The main findings of this study were that odontoid fractures usually occurred in older individuals, and type II fractures was the most frequent fracture type. Only 14% of the odontoid fractures were due to high-energy trauma. In younger individuals, motor vehicle accidents were usually reported as the injury mechanism. Conversely, in older patients, low-energy traumas were more frequent, and the fractures were predominantly caused by low-level falls. Males were commonly injured in high energy-related accidents. Lastly, non-surgical treatment was the preferred choice of management in 81% of cases.

Data from the SFR demonstrate that odontoid fractures were frequent in elders, with a mean age of 77.2 years and is consistent with previous studies [5, 9]. However, contrary to earlier reports, the present study found a clear unimodal age distribution, with a peak in the seventh to ninth decade of life. In a recent study from Illinois, USA, bimodal age distributions with peaks at 22 and 78 years of age were reported [10]. When comparing traffic accidents between Sweden and Illinois, USA, there were 4.6 times more fatal motor vehicle accidents per capita in Illinois in 2021 [11, 12]. In light of these findings, the peak in younger individuals may be the result of serious traffic accidents which was rather rare in the current study.

Approximately half of the patients with an odontoid fracture were male individuals, which is comparable with earlier studies [2, 4, 9, 13]. Similar to a previous cohort, the included male patients tended to be younger, while elders more commonly tended to be females [5]. Apart from being younger, males were less likely to be injured in low-energy traumas. While the exact mechanisms are not understood, young males are more likely to be risk-takers and show different travelling habits than females. In comparison, elderly females sustain fractures more easily, perhaps because of menopause and subsequently, an osteoporotic odontoid process [14]. Only 10% of the patients in this study had an osteoporosis diagnosis at the time of injury. Previous work has shown rates of osteoporosis in populations with odontoid fractures ranging between 58 and 77% which is considerably higher than this study [15, 16]. Likely explanations to this discrepancy could be that patients in this study possibly demonstrated signs of osteoporosis without having the diagnosis recorded (ICD codes), or that it was undiagnosed.

A large majority of patients had an intact neurological status (94%) whereas only 2.1% had some type of spinal cord injury. This is somewhat lower than previous reports with spinal cord injuries affecting 3–10% of patients [5, 17,18,19,20]. The relatively low rate in this study could be explained by several morphological attributes. For instance, the relatively large cross-sectional diameter of the spinal canal at the level of the odontoid process compared to the diameter of the spinal cord may result in fewer spinal cord injuries compared to other cervical spine fractures. Moreover, most odontoid fractures were associated with low-energy trauma and therefore did not contribute to large displacement causing neurological injuries.

The occurrence of odontoid fractures varied during the seasons with peaks in May and August. There is, to our knowledge, no previous research on the seasonal variation of odontoid fractures. However, other studies from the SFR have investigated seasonal variation of various fracture types. Talus fractures were usually sustained in warmer months (May through October) [21] while patella and ankle fractures in the colder months (October through March) [22, 23]. Speculatively, individuals are more physically active and spend more time outdoors during the summer months, therefore falling more often. Moreover, higher temperatures are associated with hypovolemia, syncope and other cardiac events in the elderly causing falls, and ultimately, odontoid fractures. In contrast, in the winter months, the streets are often slippery in Sweden which could be the cause of the increased number of patella and ankle fractures. One could only speculate as to why this increase was not seen in odontoid fractures. Perhaps those with odontoid fractures are generally older with more co-morbidities than those sustaining patella or ankle fractures. This could result in patients remaining inside when they know that the ground is unsafe to walk on.

Most of the included patients were treated non-surgically (81%) which is consistent with other publications [20, 24]. However, a previous study based on a Swedish population reported that 53% with a type II odontoid fracture underwent surgery with an annual increase between 2002 and 2014 [25]. This study included urban hospitals whereas the SFR receives data from hospitals all over Sweden. Moreover, the SFR has only collected data on spine fractures since 2015 which could provide another explanation to the different managements of type II fractures. Type II fractures are generally prone to pseudoarthrosis and non-union, and although guidelines on the management of odontoid fractures are absent, the risk of impaired fracture healing in type II fractures is an indication for surgical stabilization. This is consistent with our study since significantly more patients with type II fractures underwent surgery than the other fracture types. The overall aim of surgical treatment is to preserve the neurological function, relieve pain and establish a stable fusion. As only 3.5% of the included patients suffered neurological deficits, this could partly explain why most of the patients received non-surgical treatment. Other possible reasons may include high age and major co-morbidities.

Lastly, in this study, 87% of the fractures were caused by falls. For type II fractures, the corresponding number was 90% and for type III fractures 85%, respectively. This is similar to previous publications from Scandinavia [5, 17]. Consistent with earlier findings, older patients were more often injured after falls from a standing height or less, while younger patients were more often injured in a motor vehicle accident [10, 13]. Furthermore, older patients were more likely to sustain an odontoid fracture by a low-energy trauma than younger patients which could be explained by altered body composition, neurological dysfunction and the increasing number of co-morbidities that parallel with aging. For instance, physical inactivity, impaired balance, and neurological disabilities may result in unintended falls. In addition, polypharmacy is common in elders, further increasing the risk of falls [5]. Others have proposed that increasing age is associated with a stiffer cervical spine, and a reduced bone density, which can explain why elderly need less energy for a fracture to develop [25].

The SFR contains data from patients within various geographic locations, representing a heterogenous population which enables a broad generalizability of the results from this study. Additionally, compared to traditional epidemiologic studies based on a sample of the population, the SFR is a population-based register minimizing the risk of selection bias. Moreover, the SFR contains one of the largest datasets on odontoid fractures, enabling the conduction of reliable subgroup analyses. The SFR also contains detailed records pertinent to injury patterns and the data are prospectively collected, which mitigates the risk of recall bias. On the other hand, the completeness of odontoid fracture coverage in the SFR is unknown. Fractures are registered by physicians with various levels of expertise, which may introduce bias due to incorrectly diagnosed or classified fractures. While the reliability of the Anderson and D’Alonzo classification [8] in the SFR has not been evaluated, classification of other fractures including thoracolumbar [26], ankle [27], and humerus [28] fractures have been shown to be highly reliable.