© 2015 John Wiley & Sons A/S.
Scand J Med Sci Sports 2015: ••: ••–•• doi: 10.1111/sms.12519
Published by John Wiley & Sons Ltd
Back injuries in a cohort of schoolchildren aged 6–12: A 2.5-year prospective study C. Franz1, E. Jespersen1, C. T. Rexen1, C. Leboeuf-Yde2,3, N. Wedderkopp1,3,4 1
Research in Childhood Health, Department of Sports Science and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark, 2Research Department, Spine Center of Southern Denmark, Hospital Lillebaelt, Middelfart, Denmark, 3Institute of Regional Health Services Research, University of Southern Denmark, Odense, Denmark, 4The Sport Medicine Clinic, Orthopaedic Department, Hospital of Lillebaelt, Middelfart, Denmark Corresponding author: Claudia Franz, Research in Childhood Health, Department of Sports Science and Clinical Biomechanics, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark. Tel: +45 65504478, Fax: +45 65503480, E-mail: [email protected] Accepted for publication 2 June 2015
The aims of this prospective school cohort study were to describe the epidemiology of diagnosed back pain in childhood, classified as either nontraumatic or traumatic back injury, and to estimate the association with physical activity in different settings. Over 2.5 years, 1240 children aged 6–12 years were surveyed weekly using mobile text messages to ask about the presence or absence of back pain. Pain was clinically diagnosed and injuries were classified using the International Classification of Diseases version 10. Physical activity data were obtained from text messages and accelerometers. Of the 315 back injuries diagnosed, 186 injuries were nontraumatic and 129 were traumatic. The incidence rate ratio was 1.5 for a
nontraumatic back injury compared with a traumatic injury. The overall estimated back injury incidence rate was 0.20 per 1000 physical activity units (95% confidence interval 0.18–0.23). The back injury incidence rates were higher for sports when exposure per 1000 physical activity units was taken into consideration and especially children horse-riding had a 40 times higher risk of sustaining a traumatic back injury compared to the risk during non-organized leisure time physical activity. However, the reasonably low injury incidence rates support the recommendations of children continuously being physically active.
Back pain (BP) is a commonly reported disabling condition in the general population (Gabbay et al., 2011). It can occur already in childhood (Balague et al., 1988; Troussier et al., 1994) and its frequency increases with age, especially from around time of puberty (Balague et al., 1988; Leboeuf-Yde & Kyvik, 1998). As low back pain (LBP) in childhood is a risk factor for LBP in adulthood (Hestbaek et al., 2006), it is important to try to prevent early onset of BP. Factors commonly thought to be responsible for LBP in early life include inactivity (Balague et al., 1988; Troussier et al., 1994; Skoffer & Foldspang, 2008), low levels of physical activity (PA; Salminen et al., 1993), or excessive PA (Newcomer & Sinaki, 1996). Other studies found no association (Taimela et al., 1997) or mixed findings (Burton et al., 1996). However, these studies were based on self-reported PA, which might explain the differing conclusions. When PA was objectively measured with an accelerometer in a cross-sectional study, the agreement between self-reported levels of activity and objectively measured levels was poor and there was no association between level of PA and reported BP (Wedderkopp et al., 2003). However, a prospective
cohort study suggested that high overall PA levels protect against future BP (Wedderkopp et al., 2009). Reports on the influence of sports participation on childhood BP have also varied, from being injurious (Elliott et al., 1993) to BP being more common in those who avoided sports (Fairbank et al., 1984) – as well as an absence of association between participation in sport activities in general and self-reported LBP (Mogensen et al., 2007; Skoffer & Foldspang, 2008). Previous studies have reported both positive (Mogensen et al., 2007; Skoffer & Foldspang, 2008; Sato et al., 2011) and negative correlations (Mogensen et al., 2007; Auvinen et al., 2008; Skoffer & Foldspang, 2008) between BP and specific sport types. Only a few studies have investigated the risk of PA-related injuries associated with the entire scope of PA modalities in school-based cohorts (Spinks et al., 2006; Verhagen et al., 2009; Jespersen et al., 2015), and Jespersen et al. only reported injuries in the extremities. The aims of this prospective school cohort study were to describe the epidemiology of diagnosed BP in childhood, classified as either nontraumatic or traumatic back injury, and to estimate the association with physical
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Franz et al. activity in different settings including school physical education, organized sports activities, and non-organized leisure time PA.
Traumatic back injuries were further classified as occurring during school physical education (PE), organized sports activities, or non-organized leisure time PA.
Methods
Physical education
Study design
The weekly amount of PE was 4.5 h for sports schools and 1.5 h for normal schools, corresponding to three and one double lesson per week, respectively. Children at sports and normal schools were therefore “assigned” three and one sport exposure unit per week, respectively. Nonattendance was not recorded.
Data from the Childhood Health, Activity, and Motor Performance School Study Denmark (CHAMPS Study–DK) between August 2008 and July 2011 were used. The CHAMPS study was a large, prospective, controlled, school-based study in Denmark in the form of a natural experiment (Craig et al., 2012). Six schools were assigned to become sport schools with six physical education lessons per week, and four normal schools served as controls with two physical education lessons per week. The project is described in detail elsewhere (Wedderkopp et al., 2012). The regional ethics committee approved the project (ID S20080047), which was registered with the Danish Data Protection Agency (J.nr. 2008-41-2240). Written informed consent was obtained from parents, and the child gave verbal acceptance prior to each clinical examination. All participation was voluntary with the option to withdraw at any time.
Participants and data collection All children from preschool (grade 0) up to fourth grade in 10 state schools participating in the CHAMPS Study-DK were invited to participate in the weekly registration of BP and back injuries. Weekly information on BP (neck, mid-back, and/or lower back) was collected using text messages (SMS-track) for 2.5 school years. Each Sunday, parents answered a text message that asked questions on the presence or absence of BP during the previous week. Parents who reported that their child had BP in the previous week were contacted at the beginning of the subsequent week by one of four clinicians. If the pain was not musculo-skeletal, had disappeared, or was explained by the previous medical history, there was no further personal contact before the next pain reporting (if any). If the pain was still present and unexplained, a clinical examination was scheduled. Physiotherapists, chiropractors, and a medical practitioner were responsible for the clinical examination of the children within the next fortnight. If necessary, the child was referred for further examination by a medical specialist, and paraclinical investigations (such as X-ray, ultrasound or magnetic resonance imaging) were performed if relevant. Information on children seen or treated elsewhere (e.g., emergency department, general practitioner, medical specialist) during the study period was collected concurrently to obtain a complete picture of the clinical course. Data collection was suspended during the summer holidays.
Classification of BP into nontraumatic and traumatic injuries BP can have a traumatic or nontraumatic onset (Silverstein et al., 2002). However, the term “injury” or “sports injury” is often used as a collective name for all types of damage that can occur to the musculo-skeletal system in relation to sporting activities (van Mechelen et al., 1992). We defined a traumatic back injury as an injury resulting from a specific, identifiable event (Fuller et al., 2006). BP not fulfilling the criteria for a traumatic back injury was considered nontraumatic. Injuries were classified into these two categories using the medical records and the International Classification of Diseases version 10 (ICD-10; World Health Organization, 1992). S-codes were used to classify traumatic back injuries and M-codes were used for nontraumatic back injuries.
