Abstract
Background
Participating in sports is a great way to gain physical, psychological, and social benefits. However, it also carries the risk of injury. Soccer is one of the most popular sports worldwide, and in recent years, there have been concerns about potential vulnerabilities to head injuries.
Objectives
To investigate soccer-related head injuries (SRHIs), using data from the electronic Canadian Hospitals Injury Reporting and Prevention Program (eCHIRPP) surveillance system. Specifically, we aim to compare characteristics of SRHI cases to all head injury cases within the eCHIRPP database.
Methods
Descriptive analyses of emergency department (ED) injury surveillance data (2011 to 2017) for individuals aged 5 to 29 years from all participating eCHIRPP sites. Computation of proportionate injury ratios (PIR) comparing SRHIs to all head injuries reported to eCHIRPP, and 95% confidence intervals (CI).
Results
A total of 3,970 SRHIs were reported to eCHIRPP. Injuries were from contact with another player, the ball, ground, goal-post, and other causes. Of the injuries caused by contact with the ball, 9% were from purposely directing the ball with the head (heading). A higher proportion of concussions (PIR=1.32, 95% confidence interval [CI]: 1.27 to 1.37) and minor closed head injuries (PIR=1.20, 95% CI: 1.15 to 1.26) were observed in soccer players. Higher proportions of head injuries occurred in organized soccer and soccer played outdoors. However, admission to the ED for a SRHI was rare (PIR=0.40, 95% CI: 0.30 to 0.55).
Conclusions
Overall, elevated proportions of brain injuries were observed among soccer players, however, these injuries were unlikely to result in a hospital admission. Moreover, purposely heading the ball contributed to few ED visits.
Keywords: Brain injuries, Emergency departments, Soccer
Traumatic brain injury (TBI) is a growing global health concern and is among the leading causes of death and disability worldwide (1,2), with an estimated global incidence rate of 200 per 100,000 people per year (3). Sports or recreation are one major cause of TBI, however, the underreporting of injuries, limited recognition of milder injuries, and a lack of consensus definitions likely yields severe underestimations of the true incidence in national data sets (4–6). Concussions, often referred to as mild traumatic brain injuries (mTBI), are induced by biomechanical forces caused by a direct blow to the head or to the body with an impulsive force transmitted to the head, resulting in rapid onset of short-lived impairment of neurologic function that resolves spontaneously (7–10).
Recently, the Centre for Disease Control has declared that sport-related concussions have reached epidemic levels (8). In the USA, an estimated 1.6 to 3.8 million sport-related concussions occur annually (11,12). Researchers have investigated differences in sport-related concussions between age groups, sex, and type of sport. For example, younger persons were found to be at increased risk for sport-related TBIs, with a higher severity and prolonged recovery (13). Additionally, although up to 70% of all sport-related TBI emergency department (ED) visits are among males (4), new studies suggest that females are at a higher risk for concussions and prolonged recovery (14). Moreover, TBI-related ED visits are also more commonly associated with specific sports, including football, hockey, and soccer (4).
Soccer is one of the fastest growing and most popular sports around the world (8). According to the 2006 Fédération Internationale de Football Association (FIFA) ‘Big Count’ survey, there were approximately 265 million soccer players worldwide (15). In Canada, the highest proportion of soccer players is in the youth ages under 19 (16,17). Although participating in sports is an effective way to gain physical, social, and psychological benefits, the increased exposure to sports ultimately increases the risk for injury (18,19). Soccer players may be particularly vulnerable to head and neck injuries, not only due to the physical nature of the sport, but also because of the unique aspect of purposely using the head to direct the ball, otherwise known as ‘heading’ (8,20,21).
The specific aims of the current study were to first investigate soccer-related head injuries (SRHIs) reported in select EDs across Canada. Next, we aim to compare specific characteristics of SRHI cases to all head injury cases reported to the electronic Canadian Hospitals Injury Reporting and Prevention Program (eCHIRPP).
METHODS
Data source
Analyses were based on the data collected by eCHIRPP from April 2011 to July 2017 (total records searched: N=796,119) where data were available electronically. CHIRPP is an ED-based injury and poisoning sentinel surveillance system which was established in 1990, and recently went electronic (eCHIRPP) in 2011. CHIRPP is managed by the Public Health Agency of Canada (PHAC) and currently collects preinjury event information from EDs of 11 paediatric hospitals and 8 general hospitals across Canada. Upon visiting the ED of a participating CHIRPP hospital, patients are asked to complete a form regarding the circumstances of the injury/poisoning event. This form is later completed by hospital staff to provide additional information on the clinical features of the injury/poisoning.
