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Copyright © 2015 American College of Sports Medicine

Risk Factors for Knee Injuries in Children 8-15 Years:

The CHAMPS-Study DK

Tina Junge

1,2,3, Lisbeth Runge

1,4, Birgit Juul-Kristensen

5,6, and Niels Wedderkopp

1,7,8

1Institute of Regional Health Research, University of Southern Denmark, Odense, Denmark;

2Department of Physiotherapy,

University College Lillebaelt, Odense, Denmark;

3Health

Sciences Research Centre, University College Lillebaelt, Odense, Denmark; 4Centre for

Welfare Technology Research and Development, University College Lillebaelt, Odense,

Denmark; 5Department of Sports Science and Clinical Biomechanics, University of Southern

Denmark, Odense, Denmark; 6Institute of Occupational Therapy, Physiotherapy and

Radiography, Bergen University College, Bergen, Norway; 7Sports Medicine Clinic,

Department of Orthopaedics, Hospital Lillebaelt, Middelfart, Denmark; 8School of

Psychology and Exercise Science, Murdoch University, Perth, Australia

Accepted for Publication: 2 November 2015

ACCEPTED

Risk Factors for Knee Injuries in Children 8-15 Years: The CHAMPS-Study DK

Tina Junge1,2,3

, Lisbeth Runge1,4

, Birgit Juul-Kristensen5,6

, and Niels Wedderkopp1,7,8

1Institute of Regional Health Research, University of Southern Denmark, Odense,

Denmark; 2Department of Physiotherapy,

University College Lillebaelt, Odense,

Denmark; 3Health Sciences Research Centre, University College Lillebaelt, Odense,

Denmark; 4Centre for Welfare Technology Research and Development, University

College Lillebaelt, Odense, Denmark; 5Department of Sports Science and Clinical

Biomechanics, University of Southern Denmark, Odense, Denmark; 6Institute of

Occupational Therapy, Physiotherapy and Radiography, Bergen University College,

Bergen, Norway; 7Sports Medicine Clinic, Department of Orthopaedics, Hospital

Lillebaelt, Middelfart, Denmark; 8School of Psychology and Exercise Science, Murdoch

University, Perth, Australia

Running title: Risk factors for knee injuries in children

Corresponding author

Tina Junge, IRS, SDU, Winsløwparken 19,3. 5000 Odense C, Denmark.

Email: [email protected]

The authors gratefully acknowledge the following for funding individual researchers and for funding the

CHAMPS Study Denmark part II: The Nordea Foundation, The TRYG Foundation, The IMK Foundation,

The Region of Southern Denmark, The Egmont Foundation, The A.J. Andersen Foundation, The Danish

Rheumatism Association, Østifternes Foundation, Brd. Hartmanns Foundation and TEAM Denmark,

University College Lillebaelt Department of Physiotherapy, University of Southern Denmark, The Danish

Chiropractic Research Foundation, and the Nordic Institute of Chiropractic and Clinical Biomechanics and

Research in Childhood Health for providing office space, The Svendborg Project by Sport Study

Svendborg as well as The Municipality of Svendborg. There were no potential conflicts of interest and/or

sources of funding for all authors involved in the writing of this manuscript. The results of the present study

do not constitute endorsement by ACSM.

Medicine & Science in Sports & Exercise, Publish Ahead of PrintDOI: 10.1249/MSS.0000000000000814

ACCEPTED

Copyright © 2015 by the American College of Sports Medicine. Unauthorized reproduction of this article is prohibited.

Abstract

Introduction Knee injuries are frequent in children, with most studies reporting traumatic knee injuries.

Evidence of risk factors for knee injuries in children is sparse. The purpose of this study was to report the

extent of traumatic and overuse knee injuries in children and to evaluate intrinsic and extrinsic factors for

risk of these injuries. Methods Weekly musculoskeletal pain, sport participation and sports type were

reported by 1326 school children (8-15 years). Knee injuries were classified as traumatic or overuse.

Multinomial logistic regression was used for analyses. Results During the study period, 952 (15%

traumatic, 85% overuse) knee injuries were diagnosed. Period prevalence for traumatic and overuse knee

injuries were 0.8/1000 and 5.4/1000 sport participations, respectively. Participation in tumbling gymnastics

was a risk factor for traumatic knee injuries (OR 2.14). For overuse knee injuries, intrinsic risk factors were

sex (girls OR 1.38), and previous knee injury (OR 1.78), while participation in soccer (OR 1.64), handball

(OR 1.95), basket (OR 2.07), rhythmic (OR 1.98), and tumbling gymnastics (OR 1.74) were additional risk

factors. For both injury types, sport participation above two times/week increased odds (OR 1.46-2.40).

