INTRODUCTION: Acromioclavicular (AC) joint separation is a common injury that results from direct or indirect trauma to the shoulder. The purpose of this study was to explore the incidence, demographic variations, and injury mechanisms of AC joint separations in the United States from 2004 to 2023. METHODS: The National Electronic Injury Surveillance System database was queried to identify all patients who visited participating emergency departments between 2004 and 2023 and were diagnosed with an AC joint separation. Examined variables included age, sex, year, and injury mechanisms. National injury estimates were calculated using sample weights. Annual incidence rates were calculated using U.S. Census population estimates. RESULTS: A total of 4163 AC joint separations were recorded, representing an estimated 196,983 injuries. The overall incidence rate was 3.1 per 100,000 person-years. The highest incidence of AC joint separations was among patients aged 10 to 30 years (47.6%), and the majority of patients were male (83.9%). Sports-related injuries had a significantly greater incidence rate compared to transportation-related and fall-related injuries (p <
0.001). The most common injury mechanisms involved bicycles (15.9%), football (10.5%), stairs/steps (6.0%), and snowboarding (5.8%). CONCLUSION: The incidence of AC joint separations has increased over the past two decades, with the majority caused by sports-related and transportation-related injuries.