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Sports Organized sports activities were assessed using automated weekly text messages (SMS-track). The first question addressed the number of times the child had participated in sports during the previous week: “How many times has [NAME OF CHILD] participated in organised sports activities in the past week? 0 = 0, 1 = 1, 2 = 2, 3 = 3, 4 = 4, 5 = 5, 6 = 6, 7 = 7, 8 = more than 7 times.” If the response was 1 or more, a second question was sent: “Which type/s of sport/s: 1 = soccer, 2 = handball, 3 = basketball, 4 = volleyball, 5 = rhythmic gymnastics, 6 = tumbling gymnastics, 7 = swimming, 8 = horse-riding, 9 = dancing, 10 = other sports” (these being the most common sports among Danish children). The parents were instructed to type the relevant number(s).
Leisure time physical activity Besides PE lessons and organized sport activities, the children were also physically active in leisure time, hereafter named nonorganized leisure time PA. Non-organized leisure time PA thus comprised all daily activity of the child including transport, recess, other school activities, free play etc., except for PE lessons and organized sport activities. Data from accelerometer measurements were used as a proxy measure to estimate the amount of exposure to non-organized leisure time PA. Accelerometer measurements were assessed with the GT3X Actigraph accelerometer (Pensacola, Florida, USA). The children were instructed to wear the device for a full week from the time they woke up in the morning until bedtime to assess their entire PA for each day. The only exception was to remove the monitor when showering or swimming to prevent damage to the device. The assessments were performed from November 2009 to January 2010 in the 10 schools, when the children were in 1st–5th grade.
Statistical analysis A customized software program (Propero, version 1.0.18, University of Southern Denmark, Odense, Denmark) was used to process accelerometer data using information on PA at 10-s intervals. To distinguish between recorded “true intervals of inactivity” and “false intervals of inactivity,” consecutive strings of zeros of 30 min or longer were interpreted as “accelerometer nonworn.” Activity data were included for further analysis if the child had accumulated at least 10 h of activity per day for at least 4 days. Cut-off points for four activity intensity levels (sedentary, light, moderate, and vigorous) were determined according to Evenson et al. (2008). Time spent in moderate and vigorous activity was regarded as the “time at risk of back-injury.” Data were used at a sample level and a mean estimate of the total number of exposure units was extrapolated to cover the total study period. An exposure unit was chosen to equal 30 min, and two double lessons of PE and two sessions of sport participation were each equal to two units of exposure. Exposure time for the amount of leisure time PA was
Back injuries in schoolchildren thus the total number of exposure units (as estimated from accelerometer measurements) with subtraction of the number of exposure units in PE and sports. The data on diagnosed nontraumatic and traumatic back injuries were analyzed using STATA 13.0 (StataCorp, College Station, Texas, USA). To allow the study participants to become familiar with the novel data collection method, the first 9 weeks were considered a run-in period and these data were not included in the analyses. Calculation of injury incidence rates was the total sum of back injuries across all participating children divided by the sum of exposure across all participating children expressed in 1000 PA units. These comprised the PE, the sports and the non-organized leisure time PA exposures. The test for proportions and rates was used to test for differences between injury incidence rates. The significance level was set to P < 0.05. Injury incidence rates for traumatic back injuries were presented according to the average time in each specific setting of 10 types of sports, PE and non-organized leisure time PA for the whole sample. Average time spent being physically active across all settings and across the whole sample was used to estimate injury incidence rates for both nontraumatic and traumatic back injuries according to spinal region. Potential patterns for the missing values were addressed by logistic regression analysis controlling for effects of age, sex, school type, and sports. Missing values because of changed or incorrect mobile telephone numbers were excluded from the analyses.
Results Schools, SMS-track, and accelerometers Inclusion into the study was gradual, starting with 231 children from three schools, and after 8 months, children from all 10 schools had been included. A total of 1240 children participated in the study. The length of participation was 1–109 weeks with an average of 94.3 weeks. Dropouts were due to children moving away from the municipality or changing to nonproject schools and were counterbalanced by new children moving into project schools because of normal demographic mobility and a flow chart of the participating children in the CHAMPS-Study DK was previously described (Wedderkopp et al., 2012). Fifteen children
dropped out for other reasons, mainly because answering weekly text messages was considered too bothersome. A mean weekly response rate of 96.5% was recorded for pain reporting and 96.4% for sports participation. On average, 3.5% of 1240 children had missing values each week. Analysis of missing data showed no patterns according to age, sex, school type, or sports. 87% (1083 children) attained valid accelerometer recording. Diagnosed BP classified as nontraumatic or traumatic back injury During the study period, 18% (218 children) experienced a total of 315 back injuries. Of these, 186 injuries were nontraumatic and 129 traumatic, with an incidence rate ratio of 1.5 for a nontraumatic back injury compared with a traumatic injury (Table 1). The overall back injury incidence rate was 0.20 per 1000 PA units (95% CI 0.18 to 0.23). This would correspond to a child sustaining a back injury every 6.8 years, when the mean number of hours in moderate to vigorous activity per week was calculated. The cervical spine was the most common site for nontraumatic injuries, while the thoracic spine was the most common site for traumatic back injuries, although not significant compared to the cervical spine (Table 1). “Cervicalgia” was the most common ICD-10 diagnosis for nontraumatic back injuries. Common diagnoses for traumatic back injuries were “contusion of the back wall of thorax” and “sprain of the ligaments in the thoracic spine” (Table 2). Injury incidence rates of back injuries by sex, school type, and setting Girls had a significantly higher risk of sustaining a nontraumatic or traumatic back injury and thus also an overall higher risk of back injury compared to boys (Table 3). The number of back injuries was highest in sports schools (Table 4). When exposure per 1000 PA units was
Table 1. Number of back injuries and incidence rates in 1240 schoolchildren by injury type and spinal region
Cervical Thoracic Lumbar Cervico/thoracic Cervico/lumbar Cervical/thoracic/lumbar Thoraco/lumbar Total
Diagnosed nontraumatic back injuries
Diagnosed traumatic back injuries
Total back injuries
n
IR (95% CI)
n
IR (95% CI)
n
IR (95% CI)
71 29 40 4 0 6 36 186
0.05*1 (0.04–0.06) 0.02 (0.01–0.03) 0.03 (0.02–0.03) 0.01 (0.00–0.01) 0.00 0.00 (0.00–0.01) 0.02 (0.02–0.03) 0.12 (0.10–0.14)
31 41 26 10 4 3 14 129
0.02 (0.01–0.03) 0.03 (0.02–0.03) 0.02 (0.01–0.02) 0.01 (0.00–0.01) 0.00 (0.00–0.01) 0.00 (0.00-0.00) 0.01 (0.00–0.01) 0.08 (0.07–0.10)
102 70 66 14 4 9 50 315
0.07*2 (0.05–0.08) 0.05 (0.03–0.06) 0.04 (0.03–0.05) 0.01 (0.00–0.01) 0.00 (0.00–0.01) 0.01 (0.00–0.01) 0.03 (0.02–0.04) 0.20 (0.18–0.23)
IRs are per 1000 physical activity units; values in parenthesis are 95% CI. Physical activity units = 1 545 848. *1Significant difference of a nontraumatic injury in the cervical spine compared with the thoracic (P < 0.0001) or the lumbar spine (P = 0.003). *2Significant difference of an overall injury in the cervical compared with an injury in the lumbar spine (P = 0.0055). CI, confidence interval; IR, incidence rate; n, number.