Conclusion Overuse knee injuries were the most frequent injury type. For traumatic knee injuries,

participation in tumbling gymnastics was a risk factor. Risk factors for overuse knee injuries were being a

girl, previous knee injury and participation in soccer, handball, basket, rhythmic and tumbling gymnastics.

Further risk factors for both types of injury were participation in sports above two times/week. Although

growth-related overuse knee injuries are a self-limiting condition, a major part of children are affected by

these injuries with unknown short and long-term consequences. Key words: Knee injuries, children, risk

factors, youth sports

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Introduction

In children, engaging in physical activity and sport have important implications for individual and public

health benefits, but it also involves a risk of musculoskeletal injury (47). An increasing number of children

are reported to have musculoskeletal injuries related to physical activity and sport (12, 16, 39), and in

Denmark, the latter is the cause of 25% of all children and adolescents (10-19 years) being treated each

year in hospital emergency departments (35). The knee is one of the most frequent sites of injuries in

children and adolescents, with most epidemiological studies reporting traumatic knee injuries, mainly

derived from hospital or sport specific settings (2, 7, 19). However, these data may reflect only part of the

overall knee injury representation in children, since overuse injuries typically are not registered (10, 13,

19), illustrated in a recent study with 2.5 times more overuse (growth-related) than traumatic injuries of the

lower extremities (22). Since it is hypothesized that overuse knee injuries are under estimated, it is

important to establish the actual extent of knee injuries in a general population of children, including both

traumatic and overuse knee injuries.

Knee injuries result from a complex interaction of multiple factors and events (4), which may be one of the

reasons why knowledge of risk factors for knee injuries in children and adolescents is sparse and

inconclusive, besides limitations in research design and concerns with internal validity (15, 30, 42).

Controversy exists for intrinsic non-modifiable risk factors for knee injuries in children such as sex, age,

previous injuries and potentially modifiable risk factors like body composition and joint hypermobility,

which may be depending on age and sports type (7, 14, 15, 30). Knowledge of extrinsic risk factors for

knee injuries in children is inconclusive, but may involve increasing amount of time in sports and

participation in organised sport, especially high-load sports (7, 15, 30).

Correspondingly, we assume that sex, age, height, BMI, GJH, previous knee injuries, amount of organised

sport participation and sports type can be factors that may constitute a risk of traumatic and overuse knee

injuries. In order to evaluate a risk profile in children, factors that could constitute a risk has to be measured

repeatedly to account for changes over time (31, 46), and collected in close proximity to the injury to be

established as actual risk factors.

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Traumatic as well as overuse knee injuries in children may be more or less unavoidable, however, it is

important to establish evidence on how to prevent and reduce the numbers of these injuries (7), as injuries

may cause short and long-term disability, absence from sports and/or loss of enthusiasm for participating in

physical activity and sport (7, 17). Moreover, joint injuries may also increase the risk of accelerated

development of (secondary) osteoarthritis in adulthood (38). Therefore, evaluation of both intrinsic and

extrinsic factors can help determine causes of traumatic and overuse knee injuries in a general child

population and thereby provide more precise directions for adequate preventive recommendations.

The objectives of this study were to

1) Report the extent and severity of traumatic and overuse knee injuries in school children aged 8-15 years

old.

2) Examine the intrinsic factors of sex, age, height, BMI, GJH, previous knee injuries, and the extrinsic

factors of amount of organised sport participation and children participating in different sports types,

measured repeatedly over time, to determine if any of them constituted a risk factor in this population.

Methods

Design

This study was nested in The Childhood Health, Activity and Motor Performance School Study Denmark

(the CHAMPS-study DK), a longitudinal, open cohort study launched in 2008 following school-aged

children in the Municipality of Svendborg, Denmark, as previously described (49). Data for the current

study are collected during the period spring 2011 to summer 2014.

The Regional Scientific Ethics Committee for Southern Denmark approved the experimental protocol (jnr.

S-20080047 HJD/csf), and the study was reported to the Danish Data Protection Agency. Written and oral

information about participation in the study was provided to the parents or guardians of each child

according to the Declaration of Helsinki. Written informed consent for participation was received, and all

participation was explained to be voluntary with the option to withdraw from the project at any time. Prior

to every clinical examination for musculoskeletal injuries, a verbal agreement was further obtained from

both children and parents.