Study variables
The primary outcome measure in this study was any SRHI reported by a patient aged 5 to 29 years and evaluated in EDs participating in eCHIRPP. We excluded children under the age of 5 (n=90), because the head injuries mainly occurred while spectating, or from playing with a soccer-ball inside a home. Furthermore, SRHIs in those over the age of 29 (n=51), were excluded due to the small number of reported cases in eCHIRPP. Variables of interest included age, gender, location (indoor/outdoor), the nature of injury, direct cause of injury, organized/informal sport, and the treatment received. Notably, eCHIRPP collects data on up to three different injuries per patient’s ED visit, whereby the attending physician or other hospital staff triage the injuries and record them in decreasing order of severity. This study examined the first listed injury only, as most soccer-related head injury cases only had one injury listed and in the case of multiple injuries the most serious/severe injury should have been listed first. There were 121,239 head injury cases sustained by individuals 5 to 29 years of age and reported to eCHIRPP within the study period. Head injury was defined as an injury to the scalp and skull; an injury to the face including eyelid and periocular area, ear, nose, external mouth, jaw, and facial bones; an injury to the internal mouth including dental injury; or a specified head injury including intracranial injury, concussion, minor closed head injury (MCHI), inner ear, eye injury, or foreign body in the eye, ear, or nose. Intracranial injuries are typically associated with serious structural damage. MCHI is a less specific code and captures undiagnosed concussions and events involving sub-concussive impacts. Finally, only head injuries recorded as ‘soccer-related’ were included in this study (N=3,970).
Statistical analysis
Descriptive statistics were generated to examine the distribution of selected SRHI characteristics, overall and stratified by gender. Frequency distributions (counts and percentages) were calculated for all variables. Proportionate injury ratios (PIRs) and 95% confidence intervals (CI) were computed to compare specific characteristics of SRHI cases (N=3,970) to all head injury cases (N=121,239). The PIR was calculated as a ratio of the observed number of SRHI cases for a given characteristic (i.e., concussion) to the expected number of cases based on age- and gender-specific proportions of that characteristic among all eCHIRPP head injury cases (22). Additional analyses were completed by the first author and included investigating the direct causes of SRHIs, the types of injuries resulting in hospital admission, as well as conducting a manual review (n=998) of the narratives describing head injuries caused by contact with the soccer-ball. Of the 1,047 head injuries caused by the ball, the cases lacking a narrative (n=41) and those which weren’t directly soccer-ball related (n=8) were excluded. The narratives of 998 cases were searched for descriptions of intentionally heading the ball (‘headed the ball’, ‘hit the ball with the head’) compared to unintentionally being hit by the ball (‘ball hit head or face’, ‘struck by the ball’).
RESULTS
Case characteristics
A total of 3,970 SRHI cases were reported to eCHIRPP between 2011 and 2017. Table 1 presents basic demographic and injury characteristics of cases, by gender, and overall. Males (n=2,475) sustained more SRHIs than females (n=1,495) in all age groups, except for 15 to 19 years of age. Most SRHIs occurred between the ages of 10 to 14 years (n=1,914). Within the 5 to 9 age group, males experienced almost six times more SRHIs than females. The top direct causes of SRHI were contact with another player (n=1,225), the ball (n=1,047), ground (n=1,027), and goal-post (n=175). Soccer resulted in 2,745 brain injuries, whereby concussions accounted for 40% of all SRHIs. Notably, seven soccer-related intracranial head injury cases were reported to eCHIRPP; all of which were reported by males. Direct causes of the intracranial cases were the ground (n=3), the goal-post (n=2), and another player (n=2). Among the 998 head injuries included in the manual review, 89 cases (9%) were from purposely heading the ball. Additionally, 63% of head injuries were associated with organized soccer and 68% occurred outdoors. Most SRHIs were treated in the ED with follow up as needed (n=1,592) or received advice only (n=1,369). Forty-two cases were admitted to a hospital, 2.5 times more males were admitted than females. The types of SRHI which resulted in hospital admission (n=42) included fractures (n=16), concussions (n=15), intracranial injuries (n=6), minor closed head injuries (n=3), open-wound and superficial injury (n=1), and dental injury (n=1). The direct causes of the hospital admissions (n=42) were another person (n=19), the floor/ground/unknown surface (n=12), the goal-post (n=4), the soccer ball (n=2), sports footwear (n=2), walls (n=1), and due to an unknown factor (n=2).