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Franz et al. Table 2. Number of diagnosed back injuries and ICD-10 codes in 1240 schoolchildren
Number of nontraumatic injuries
ICD-10 codes
Number of traumatic injuries
Cervical spine
76
Cervicalgia Torticollis
39
Thoracic spine
52
54
Lumbo-/pelvic spine
58
Thoracic spinal pain Intervertebral disc degeneration Juvenile idiopathic scoliosis Syringomyelia Lumbago Intervertebral disc degeneration Schmorl’s nodes Spondylolysis Juvenile idiopathic scoliosis Limbus vertebra
Total
186
ICD-10 codes Sprain Facet syndrome Traumatic rupture of intervertebral disc Sprain Contusion Facet syndrome Fracture of vertebra Sprain Contusion Facet syndrome Fracture of vertebra Traumatic rupture of intervertebral disc
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129
ICD-10, International Classification of Diseases version 10. Table 3. Number of back injuries and incidence rates in 1240 schoolchildren by injury type and sex
Girls (n = 654) Boys (n = 586) Total
Nontraumatic back injuries (M-diagnosis)
Traumatic back injuries (S-diagnosis)
Total back injuries
n
IR (95% CI)
n
IR (95% CI)
n
IR (95% CI)
121 65 186
0.16*1 (0.13–0.19) 0.08 (0.06–0.10) 0.12 (0.10–0.14)
81 48 129
0.11*2 (0.09–0.13) 0.06 (0.04–0.08) 0.08 (0.07–0.10)
202 113 315
0.27*3 (0.24–0.31) 0.14 (0.11–0.17) 0.20 (0.18–0.23)
IRs are per 1000 physical activity units; values in parenthesis are 95% CI. Physical activity units girls = 738 894, physical activity units boys = 806 954. *1Significant difference (P < 0.0001) of girls sustaining a nontraumatic back injury compared with boys. *2Significant difference (P = 0.0007) of girls sustaining a traumatic back injury compared with boys. *3Significant difference (P < 0.0001) of girls sustaining an overall back injury compared with boys. CI, confidence interval; IR, incidence rate; n, number. Table 4. Number of back injuries and incidence rates in 1240 schoolchildren by injury and school type
Sports schools (n = 728) Normal schools (n = 512) Total (n = 1240)
Nontraumatic back injuries (M-diagnosis)
Traumatic back injuries (S-diagnosis)
Total back injuries
n
IR (95% CI)
n
IR (95% CI)
n
IR (95% CI)
122 64 186
0.14 (0.11–0.16) 0.10 (0.07–0.12) 0.12 (0.10–0.14)
74 55 129
0.08 (0.06–0.10) 0.08 (0.06–0.11) 0.08 (0.07–0.10)
196 119 315
0.22 (0.19–0.25) 0.18 (0.15–0.22) 0.20 (0.18–0.23)
IRs are per 1000 physical activity units; values in parenthesis are 95% CI. Physical activity units sports schools = 897 130, physical activity units normal schools = 648 718. CI, confidence interval; IR, incidence rate; n, number.
taken into consideration, no significant difference in the risk of sustaining a nontraumatic or traumatic back injury was found between school types (Table 4). The number of traumatic back injuries was highest in leisure time PA (Table 5). However, when exposure per 1000 PA units was taken into consideration, the back injury incidence rates were highest in sports, for example, rates of 2.45 (95% CI 1.28–3.61) in horseriding and 1.25 (95% CI 0.48–2.03) in tumbling gymnastics. Children who participated in horse-riding had about a 40 times higher risk of sustaining a traumatic
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back injury compared with the risk during non-organized leisure time PA and the most common cause of back injury was fall from horse. Discussion This is the first longitudinal study to report both nontraumatic and traumatic back injuries in children using a “momentary data assembly technique” (the SMS-track tool) to capture all symptoms indicative of BP in children and supplemented by clinical diagnosis.
Back injuries in schoolchildren Table 5. Number of exposure units, traumatic back injuries, and incidence rates in different settings
Sports Horse-riding Basketball Tumbling gymnastics Handball Swimming Soccer Other sports Rhythmic gymnastics Dance Volleyball Total sports PE lessons Leisure time PA (non-organized) Total
Physical activity units PA units
Number of traumatic back injuries n
Incidence rate per 1000 physical activity units IR (95% CI)
6950 1267 7974 16 301 13 220 25 222 14 855 4838 3374 1892 95 893 239 301 1 210 654 1 545 848
17 2 10 5 4 5 3 0 0 0 46 13 70 129
2.45 (1.28–3.61) 1.58 (0.00–3.77) 1.25 (0.48–2.03) 0.31 (0.04–0.58) 0.30 (0.01–0.60) 0.20 (0.02–0.37) 0.20 (0.00–0.43) 0 0 0 0.48 (0.34–0.62) 0.05 (0.02–0.08) 0.06 (0.04–0.07) 0.08 (0.07–0.10)
CI, confidence interval; IR, incidence rate; n, number.
When exposure per 1000 physical activity units was taken into consideration, children were at greatest risk of sustaining a traumatic back injury while doing sports, especially horse-riding and tumbling gymnastics. Children who participated in horse-riding had a 40 times higher risk of sustaining a traumatic back injury compared with the risk during non-organized leisure time PA. The total time-at-risk was associated with an overall injury incidence rate of 0.20 injuries per 1000 PA units (95% CI 0.18–0.23). Based on the mean number of hours in moderate to vigorous activity per week, this would correspond to a child sustaining a back injury every 6.8 years. As in most descriptive studies, direct comparisons of incidence figures are hampered by differences in the injury definition and data collection methods. Previous exposure-based injury rates in children mostly focus on sports-specific injury risk (Spinks & McClure, 2007). However, three somewhat comparable cohort studies of schoolchildren were identified. A Swiss study reported an injury incidence rate of 0.43/1000 h (Martin-Diener et al., 2013), which was comparable with Dutch rates of 0.48 injuries/1000 h (Verhagen et al., 2009) and Australian rates of 0.59 injuries/1000 h, respectively (Spinks et al., 2006). All three studies included all musculoskeletal locations, which could partially explain the higher rates compared to our overall back injury incidence rate of 0.20/1000 PA units. The lower incidence rate in the Swiss study compared with the other two studies might be due to underestimation of injuries as the recall frame was 12 months. Even higher rates in the Dutch and Australian studies would probably have been reported, if the focus had not mainly been on traumatic injuries. No studies were found reporting injury incidence rates of spinal regional injuries/1000 h in younger children not participating in elite sport.
