Objectives
In Denmark, the use of bicycles is widespread, and head injuries are often seen in cyclists involved in collisions. Despite the well-known effects of using a helmet to reduce head injuries, using helmets is not mandatory in Denmark. The primary objective of this study was to provide data regarding injury outcomes and helmet usage.

Methods
Participants were bicyclists who sustained head injuries in bicycle collisions and were assessed by the Copenhagen Emergency Medical Services between 1 January 2016; and 15 June 2019. Patients with suspected head injury were identified in an electronic prehospital patient record. Data were linked to the Danish National Patient Registry to retrieve the diagnosis and were categorized into head injury or no head injury based on the diagnosis. Adjusted logistic regression analyses were reported with odds ratios and corresponding confidence intervals to assess the risk of head injury while adjusting for risk factors like age, sex, alcohol consumption, occurrence during weekends and traumatic brain injury.

Results
A total of 407 patients were included in this study. Within this entity, 247 (61%) had sustained a head injury. The use of a helmet was reported in one-third of the included patients. Among the head-injured patients, 13% sustained moderate to severe head injuries. Patients with suspected alcohol involvement were significantly less likely to report the use of a helmet. Helmet use reduced the risk of head injury with an odds ratio of 0.52, (95% CI 0.31 − 0.86). In high-energy trauma, the use of a helmet showed a significant reduction in the risk of sustaining a head injury with an odds ratio of 0.28, (95% CI 0.12 − 0.80).

Conclusions
In this study, using a helmet was associated with a significantly decreased risk of head injury; this association was even more significant in high-energy trauma.