Table 1.
Distribution of soccer-related head injury (SRHI) characteristics in those aged 5–29 years reported to the electronic Canadian Hospitals Injury Reporting and Prevention Program (eCHIRPP) from across Canada, overall and by gender, 2011–2017
| Characteristics | Males | Females | All | |||
|---|---|---|---|---|---|---|
| n | (%) | n | (%) | N | (%) | |
| Age Group (years) | ||||||
| 5–9 | 718 | (29.01) | 127 | (8.49) | 845 | (21.28) |
| 10–14 | 1,161 | (46.91) | 753 | (50.37) | 1,914 | (48.21) |
| 15–19 | 529 | (21.37) | 580 | (38.80) | 1,109 | (27.93) |
| 20–29 | 67 | (2.71) | 35 | (2.34) | 102 | (2.57) |
| Nature of Injury | ||||||
| Brain Injury | 1,585 | (64.04) | 1160 | (77.59) | 2,745 | (69.14) |
| Minor closed head injury | 739 | (29.86) | 405 | (27.09) | 1,144 | (28.82) |
| Concussion | 839 | (33.90) | 755 | (50.50) | 1,594 | (40.15) |
| Intracranial injury | 7 | (0.28) | 0 | (0.00) | 7 | (0.18) |
| Head Injury | 890 | (35.96) | 335 | (22.41) | 1,225 | (30.86) |
| Eye Injury | 157 | (6.34) | 57 | (3.82) | 214 | (5.39) |
| Fractures | ||||||
| Facial | 88 | (3.56) | 44 | (2.94) | 132 | (3.32) |
| Skull | 8 | (0.32) | 3 | (0.20) | 11 | (0.28) |
| Open Wound Head Injury | 369 | (14.91) | 62 | (4.15) | 431 | (10.86) |
| Other Head Injuries | 268 | (10.83) | 169 | (11.30) | 437 | (11.01) |
| Direct Cause | ||||||
| Person | 778 | (31.43) | 447 | (29.90) | 1,225 | (30.86) |
| Ball | 509 | (20.57) | 538 | (35.99) | 1,047 | (26.37) |
| Ground | 666 | (26.91) | 361 | (24.15) | 1,027 | (25.87) |
| Goal-post | 127 | (5.13) | 48 | (3.21) | 175 | (4.41) |
| Other | 395 | (15.96) | 101 | (6.76) | 496 | (12.49) |
| Organized Sport | ||||||
| Yes | 1,336 | (53.98) | 1149 | (76.86) | 2,485 | (62.59) |
| No | 740 | (29.90) | 133 | (8.90) | 873 | (21.99) |
| Not Applicable/Unknown | 399 | (16.12) | 213 | (14.25) | 612 | (15.42) |
| Indoor/Outdoor | ||||||
| Indoor | 594 | (24.00) | 485 | (32.44) | 1,079 | (27.18) |
| Outdoor | 1,749 | (70.67) | 940 | (62.88) | 2,689 | (67.73) |
| Missing/Blank | 132 | (5.33) | 70 | (4.68) | 202 | (5.09) |
| Treatment of Injury | ||||||
| Left without being seen by physician | 4 | (0.16) | 4 | (0.27) | 8 | (0.20) |
| Admitted due for reason other than injury treatment | 2 | (0.08) | 0 | (0.00) | 2 | (0.05) |
| Advice, diagnostic testing, referred to GP (no treatment in ED) | 838 | (33.86) | 531 | (35.52) | 1,369 | (34.48) |
| Treated in ED, follow-up PRN | 1,026 | (41.45) | 566 | (37.86) | 1,592 | (40.10) |
| Observation in ED, follow-up PRN | 220 | (8.89) | 144 | (9.63) | 364 | (9.17) |
| Observation in ED, follow-up required | 59 | (2.38) | 51 | (3.41) | 110 | (2.77) |
| Treated in ED, follow-up required, referred to other hospital or clinic | 296 | (11.96) | 187 | (12.51) | 483 | (12.17) |
| Admitted to this or another hospital | 30 | (1.21) | 12 | (0.80) | 42 | (1.06) |
| Total | 2,475 | (100.00) | 1,495 | (100.00) | 3,970 | (100.00) |
ED Emergency department; GP General physician; PRN Pro re nata (as needed).