Injury incidence rates of back injuries by sex and school type In contrast to what is previously reported in the literature (Spinks et al., 2006), girls were found to be at significantly higher risk of a back injury 0.27 (95% CI 0.24 to 0.31) compared with boys 0.14 (95% CI 0.11 to 0.17). Girls also had the highest total injury incidence rates of 0.58 in the Swiss (Martin-Diener et al., 2013) and 0.60 in Dutch (Verhagen et al., 2009) school studies, as compared with boys 0.32 in the Swiss (Martin-Diener et al., 2013) and 0.37 in Dutch (Verhagen et al., 2009) studies, respectively. One reason for the overall higher injury incidence rates in these studies compared to our study could be the inclusion of all musculoskeletal locations. Contributions to some of these sex differences might be that girls are more likely to express distress in response to pain and injury compared with boys (Fearon et al., 1996) or maybe specific hormonal and biochemical mechanisms (Stanford et al., 2008) such as growth spurt appearing earlier in girls might explain the higher risk. Even though the number of back injuries was highest in sports schools, children in sports schools did not have a significantly higher risk of sustaining a nontraumatic or traumatic back injury compared with children in normal schools, when exposure per 1000 PA units was taken into consideration. No comparable studies were found. Injury incidence rates of traumatic back injuries in different settings The number of traumatic back injuries was highest in non-organized leisure time PA, similar to findings from a Swedish study (Sundblad et al., 2005). However, when exposure time was taken into account, the highest incidence rate of back injuries was in sports. The same injury location pattern was also seen in a Danish cohort study (Jespersen et al., 2015), where the highest injury
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Franz et al. incidence rate of 1.57 was during sports participation. Contributions to the higher rates could be the inclusion of musculoskeletal injuries in both the upper and lower extremities and also their exposure unit was equal to 60 min as opposed to our unit of 30 min in moderate to vigorous physical activity (MVPA). An explanation for the increased risk of injury in sports could be the higher level of competition, which requires more complex motor skills and thus development of individual strategies for training and performance. Another reason might be that many Danish sports clubs rely on voluntary trainers, who may be less experienced or less qualified for training than professional instructors. A different injury location pattern within the different modalities of PA was seen in the Dutch study (Verhagen et al., 2009), where the least common setting for back injuries was organized sports and the most common were school physical education lessons and leisure time activity. Methodological differences may explain this finding, as the school physical education teachers in the Dutch study provided the children with injury registration forms and the children themselves filled in questionnaires at baseline and at the end of the school year regarding individual exposure to sports and leisure time activity. For sports, the risk for sustaining a back injury was highest for horse-riding, which is in agreement with a Swedish school study (Sundblad et al., 2005). Previous research has also shown an injury peak for riders aged 10–14 years and, consistent with our study, falls were the most common mechanism of injury (Thomas et al., 2006). Other comparable injury rates for horse-riding in primary schoolchildren are difficult to find because previous studies include wider age ranges and mainly use data collected from hospital emergency departments, which tends to underestimate the extent of horse-riding injuries (Christey et al., 1994). We found a back injury incidence rate of 1.25 (95% CI 0.48–2.03) in tumbling gymnastics. No directly comparable studies were found for younger children who were not top-level or elite performers. Strengths and limitations The major strengths of this study are that the study sample was taken from the real world in a natural experiment, and that the sample size was fairly substantial. Memory decay would be unlikely, as data were collected weekly. Bias in reporting was also unlikely, as there was an exceptionally high response rate (96.5%) and BP data were supplemented by telephone consultation and clinical evaluation and were collected consistently over 2.5 school years, which provided a unique opportunity to follow these children very closely over time. A general limitation to the injury incidence rates is the use of sample-level exposure data for feasibility reasons. Calculating the point estimate for the injury incidence
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rate in the sample as the sum of injuries across all individuals divided by the sum of exposures across all individuals is the method used in most research, but this assumes a fixed overall injury rate that is the same for every individual, which is rarely the case (Verhagen & Van Mechelen, 2010). Another weakness is that with a follow-up of 2.5 school years both injury incidence rates and exposure time might have varied in a way that makes causality more uncertain, that is, a child might have had no injuries and a high level of sports participation one year and several injuries and a low level of sports participation the following year. Another limitation is that the PA assessment was based on extrapolation of estimates from 1 week of accelerometer measurements, that is, the activity patterns shown across one week in winter might not reflect the individual child’s activity level in general. Also, the risk of sustaining a traumatic back injury was not reported separately for match-playing and training, as we in general had very few back injuries in this age group of children. This could be a limitation for some sports. Finally, the PA activity level in children participating in swimming could be biased with a “false” low PA level because of nonwear time while being in the water. However, only few traumatic back injuries were diagnosed in this category. Exposure time based on exact hours of participation instead of units of participation would have been more accurate to account for the variance in time-at-risk. Injury incidence expressed as incidence rates per 1000 participation hours has been the preferred measure in sports injury studies (Spinks et al., 2006; Caine et al., 2008). Even so, it has been argued that the content of a training/match session or a leisure time activity may be just as important for injury risk as the length of time spent in the activity (Kopjar & Wickizer, 1995; Stuart et al., 2002). This study lacks analysis investigating other possible explanatory risk factors besides age, sex, school type, and organized sport activity. This will be investigated in a future manuscript. However, we here present an overall picture of musculoskeletal back injuries in a generic sample of 6–12-year-old school children with a broad diversity in choice of sports and physical activity behavior in general. Perspectives This study has added a broader perspective to incidence rates of diagnosed BP in different spinal regions and activity settings in schoolchildren aged 6–12 years. The use of SMS-track to capture all symptoms indicative of BP, with subsequent diagnosis by a clinician, increases the likelihood of recording both severe and less severe nontraumatic and traumatic back injuries. From a public health viewpoint, our data indicate that most children in this age group do not sustain neck or back injuries. This supports the recommendations for
Back injuries in schoolchildren children to be physically active so as to gain the many health benefits later in life (Blair et al., 2001). From a research perspective, further understanding of injury epidemiology may be needed for some subgroups of children (e.g., those participating in sports such as horseriding or tumbling gymnastics) to allow development of preventive strategies at all responsible levels (Emery et al., 2006). Key words: Child, back pain, injury, spinal regions, incidence rate.
Acknowledgements The authors wish to acknowledge K. Froberg and L. B. Andersen, Center for Research in Childhood Health, University of Southern
Denmark. The authors thank the participants, their parents, and the participating schools, the Svendborg Project, and the municipality of Svendborg. Finally, the authors would like to acknowledge the members of the CHAMPS Study-DK not listed as coauthors of this paper: H. Klakk, T. Junge, and N. C. Møller.
Funding The study was supported by grants from the Danish Chiropractors’ Foundation, the IMK Foundation, the Nordea Foundation, the Tryg Foundation – all private, nonprofit organizations that support research in health prevention and treatment. TEAM Denmark, the elite sport organization in Denmark, provided the grant for the text messaging system.
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pain among children and adolescents. A nationwide, cohort-based questionnaire survey in Finland. Spine 1997: 22: 1132–1136. Thomas KE, Annest JL, Gilchrist J, Bixby-Hammett DM. Non-fatal horse related injuries treated in emergency departments in the United States, 2001–2003. Br J Sports Med 2006: 40: 619–626. Troussier B, Davoine P, de Gaudemaris R, Fauconnier J, Phelip X. Back pain in school children. A study among 1178 pupils. Scand J Rehabil Med 1994: 26: 143–146. van Mechelen W, Hlobil H, Kemper HC. Incidence, severity, aetiology and prevention of sports injuries. A review of concepts. Sports Med 1992: 14: 82–99. Verhagen E, Collard D, Paw MC, van Mechelen W. A prospective cohort study on physical activity and sports-related injuries in 10–12-year-old children. Br J Sports Med 2009: 43: 1031–1035.
