During the Tokyo 2020 Olympic Games, 29 athletes and MRI scans (0.3% of all participants, 5.1% of all MRIs) at the polyclinic were diagnosed with bone stress injuries. Although there was no significant difference between males and females, bone stress injuries were most common in females (55%), in the lower extremities (66%), and among track and field athletes (45%). These results were very similar to those from Rio de Janeiro 2016, in which there were a total of 25 athletes (approximately 2% of all injuries, 0.2% of all participants) with bone stress injuries (9 stress fractures, 16 stress reactions), 72% of which were in women, 84% of which were in the lower extremities, and 44% of which were in track and field athletes . Bennell et al. summarize the epidemiological characteristics of stress fractures in different sporting populations , indicating that bone stress injuries comprised between 0.7 to 15.6% of all athletic injuries and were most common in women and track and field athletes. These results suggest that Olympic athletes could show similar epidemiologic trends in bone stress injury as non-Olympic athletes.
This study describes the incidence of bone stress injuries by continent. It showed that athletes from the African continent were diagnosed with bone stress injuries at a significantly higher frequency. The result was insufficient to conclude a continent-specific incidence trend, as we did not consider the proportion of sports events, gender, and age. At the Rio Olympics, bone stress injuries in African athletes were second only to those in Europe, but the incidence was not evaluated . Multifactorial evaluations of the incidence and risks of bone stress injuries among continents might be worth considering.
The current study provided additional details within the track and field category, separately providing data on road events (marathon and race walking). Although we did not calculate the total number of athletes in each category, about half of the track and field athletes with bone stress injuries (n = 6, 46%) competed in track events. Since the anatomical sites exposed to physical loading and the content and intensity of training differ among the events in track and field, it may be helpful to accumulate more detailed epidemiological information on bone stress injuries in each event of track and field.
High-intensity training is unavoidable for Olympic athletes, meaning that delayed diagnosis can result in the progression of the MRI grade of bone stress injury. According to the literature, the longer the time between symptom onset and diagnosis of the stress fracture, the longer the recovery time . Hoenig et al., in their systematic review and meta-analysis , reported that higher MRI-based grading of bone stress injury was associated with an increased time to return to competition. Therefore, the finding that more than half of the cohort in our study had high-grade injuries emphasises the importance of prevention and early diagnosis in Olympic athletes.
We also described the details of injury onset and DNS/DNF athletes with imaging-detected bone stress injuries. Most athletes with bone stress injuries (72%) had symptoms before their arrival at the Olympic Village. Furthermore, six of 29 athletes with bone stress injuries (21%) did not start or did not finish their competitions, and four of the six athletes who withdrew had symptoms before entering the Olympic Village. In Olympic athletes, early MRI examination of symptomatic athletes, even before their arrival at the Games, could reduce the risk of withdrawal and stress injury progression. This emphasis on early diagnosis clearly extends to athletes of all levels. Although preparticipation evaluation has been performed at the athlete level regarding sports injury prevention and screening, no effective means of providing risk factors and early diagnosis for bone stress injury has been established . Further investigation of early diagnostic strategies, including the validity of early MRI examination, especially at the level of elite athletes, is required.
Our study had several limitations. First, many of the athletes with bone stress injuries in this study did not undergo radiography; therefore we could not assess the utility of MRI compared with that of radiographs. Second, we could not obtain information on risk factors for bone stress injury such as eating disorders, menstrual dysfunction, medication use, or underlying medical conditions [20,21,22,23]. IOC policy states that no research can disrupt athlete training and competition, and therefore questionnaires regarding risk factors including eating disorders and menstrual function were not possible. Another limitation is that only athletes who visited the polyclinic and underwent radiology examinations were included in this study; potential patients with more subtle symptoms that did not warrant imaging examinations or did not visit the polyclinic were not evaluated. Finally, we relied on imaging reports in the EMR and PACS of the polyclinic to identify athletes with bone stress injuries; it is possible that some athletes with more subtle symptoms and imaging findings could have been missed.