Comparisons with all head injury cases in eCHIRPP
Table 2 presents PIRs comparing selected characteristics of SRHI cases to all head injury cases reported to eCHIRPP. Females experienced slightly elevated PIRs for SRHIs (PIR=1.05, 95% CI: 1.01 to 1.09). Significantly elevated proportions of overall concussions (PIR=1.32, 95% CI: 1.27 to 1.37) and MCHI (PIR=1.20, 95% CI: 1.15 to 1.26) were observed among soccer players. Females showed the highest PIR for concussions (PIR=1.38, 95% CI: 1.31 to 1.45). The PIR for eye injury was also elevated, but these injuries were considered to be less severe. The PIR for those competing in organized soccer was significantly higher (PIR=2.20, 95% CI: 2.15 to 2.25). This was true in males (PIR=2.00, 95% CI: 1.93 to 2.07), but even more so for females (PIR=2.52, 95% CI: 2.45 to 2.59). Notably, the PIR for males but not females, playing informal soccer was still significantly elevated (PIR=1.49, 95% CI: 1.41 to 1.58). The proportion of SRHIs sustained outdoors (PIR=1.45, 95% CI: 1.41 to 1.48) was also significantly elevated.
Table 2.
Age- and gender-adjusted proportionate injury ratios (PIR) and confidence intervals (95% CI) for soccer-related head injury (SRHI) cases compared to all head injury cases reported to eCHIRPP, overall and by gender, 2011–2017
| Characteristics | Males | Females | All | |||
|---|---|---|---|---|---|---|
| PIR | (95% CI) | PIR | (95% CI) |
PIR | (95% CI) |
|
| Gender | ||||||
| Male | NAa | NAa | 0.97 | (0.95, 0.99) | ||
| Female | NAb | NAb | 1.05 | (1.01, 1.09) | ||
| Nature of Injury | ||||||
| Brain Injury | ||||||
| Minor closed head injury | 1.29 | (1.22, 1.37) | 1.03 | (0.95, 1.12) | 1.20 | (1.15, 1.26) |
| Concussion | 1.25 | (1.18, 1.32) | 1.38 | (1.31, 1.45) | 1.32 | (1.27, 1.37) |
| Intracranial injury | 0.34 | (0.16, 0.71) | 0.00 | (NA)c | NAc | (NA)c |
| Head Injury | ||||||
| Eye Injury | 1.31 | (1.13, 1.53) | 0.91 | (0.71, 1.18) | 1.17 | (1.03, 1.34) |
| Fractures | ||||||
| Facial | 1.16 | (0.95, 1.43) | 1.13 | (0.84, 1.50) | 1.10 | (0.94, 1.30) |
| Skull | 0.67 | (0.34, 1.35) | 0.55 | (0.18, 1.71) | 0.63 | (0.35, 1.13) |
| Open Wound Head Injury | 0.59 | (0.54, 0.65) | 0.32 | (0.25, 0.41) | 0.52 | (0.48, 0.57) |
| Other Head Injuries | 0.71 | (0.63, 0.79) | 1.21 | (1.09, 1.34) | 0.70 | (0.64, 0.76) |
| Organized Sport | ||||||
| Yes | 2.00 | (1.93, 2.07) | 2.52 | (2.45, 2.59) | 2.20 | (2.15, 2.25) |
| No | 1.49 | (1.41, 1.58) | 0.64 | (0.55, 0.75) | 1.23 | (1.16, 1.30) |
| Not Applicable/Unknown/Blank | 0.30 | (0.28, 0.33) | 0.26 | (0.23, 0.29) | 0.29 | (0.27, 0.31) |
| Indoor/Outdoor | ||||||
| Indoor | 0.54 | (0.51, 0.58) | 0.65 | (0.61, 0.70) | 0.59 | (0.56, 0.62) |
| Outdoor | 1.45 | (1.42, 1.49) | 1.46 | (1.40, 1.51) | 1.45 | (1.41, 1.48) |
| Missing/Blank | 0.76 | (0.64, 0.89) | 0.66 | (0.52, 0.83) | 0.72 | (0.63, 0.82) |
| Treatment of Injury | ||||||
| Left without being seen by physician | 0.64 | (0.24, 1.69) | 1.11 | (0.42, 2.94) | 0.80 | (0.40, 1.59) |
| Admitted due for reason other than injury treatment | 1.51 | (0.38, 6.03) | 0.00 | (NA)d | NAd | (NA)d |
| Advice, diagnostic testing, referred to GP (no treatment in ED) | 1.18 | (1.12, 1.25) | 1.10 | (1.03, 1.18) | 1.16 | (1.11, 1.21) |
| Treated in ED, follow-up PRN | 0.93 | (0.89, 0.98) | 0.92 | (0.86, 0.98) | 0.93 | (0.89, 0.97) |