Verhagen E, Van Mechelen W. Sports injury research. New York: Oxford University Press, 2010. Wedderkopp N, Jespersen E, Franz C, Klakk H, Heidemann M, Christiansen C, Moller NC, Leboeuf-Yde C. Study protocol. The childhood health, activity, and motor performance school study Denmark (The CHAMPS-study DK). BMC Pediatr 2012: 12: 128–135. Wedderkopp N, Kjaer P, Hestbaek L, Korsholm L, Leboeuf-Yde C. High-level physical activity in childhood seems to protect against low back pain in early adolescence. Spine J 2009: 9: 134–141. Wedderkopp N, Leboeuf-Yde C, Bo Andersen L, Froberg K, Steen Hansen H. Back pain in children: no association with objectively measured level of physical activity. Spine 2003: 28: 2019–2024. World Health Organization WHO. International statistical classification of diseases and related health problems. ICD-10. Tenth revision. Geneva: World Health Organization, 1992.
Scand J Med Sci Sports 2015: ••: ••–•• doi: 10.1111/sms.12519
Published by John Wiley & Sons Ltd
Back injuries in a cohort of schoolchildren aged 6–12: A 2.5-year prospective study C. Franz1, E. Jespersen1, C. T. Rexen1, C. Leboeuf-Yde2,3, N. Wedderkopp1,3,4 1
Research in Childhood Health, Department of Sports Science and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark, 2Research Department, Spine Center of Southern Denmark, Hospital Lillebaelt, Middelfart, Denmark, 3Institute of Regional Health Services Research, University of Southern Denmark, Odense, Denmark, 4The Sport Medicine Clinic, Orthopaedic Department, Hospital of Lillebaelt, Middelfart, Denmark Corresponding author: Claudia Franz, Research in Childhood Health, Department of Sports Science and Clinical Biomechanics, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark. Tel: +45 65504478, Fax: +45 65503480, E-mail: [email protected] Accepted for publication 2 June 2015
The aims of this prospective school cohort study were to describe the epidemiology of diagnosed back pain in childhood, classified as either nontraumatic or traumatic back injury, and to estimate the association with physical activity in different settings. Over 2.5 years, 1240 children aged 6–12 years were surveyed weekly using mobile text messages to ask about the presence or absence of back pain. Pain was clinically diagnosed and injuries were classified using the International Classification of Diseases version 10. Physical activity data were obtained from text messages and accelerometers. Of the 315 back injuries diagnosed, 186 injuries were nontraumatic and 129 were traumatic. The incidence rate ratio was 1.5 for a
nontraumatic back injury compared with a traumatic injury. The overall estimated back injury incidence rate was 0.20 per 1000 physical activity units (95% confidence interval 0.18–0.23). The back injury incidence rates were higher for sports when exposure per 1000 physical activity units was taken into consideration and especially children horse-riding had a 40 times higher risk of sustaining a traumatic back injury compared to the risk during non-organized leisure time physical activity. However, the reasonably low injury incidence rates support the recommendations of children continuously being physically active.
Back pain (BP) is a commonly reported disabling condition in the general population (Gabbay et al., 2011). It can occur already in childhood (Balague et al., 1988; Troussier et al., 1994) and its frequency increases with age, especially from around time of puberty (Balague et al., 1988; Leboeuf-Yde & Kyvik, 1998). As low back pain (LBP) in childhood is a risk factor for LBP in adulthood (Hestbaek et al., 2006), it is important to try to prevent early onset of BP. Factors commonly thought to be responsible for LBP in early life include inactivity (Balague et al., 1988; Troussier et al., 1994; Skoffer & Foldspang, 2008), low levels of physical activity (PA; Salminen et al., 1993), or excessive PA (Newcomer & Sinaki, 1996). Other studies found no association (Taimela et al., 1997) or mixed findings (Burton et al., 1996). However, these studies were based on self-reported PA, which might explain the differing conclusions. When PA was objectively measured with an accelerometer in a cross-sectional study, the agreement between self-reported levels of activity and objectively measured levels was poor and there was no association between level of PA and reported BP (Wedderkopp et al., 2003). However, a prospective
cohort study suggested that high overall PA levels protect against future BP (Wedderkopp et al., 2009). Reports on the influence of sports participation on childhood BP have also varied, from being injurious (Elliott et al., 1993) to BP being more common in those who avoided sports (Fairbank et al., 1984) – as well as an absence of association between participation in sport activities in general and self-reported LBP (Mogensen et al., 2007; Skoffer & Foldspang, 2008). Previous studies have reported both positive (Mogensen et al., 2007; Skoffer & Foldspang, 2008; Sato et al., 2011) and negative correlations (Mogensen et al., 2007; Auvinen et al., 2008; Skoffer & Foldspang, 2008) between BP and specific sport types. Only a few studies have investigated the risk of PA-related injuries associated with the entire scope of PA modalities in school-based cohorts (Spinks et al., 2006; Verhagen et al., 2009; Jespersen et al., 2015), and Jespersen et al. only reported injuries in the extremities. The aims of this prospective school cohort study were to describe the epidemiology of diagnosed BP in childhood, classified as either nontraumatic or traumatic back injury, and to estimate the association with physical
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Franz et al. activity in different settings including school physical education, organized sports activities, and non-organized leisure time PA.
Traumatic back injuries were further classified as occurring during school physical education (PE), organized sports activities, or non-organized leisure time PA.
Methods
Physical education
Study design
The weekly amount of PE was 4.5 h for sports schools and 1.5 h for normal schools, corresponding to three and one double lesson per week, respectively. Children at sports and normal schools were therefore “assigned” three and one sport exposure unit per week, respectively. Nonattendance was not recorded.
Data from the Childhood Health, Activity, and Motor Performance School Study Denmark (CHAMPS Study–DK) between August 2008 and July 2011 were used. The CHAMPS study was a large, prospective, controlled, school-based study in Denmark in the form of a natural experiment (Craig et al., 2012). Six schools were assigned to become sport schools with six physical education lessons per week, and four normal schools served as controls with two physical education lessons per week. The project is described in detail elsewhere (Wedderkopp et al., 2012). The regional ethics committee approved the project (ID S20080047), which was registered with the Danish Data Protection Agency (J.nr. 2008-41-2240). Written informed consent was obtained from parents, and the child gave verbal acceptance prior to each clinical examination. All participation was voluntary with the option to withdraw at any time.
Participants and data collection All children from preschool (grade 0) up to fourth grade in 10 state schools participating in the CHAMPS Study-DK were invited to participate in the weekly registration of BP and back injuries. Weekly information on BP (neck, mid-back, and/or lower back) was collected using text messages (SMS-track) for 2.5 school years. Each Sunday, parents answered a text message that asked questions on the presence or absence of BP during the previous week. Parents who reported that their child had BP in the previous week were contacted at the beginning of the subsequent week by one of four clinicians. If the pain was not musculo-skeletal, had disappeared, or was explained by the previous medical history, there was no further personal contact before the next pain reporting (if any). If the pain was still present and unexplained, a clinical examination was scheduled. Physiotherapists, chiropractors, and a medical practitioner were responsible for the clinical examination of the children within the next fortnight. If necessary, the child was referred for further examination by a medical specialist, and paraclinical investigations (such as X-ray, ultrasound or magnetic resonance imaging) were performed if relevant. Information on children seen or treated elsewhere (e.g., emergency department, general practitioner, medical specialist) during the study period was collected concurrently to obtain a complete picture of the clinical course. Data collection was suspended during the summer holidays.
