rest, changes in footwear, anti-inflammatory medication, and PT. In recalcitrant cases, US guided tendon sheath injections, extracorporeal shock wave therapy and radiofrequency ablation have been documented in the literature as an appropriate next step in treatment. Characteristic US findings include thickening of the tendon, thickened synovial sheath, changes in tendon echogenicity and increased fluid within the tendon sheath. Hyperemia can be present and visualized via Doppler imaging. References Maquirriain J, Sammartino M, Ghisi JP, Mazzuco J. Tibialis anterior tenosynovitis: Avoiding extensor retinaculum damage during endoscopic debridement. Arthroscopy. 2003 Feb;19(2):E9. doi: 10.1053/jars.2003.50059. PMID: 12579138. Lowell DL, Jean-Claude JM, Fuchs L. Infectious Tenosynovitis of the Tibialis Anterior Due to Mycobacterium chelonae After Intravenous Heroin Injection. J Foot Ankle Surg. 2020 Mar-Apr;59(2):413-417. doi: 10.1053/j.jfas.2019.08.017. PMID: 32131013. Varghese A, Bianchi S. Ultrasound of tibialis anterior muscle and tendon: anatomy, technique of examination, normal and pathologic appearance. J Ultrasound. 2013 Dec 19;17(2):113-23. doi: 10.1007/s40477-013-0060-7. PMID: 24883132; PMCID: PMC4033720. Harkin E, Pinzur M, Schiff A. Treatment of Acute and Chronic Tibialis Anterior Tendon Rupture and Tendinopathy. Foot Ankle Clin. 2017 Dec;22(4):819-831. doi: 10.1016/j.fcl.2017.07.009. Epub 2017 Sep 28. PMID: 29078830.