| Observation in ED, follow-up PRN | 1.20 | (1.06, 1.36) | 1.11 | (0.95, 1.30) | 1.17 | (1.06, 1.29) |
| Observation in ED, follow-up required | 1.03 | (0.80, 1.33) | 1.06 | (0.81, 1.40) | 1.05 | (0.88, 1.27) |
| Treated in ED, follow-up required, referred to other hospital or clinic | 0.85 | (0.77, 0.95) | 1.03 | (0.90, 1.18) | 0.90 | (0.83, 0.98) |
| Admitted to this or another hospital | 0.44 | (0.31, 0.63) | 0.36 | (0.21, 0.64) | 0.40 | (0.30, 0.55) |
Counts of all head injury reported to eCHIRPP: Male 78,035; Female 43,204; All 121,239.
Counts of soccer-related head injury reported to eCHIRPP: Male 2,475; Female 1,495; All 3,970.
ED Emergency department; GP General physician; NA Not applicable; PRN Pro re nata (as needed).
aPIR calculated in the Total column only as both males and females are included in the total and can be compared one relative to another.
bPIR calculated in the Total column only as both males and females are included in the total and can be compared one relative to another.
cThere were no female intracranial head injury cases.
dThere were no female cases which were admitted for reason other than injury treatment.
The proportion of SRHIs given no treatment in the ED (PIR=1.16, 95% CI: 1.11 to 1.21) and those observed in the ED with follow up as needed (PIR=1.17, 95% CI: 1.06 to 1.29) were significantly elevated. Notably, the proportion of SRHI cases being admitted to a hospital was significantly lower (PIR=0.40, 95% CI: 0.30 to 0.55).
DISCUSSION
The participation in almost any sport comes with the risk of injury. Unlike other sports, soccer bears the unique aspect of heading, which has been suggested to increase the associated risk of head injuries (23). In this study, we described the characteristics of SRHIs of children and youth 5 to 29 years of age, presenting to select EDs across Canada. Based on the analysis of eCHIRPP (2011 to 2017), our findings showed higher proportions of MCHI and concussions among soccer players. After stratifying by sex, male soccer players were found to have a significantly elevated PIR for MCHI (PIR=1.29, 95% CI: 1.22 to 1.37) and female soccer players showed a significantly elevated PIR for concussion (PIR=1.38, 95% CI: 1.31 to 1.45). Female soccer players may be disproportionately affected by concussion due to various anthropometric, biomechanical, and physiological factors such as smaller head mass and neck girth, weaker neck strength, and higher head acceleration upon impact (24–29). However, it is also possible that female athletes are more likely to report injuries compared to male athletes, whereby such reporting bias may result in a greater proportion of male concussions going undiagnosed (25).
We found higher proportions of head injuries occurring in organized, compared to informal, soccer. One would suspect that organized sport is safer due to the availability of equipment, the presence of rules or referees, or more skilled players (30). However, organized youth sports today seem to encompass more specialized and intensive training, higher levels of competitiveness at younger ages and being more serious or organized, compared to informal sport (31,32). Within the organized sporting culture, experiencing and playing through injury and pain seems to be a central principle (31).