Classification of BP into nontraumatic and traumatic injuries BP can have a traumatic or nontraumatic onset (Silverstein et al., 2002). However, the term “injury” or “sports injury” is often used as a collective name for all types of damage that can occur to the musculo-skeletal system in relation to sporting activities (van Mechelen et al., 1992). We defined a traumatic back injury as an injury resulting from a specific, identifiable event (Fuller et al., 2006). BP not fulfilling the criteria for a traumatic back injury was considered nontraumatic. Injuries were classified into these two categories using the medical records and the International Classification of Diseases version 10 (ICD-10; World Health Organization, 1992). S-codes were used to classify traumatic back injuries and M-codes were used for nontraumatic back injuries.
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Sports Organized sports activities were assessed using automated weekly text messages (SMS-track). The first question addressed the number of times the child had participated in sports during the previous week: “How many times has [NAME OF CHILD] participated in organised sports activities in the past week? 0 = 0, 1 = 1, 2 = 2, 3 = 3, 4 = 4, 5 = 5, 6 = 6, 7 = 7, 8 = more than 7 times.” If the response was 1 or more, a second question was sent: “Which type/s of sport/s: 1 = soccer, 2 = handball, 3 = basketball, 4 = volleyball, 5 = rhythmic gymnastics, 6 = tumbling gymnastics, 7 = swimming, 8 = horse-riding, 9 = dancing, 10 = other sports” (these being the most common sports among Danish children). The parents were instructed to type the relevant number(s).
Leisure time physical activity Besides PE lessons and organized sport activities, the children were also physically active in leisure time, hereafter named nonorganized leisure time PA. Non-organized leisure time PA thus comprised all daily activity of the child including transport, recess, other school activities, free play etc., except for PE lessons and organized sport activities. Data from accelerometer measurements were used as a proxy measure to estimate the amount of exposure to non-organized leisure time PA. Accelerometer measurements were assessed with the GT3X Actigraph accelerometer (Pensacola, Florida, USA). The children were instructed to wear the device for a full week from the time they woke up in the morning until bedtime to assess their entire PA for each day. The only exception was to remove the monitor when showering or swimming to prevent damage to the device. The assessments were performed from November 2009 to January 2010 in the 10 schools, when the children were in 1st–5th grade.
Statistical analysis A customized software program (Propero, version 1.0.18, University of Southern Denmark, Odense, Denmark) was used to process accelerometer data using information on PA at 10-s intervals. To distinguish between recorded “true intervals of inactivity” and “false intervals of inactivity,” consecutive strings of zeros of 30 min or longer were interpreted as “accelerometer nonworn.” Activity data were included for further analysis if the child had accumulated at least 10 h of activity per day for at least 4 days. Cut-off points for four activity intensity levels (sedentary, light, moderate, and vigorous) were determined according to Evenson et al. (2008). Time spent in moderate and vigorous activity was regarded as the “time at risk of back-injury.” Data were used at a sample level and a mean estimate of the total number of exposure units was extrapolated to cover the total study period. An exposure unit was chosen to equal 30 min, and two double lessons of PE and two sessions of sport participation were each equal to two units of exposure. Exposure time for the amount of leisure time PA was
Back injuries in schoolchildren thus the total number of exposure units (as estimated from accelerometer measurements) with subtraction of the number of exposure units in PE and sports. The data on diagnosed nontraumatic and traumatic back injuries were analyzed using STATA 13.0 (StataCorp, College Station, Texas, USA). To allow the study participants to become familiar with the novel data collection method, the first 9 weeks were considered a run-in period and these data were not included in the analyses. Calculation of injury incidence rates was the total sum of back injuries across all participating children divided by the sum of exposure across all participating children expressed in 1000 PA units. These comprised the PE, the sports and the non-organized leisure time PA exposures. The test for proportions and rates was used to test for differences between injury incidence rates. The significance level was set to P < 0.05. Injury incidence rates for traumatic back injuries were presented according to the average time in each specific setting of 10 types of sports, PE and non-organized leisure time PA for the whole sample. Average time spent being physically active across all settings and across the whole sample was used to estimate injury incidence rates for both nontraumatic and traumatic back injuries according to spinal region. Potential patterns for the missing values were addressed by logistic regression analysis controlling for effects of age, sex, school type, and sports. Missing values because of changed or incorrect mobile telephone numbers were excluded from the analyses.
Results Schools, SMS-track, and accelerometers Inclusion into the study was gradual, starting with 231 children from three schools, and after 8 months, children from all 10 schools had been included. A total of 1240 children participated in the study. The length of participation was 1–109 weeks with an average of 94.3 weeks. Dropouts were due to children moving away from the municipality or changing to nonproject schools and were counterbalanced by new children moving into project schools because of normal demographic mobility and a flow chart of the participating children in the CHAMPS-Study DK was previously described (Wedderkopp et al., 2012). Fifteen children
dropped out for other reasons, mainly because answering weekly text messages was considered too bothersome. A mean weekly response rate of 96.5% was recorded for pain reporting and 96.4% for sports participation. On average, 3.5% of 1240 children had missing values each week. Analysis of missing data showed no patterns according to age, sex, school type, or sports. 87% (1083 children) attained valid accelerometer recording. Diagnosed BP classified as nontraumatic or traumatic back injury During the study period, 18% (218 children) experienced a total of 315 back injuries. Of these, 186 injuries were nontraumatic and 129 traumatic, with an incidence rate ratio of 1.5 for a nontraumatic back injury compared with a traumatic injury (Table 1). The overall back injury incidence rate was 0.20 per 1000 PA units (95% CI 0.18 to 0.23). This would correspond to a child sustaining a back injury every 6.8 years, when the mean number of hours in moderate to vigorous activity per week was calculated. The cervical spine was the most common site for nontraumatic injuries, while the thoracic spine was the most common site for traumatic back injuries, although not significant compared to the cervical spine (Table 1). “Cervicalgia” was the most common ICD-10 diagnosis for nontraumatic back injuries. Common diagnoses for traumatic back injuries were “contusion of the back wall of thorax” and “sprain of the ligaments in the thoracic spine” (Table 2). Injury incidence rates of back injuries by sex, school type, and setting Girls had a significantly higher risk of sustaining a nontraumatic or traumatic back injury and thus also an overall higher risk of back injury compared to boys (Table 3). The number of back injuries was highest in sports schools (Table 4). When exposure per 1000 PA units was
Table 1. Number of back injuries and incidence rates in 1240 schoolchildren by injury type and spinal region
Cervical Thoracic Lumbar Cervico/thoracic Cervico/lumbar Cervical/thoracic/lumbar Thoraco/lumbar Total
Diagnosed nontraumatic back injuries
Diagnosed traumatic back injuries
Total back injuries
n
IR (95% CI)
n
IR (95% CI)
n
IR (95% CI)
71 29 40 4 0 6 36 186
0.05*1 (0.04–0.06) 0.02 (0.01–0.03) 0.03 (0.02–0.03) 0.01 (0.00–0.01) 0.00 0.00 (0.00–0.01) 0.02 (0.02–0.03) 0.12 (0.10–0.14)
31 41 26 10 4 3 14 129
0.02 (0.01–0.03) 0.03 (0.02–0.03) 0.02 (0.01–0.02) 0.01 (0.00–0.01) 0.00 (0.00–0.01) 0.00 (0.00-0.00) 0.01 (0.00–0.01) 0.08 (0.07–0.10)
102 70 66 14 4 9 50 315
0.07*2 (0.05–0.08) 0.05 (0.03–0.06) 0.04 (0.03–0.05) 0.01 (0.00–0.01) 0.00 (0.00–0.01) 0.01 (0.00–0.01) 0.03 (0.02–0.04) 0.20 (0.18–0.23)
IRs are per 1000 physical activity units; values in parenthesis are 95% CI. Physical activity units = 1 545 848. *1Significant difference of a nontraumatic injury in the cervical spine compared with the thoracic (P < 0.0001) or the lumbar spine (P = 0.003). *2Significant difference of an overall injury in the cervical compared with an injury in the lumbar spine (P = 0.0055). CI, confidence interval; IR, incidence rate; n, number.
