Rockwood type III is the most common type of acromioclavicular joint dislocation: A prospective cohort study investigating the incidence and epidemiology of acute acromioclavicular joint dislocations in an urban population.

Introduction: Acromioclavicular (AC) joint dislocations are common, but basic epidemiological features and sub-classification are not well investigated. The aim of the study was to investigate the incidence and epidemiology of acute AC joint dislocations in the capital region of Denmark.

Methods: All patients with acute AC joint dislocation admitted to the emergency departments at 3 University Hospitals serving a population of 549,225 residents were prospectively registered from January to December 2019. Patients with trauma to the shoulder, pain from the AC joint and increased coracoclavicular distance on radiographs were included and classified according to Rockwood's classification. Data on age, sex and mechanism of injury were registered.

Results: A total of 106 patients, male:female ratio 8.6:1, were included. Rockwood type III was most common accounting for 59/106(55.7%) of the injuries. The incidence was 19.3 per 100,000 person-years at risk (PYRS). The age distribution was bimodal peaking at the ages of 20-24 and 55-59 years. The most common mechanism of injury was sports, 80/106, with cycling accounting for 51/106.

Discussion/conclusion: Rockwood type III was the most common type of AC joint dislocation constituting 55.7% of the injuries. The incidence was 19.3 per 100,000 PYRS. Young and middle-aged males were at highest risk and most injuries occurred during sports.