Soccer players may head the ball hundreds of times throughout their career, thus, researchers have paid particular attention to the effects of heading the soccer ball, and many controversies exist regarding regulations around heading in soccer. Some researchers suggest that heading should be banned from the game, as although cognitive symptoms might not be induced each time (8), recent studies suggest that even sub-concussive impacts are associated with alterations in cognitive functioning, similar to the effects of concussive impacts (33,34). Such impacts may be of particular concern to young athletes due to the potential for adverse effects on developing brains (35,36). On the other hand, our manual review examining the SRHI cases caused by contact with the soccer-ball suggested that most patients were struck by the ball unexpectedly. The risk of concussion is a direct function of the maximal linear and/or rotational acceleration to the head (37), whereby this risk may be higher when a player cannot anticipate or brace themselves for the impact through pre-activating the neck musculature or evasive movements (36). Head and neck size have been suggested to predict peak linear and rotational accelerations of the head (38), whereby soccer players with lower neck strength and smaller head–neck segment masses may be at higher risk for greater head acceleration and concussion (39,40). Pickett and colleagues (2005) found similar results, in which few minor head injuries resulting in emergency medical treatment could be attributed to the purposeful heading of the ball (41). This finding can serve to inform areas of focus for prevention strategies, as a small proportion of our sample reported heading as the mechanism of SRHI requiring ED care.
Prevention efforts focusing on the other common causes of SRHI, including contact with another player, the ground or the goal post, might more effectively prevent SRHIs requiring ED care. Specifically, in line with our findings, contact with another player has generally been documented to cause most SRHIs (42,43). Better enforcement of existing rules or the addition of rule changes could be implemented to reduce player–player contact (43). For example, the implementation of more red cards (sent off the field) for intentional player–player contact may deter players from engaging in aggressive play (44). The addition of protective padding surrounding the structures on the playing field such as the goal post could also be explored. Mixed results exist with regards to soccer protective headgear (45,46). Further studies are needed in this area to assess the effectiveness of such prevention strategies.
Strengths and limitations
This study is important because it represents one of the few large-scale Canadian studies describing SRHIs requiring ED care. eCHIRPP provides unique strengths to this study, such as the broad surveillance coverage across Canada. Its sentinel surveillance monitoring allows for connections to 18 hospitals Canada-wide. eCHIRPP also provides narratives, which allows for further insight into the context behind the data. Nonetheless, this study carried some limitations. First, it was not population based, thus, we could not assess head injury burden, or determine rates for the entire population of Canada. We also lacked the general exposure data describing the number of individuals participating in informal or formal soccer by age. To overcome this limitation, we calculated PIRs which compared the proportion of head injuries sustained while playing soccer to the proportion of all head injuries, reported to eCHIRPP. Second, more severe head injuries might not have been captured by eCHIRPP, as these cases may have been admitted directly to the intensive care unit, hence bypassing the ED. Additionally, the true number of individuals who do not seek medical care is unknown (11). Furthermore, we investigated the first (and likely most severe) injury per patient’s ED visit. A total of 97 cases with a brain injury listed in the second or third field were identified, however, distributed over the age groups and gender, the results were not significantly affected by the exclusion of these cases. Future research could include up to three reported injuries per patient in the analyses. Additionally, although most head injury cases did not result in an admission to the ED, the number of cases which may have had a follow-up at another point of care for postconcussion syndrome is unknown. Despite potential underestimations in head injury counts, significant findings persisted, lending to the importance of the SRHI issue.
CONCLUSION
Overall, elevated proportions of concussions and MCHI were observed among soccer players. Although a very low proportion of SRHI cases were admitted to the hospital and purposely heading the ball contributed to few hospital visits, the long-term effects of cumulative sub-concussive events are unknown. Implications from this study include firstly, the importance of maintaining efforts to increase awareness and education on safe play, the signs/symptoms of concussions, and the risks of sub-concussive and concussive impacts on cognitive functioning. Additionally, soccer coaches could teach alternative techniques to control the ball, rather than using the head, as well as proper heading techniques when appropriate. General neck strengthening exercises could be incorporated into training. Finally, better enforcement of existing rules or the addition of rule changes, as well as increasing proprioceptive awareness and balance in young players through various training, might decrease the likelihood of player-to-player contact and falls. These strategies might be useful in reducing the incidence of SRHIs and ultimately help individuals to grasp the positive health benefits associated with participating in the sport.
Funding: There are no funders to report for this submission.
Potential Conflicts of Interest: All authors: No reported conflicts of interest. All authors have submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.
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