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Franz et al. Table 2. Number of diagnosed back injuries and ICD-10 codes in 1240 schoolchildren
Number of nontraumatic injuries
ICD-10 codes
Number of traumatic injuries
Cervical spine
76
Cervicalgia Torticollis
39
Thoracic spine
52
54
Lumbo-/pelvic spine
58
Thoracic spinal pain Intervertebral disc degeneration Juvenile idiopathic scoliosis Syringomyelia Lumbago Intervertebral disc degeneration Schmorl’s nodes Spondylolysis Juvenile idiopathic scoliosis Limbus vertebra
Total
186
ICD-10 codes Sprain Facet syndrome Traumatic rupture of intervertebral disc Sprain Contusion Facet syndrome Fracture of vertebra Sprain Contusion Facet syndrome Fracture of vertebra Traumatic rupture of intervertebral disc
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129
ICD-10, International Classification of Diseases version 10. Table 3. Number of back injuries and incidence rates in 1240 schoolchildren by injury type and sex
Girls (n = 654) Boys (n = 586) Total
Nontraumatic back injuries (M-diagnosis)
Traumatic back injuries (S-diagnosis)
Total back injuries
n
IR (95% CI)
n
IR (95% CI)
n
IR (95% CI)
121 65 186
0.16*1 (0.13–0.19) 0.08 (0.06–0.10) 0.12 (0.10–0.14)
81 48 129
0.11*2 (0.09–0.13) 0.06 (0.04–0.08) 0.08 (0.07–0.10)
202 113 315
0.27*3 (0.24–0.31) 0.14 (0.11–0.17) 0.20 (0.18–0.23)
IRs are per 1000 physical activity units; values in parenthesis are 95% CI. Physical activity units girls = 738 894, physical activity units boys = 806 954. *1Significant difference (P < 0.0001) of girls sustaining a nontraumatic back injury compared with boys. *2Significant difference (P = 0.0007) of girls sustaining a traumatic back injury compared with boys. *3Significant difference (P < 0.0001) of girls sustaining an overall back injury compared with boys. CI, confidence interval; IR, incidence rate; n, number. Table 4. Number of back injuries and incidence rates in 1240 schoolchildren by injury and school type
Sports schools (n = 728) Normal schools (n = 512) Total (n = 1240)
Nontraumatic back injuries (M-diagnosis)
Traumatic back injuries (S-diagnosis)
Total back injuries
n
IR (95% CI)
n
IR (95% CI)
n
IR (95% CI)
122 64 186
0.14 (0.11–0.16) 0.10 (0.07–0.12) 0.12 (0.10–0.14)
74 55 129
0.08 (0.06–0.10) 0.08 (0.06–0.11) 0.08 (0.07–0.10)
196 119 315
0.22 (0.19–0.25) 0.18 (0.15–0.22) 0.20 (0.18–0.23)
IRs are per 1000 physical activity units; values in parenthesis are 95% CI. Physical activity units sports schools = 897 130, physical activity units normal schools = 648 718. CI, confidence interval; IR, incidence rate; n, number.
taken into consideration, no significant difference in the risk of sustaining a nontraumatic or traumatic back injury was found between school types (Table 4). The number of traumatic back injuries was highest in leisure time PA (Table 5). However, when exposure per 1000 PA units was taken into consideration, the back injury incidence rates were highest in sports, for example, rates of 2.45 (95% CI 1.28–3.61) in horseriding and 1.25 (95% CI 0.48–2.03) in tumbling gymnastics. Children who participated in horse-riding had about a 40 times higher risk of sustaining a traumatic
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back injury compared with the risk during non-organized leisure time PA and the most common cause of back injury was fall from horse. Discussion This is the first longitudinal study to report both nontraumatic and traumatic back injuries in children using a “momentary data assembly technique” (the SMS-track tool) to capture all symptoms indicative of BP in children and supplemented by clinical diagnosis.
Back injuries in schoolchildren Table 5. Number of exposure units, traumatic back injuries, and incidence rates in different settings
Sports Horse-riding Basketball Tumbling gymnastics Handball Swimming Soccer Other sports Rhythmic gymnastics Dance Volleyball Total sports PE lessons Leisure time PA (non-organized) Total
Physical activity units PA units
Number of traumatic back injuries n
Incidence rate per 1000 physical activity units IR (95% CI)
6950 1267 7974 16 301 13 220 25 222 14 855 4838 3374 1892 95 893 239 301 1 210 654 1 545 848
17 2 10 5 4 5 3 0 0 0 46 13 70 129
2.45 (1.28–3.61) 1.58 (0.00–3.77) 1.25 (0.48–2.03) 0.31 (0.04–0.58) 0.30 (0.01–0.60) 0.20 (0.02–0.37) 0.20 (0.00–0.43) 0 0 0 0.48 (0.34–0.62) 0.05 (0.02–0.08) 0.06 (0.04–0.07) 0.08 (0.07–0.10)
CI, confidence interval; IR, incidence rate; n, number.
When exposure per 1000 physical activity units was taken into consideration, children were at greatest risk of sustaining a traumatic back injury while doing sports, especially horse-riding and tumbling gymnastics. Children who participated in horse-riding had a 40 times higher risk of sustaining a traumatic back injury compared with the risk during non-organized leisure time PA. The total time-at-risk was associated with an overall injury incidence rate of 0.20 injuries per 1000 PA units (95% CI 0.18–0.23). Based on the mean number of hours in moderate to vigorous activity per week, this would correspond to a child sustaining a back injury every 6.8 years. As in most descriptive studies, direct comparisons of incidence figures are hampered by differences in the injury definition and data collection methods. Previous exposure-based injury rates in children mostly focus on sports-specific injury risk (Spinks & McClure, 2007). However, three somewhat comparable cohort studies of schoolchildren were identified. A Swiss study reported an injury incidence rate of 0.43/1000 h (Martin-Diener et al., 2013), which was comparable with Dutch rates of 0.48 injuries/1000 h (Verhagen et al., 2009) and Australian rates of 0.59 injuries/1000 h, respectively (Spinks et al., 2006). All three studies included all musculoskeletal locations, which could partially explain the higher rates compared to our overall back injury incidence rate of 0.20/1000 PA units. The lower incidence rate in the Swiss study compared with the other two studies might be due to underestimation of injuries as the recall frame was 12 months. Even higher rates in the Dutch and Australian studies would probably have been reported, if the focus had not mainly been on traumatic injuries. No studies were found reporting injury incidence rates of spinal regional injuries/1000 h in younger children not participating in elite sport.
Injury incidence rates of back injuries by sex and school type In contrast to what is previously reported in the literature (Spinks et al., 2006), girls were found to be at significantly higher risk of a back injury 0.27 (95% CI 0.24 to 0.31) compared with boys 0.14 (95% CI 0.11 to 0.17). Girls also had the highest total injury incidence rates of 0.58 in the Swiss (Martin-Diener et al., 2013) and 0.60 in Dutch (Verhagen et al., 2009) school studies, as compared with boys 0.32 in the Swiss (Martin-Diener et al., 2013) and 0.37 in Dutch (Verhagen et al., 2009) studies, respectively. One reason for the overall higher injury incidence rates in these studies compared to our study could be the inclusion of all musculoskeletal locations. Contributions to some of these sex differences might be that girls are more likely to express distress in response to pain and injury compared with boys (Fearon et al., 1996) or maybe specific hormonal and biochemical mechanisms (Stanford et al., 2008) such as growth spurt appearing earlier in girls might explain the higher risk. Even though the number of back injuries was highest in sports schools, children in sports schools did not have a significantly higher risk of sustaining a nontraumatic or traumatic back injury compared with children in normal schools, when exposure per 1000 PA units was taken into consideration. No comparable studies were found. Injury incidence rates of traumatic back injuries in different settings The number of traumatic back injuries was highest in non-organized leisure time PA, similar to findings from a Swedish study (Sundblad et al., 2005). However, when exposure time was taken into account, the highest incidence rate of back injuries was in sports. The same injury location pattern was also seen in a Danish cohort study (Jespersen et al., 2015), where the highest injury
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Franz et al. incidence rate of 1.57 was during sports participation. Contributions to the higher rates could be the inclusion of musculoskeletal injuries in both the upper and lower extremities and also their exposure unit was equal to 60 min as opposed to our unit of 30 min in moderate to vigorous physical activity (MVPA). An explanation for the increased risk of injury in sports could be the higher level of competition, which requires more complex motor skills and thus development of individual strategies for training and performance. Another reason might be that many Danish sports clubs rely on voluntary trainers, who may be less experienced or less qualified for training than professional instructors. A different injury location pattern within the different modalities of PA was seen in the Dutch study (Verhagen et al., 2009), where the least common setting for back injuries was organized sports and the most common were school physical education lessons and leisure time activity. Methodological differences may explain this finding, as the school physical education teachers in the Dutch study provided the children with injury registration forms and the children themselves filled in questionnaires at baseline and at the end of the school year regarding individual exposure to sports and leisure time activity. For sports, the risk for sustaining a back injury was highest for horse-riding, which is in agreement with a Swedish school study (Sundblad et al., 2005). Previous research has also shown an injury peak for riders aged 10–14 years and, consistent with our study, falls were the most common mechanism of injury (Thomas et al., 2006). Other comparable injury rates for horse-riding in primary schoolchildren are difficult to find because previous studies include wider age ranges and mainly use data collected from hospital emergency departments, which tends to underestimate the extent of horse-riding injuries (Christey et al., 1994). We found a back injury incidence rate of 1.25 (95% CI 0.48–2.03) in tumbling gymnastics. No directly comparable studies were found for younger children who were not top-level or elite performers. Strengths and limitations The major strengths of this study are that the study sample was taken from the real world in a natural experiment, and that the sample size was fairly substantial. Memory decay would be unlikely, as data were collected weekly. Bias in reporting was also unlikely, as there was an exceptionally high response rate (96.5%) and BP data were supplemented by telephone consultation and clinical evaluation and were collected consistently over 2.5 school years, which provided a unique opportunity to follow these children very closely over time. A general limitation to the injury incidence rates is the use of sample-level exposure data for feasibility reasons. Calculating the point estimate for the injury incidence
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rate in the sample as the sum of injuries across all individuals divided by the sum of exposures across all individuals is the method used in most research, but this assumes a fixed overall injury rate that is the same for every individual, which is rarely the case (Verhagen & Van Mechelen, 2010). Another weakness is that with a follow-up of 2.5 school years both injury incidence rates and exposure time might have varied in a way that makes causality more uncertain, that is, a child might have had no injuries and a high level of sports participation one year and several injuries and a low level of sports participation the following year. Another limitation is that the PA assessment was based on extrapolation of estimates from 1 week of accelerometer measurements, that is, the activity patterns shown across one week in winter might not reflect the individual child’s activity level in general. Also, the risk of sustaining a traumatic back injury was not reported separately for match-playing and training, as we in general had very few back injuries in this age group of children. This could be a limitation for some sports. Finally, the PA activity level in children participating in swimming could be biased with a “false” low PA level because of nonwear time while being in the water. However, only few traumatic back injuries were diagnosed in this category. Exposure time based on exact hours of participation instead of units of participation would have been more accurate to account for the variance in time-at-risk. Injury incidence expressed as incidence rates per 1000 participation hours has been the preferred measure in sports injury studies (Spinks et al., 2006; Caine et al., 2008). Even so, it has been argued that the content of a training/match session or a leisure time activity may be just as important for injury risk as the length of time spent in the activity (Kopjar & Wickizer, 1995; Stuart et al., 2002). This study lacks analysis investigating other possible explanatory risk factors besides age, sex, school type, and organized sport activity. This will be investigated in a future manuscript. However, we here present an overall picture of musculoskeletal back injuries in a generic sample of 6–12-year-old school children with a broad diversity in choice of sports and physical activity behavior in general. Perspectives This study has added a broader perspective to incidence rates of diagnosed BP in different spinal regions and activity settings in schoolchildren aged 6–12 years. The use of SMS-track to capture all symptoms indicative of BP, with subsequent diagnosis by a clinician, increases the likelihood of recording both severe and less severe nontraumatic and traumatic back injuries. From a public health viewpoint, our data indicate that most children in this age group do not sustain neck or back injuries. This supports the recommendations for
Back injuries in schoolchildren children to be physically active so as to gain the many health benefits later in life (Blair et al., 2001). From a research perspective, further understanding of injury epidemiology may be needed for some subgroups of children (e.g., those participating in sports such as horseriding or tumbling gymnastics) to allow development of preventive strategies at all responsible levels (Emery et al., 2006). Key words: Child, back pain, injury, spinal regions, incidence rate.
Acknowledgements The authors wish to acknowledge K. Froberg and L. B. Andersen, Center for Research in Childhood Health, University of Southern
Denmark. The authors thank the participants, their parents, and the participating schools, the Svendborg Project, and the municipality of Svendborg. Finally, the authors would like to acknowledge the members of the CHAMPS Study-DK not listed as coauthors of this paper: H. Klakk, T. Junge, and N. C. Møller.
Funding The study was supported by grants from the Danish Chiropractors’ Foundation, the IMK Foundation, the Nordea Foundation, the Tryg Foundation – all private, nonprofit organizations that support research in health prevention and treatment. TEAM Denmark, the elite sport organization in Denmark, provided the grant for the text messaging system.
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