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Participants

In August 2011, totally 1888 children from the second to the eighth grade, 8-15 years, from 10 public

schools, were invited to participate in the longitudinal registration of musculoskeletal injuries. The study

was kept open, with the possibility for children to leave or enter the study.

Exclusion criteria for the current study were children with a diagnosis of chronic musculoskeletal or

neurological condition, or pain in the regions being examined in the Beighton Tests for hypermobility (BT)

(5) on the day of testing at three annual test rounds. Children not being tested with BT due to e.g. illness in

a test round had a missing score, and that time point was not included in the analyses.

Outcome measures

SMS surveys

A) Knee injuries

Registration of knee injuries was performed in two steps:

1) Short Message Service (SMS) surveys was used for injury registration, as this method so far has proven

to collect accurate data of fluctuating conditions by obtaining detailed information on changes over time (1,

25), and to result in more complete injury registration data than team medical staff registration (34). Also,

this method has been shown to be satisfactory for capturing both severe and less severe, traumatic and

overuse injuries in a school child population (22). Every Sunday, except for the summer and Christmas

holidays, the parents received an SMS on their cell phone, asking “Has your child had any pain during the

past week”? The possible answer options were one of four numbers, corresponding to pain or complaints

located in 1) the back, 2) the arms, 3) the legs or 4) no pain. Every Monday, the parents, who had answered

1), 2) and/or 3) were contacted via telephone by a clinician to determine requirements for clinical

examination.

2) The children with need for further examination due to persistent pain were examined at their respective

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schools within a fortnight by clinicians. Injuries were registered by type, with a traumatic injury defined as

resulting from a specific, identifiable event, and an overuse injury as caused by repeated micro trauma

without a single, recognisable event responsible for that injury (18). The injuries were further classified

according to the ICD-10 by WHO. Information of children being diagnosed elsewhere (e.g. hospital

emergency department) during the study period was collected concurrently. Multiple injuries within single

children over the course of the study were included in the analyses.

B) Organised sports activity

The weekly amount of organised sports activity, reported by the parents to each child as the number of

times spent in organised sport activity, was also registered by the SMS survey every Sunday. The question

was: “How many times did your child participate in organised leisure time sport within the last week?”

with the possibility of answering the relevant number between 0 (none) and 8, with 8 corresponding to

more than 7 times. The weekly amount was expressed in times, which is not equivalent to hours for all

sports types. Therefore, the term ´sport participations´ is used throughout the text.

C) Type of sport

If the answer to the amount of organised sports activity was a number between 1-8, it was followed by the

question: “Which type of sport?” with 10 options for answering: 1: Soccer, 2: Handball, 3: Basketball, 4:

Volleyball, 5: Rhythmic gymnastics, 6: Tumbling gymnastics, 7: Swimming, 8: Horseback riding, 9:

Dancing and 10: Other sports.

Clinical tests for hypermobility and anthropometrics

The children and adolescents participating in the study were tested during three test rounds with baseline in

spring 2011, the first follow-up measurement in spring 2012, and the second follow-up measurement in

spring 2013.

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A) Generalised Joint Hypermobility

The BT is as an overall evaluation of joint flexibility, with a total score ranging from 0-9 (5). Children were

classified GJH at cut-point ≥5/9 (23) or controls according to the described cut-point. The child and the

parents were not informed about the status of GJH.

The children did not perform any warm-up exercises or stretching before the BT examination, and were

tested with BT in a random order by physiotherapy students in each test round. All testers were trained by

two experienced physiotherapists (TJ & LRL) in a standardised protocol for the BT, describing the test

procedures in detail (23).

B) Anthropometrics

Anthropometric measures in the form of height and weight were collected simultaneously with the BT.

Statistical analysis

In summary, data included demographics as well as prevalence and distribution by type and ICD-10

diagnoses for traumatic and overuse knee injuries. The prevalence is expressing the number of children

affected by a knee injury at any given time during the study period (3). Also, the knee injury prevalence is

expressed as injuries per 1000 sport participations, as this figure does not reflect incidence but rather the

period prevalence.

The main outcome was odds for knee injury (by injury type) being one of the four competing states: 1) no

injury, 2) a traumatic knee injury, 3) an overuse knee injury or 4) a lower extremity injury other than a knee

injury. The included explanatory variables were intrinsic risk factors from demographics in the three test

rounds as well as previous knee injuries up till two years prior to the index injury as well as the extrinsic

factors of weekly information of amount of sport participation and sports type. The SMS answers regarding

´Other sports´ were included in the analyses, but not interpreted as a result, as this category consisted of too

many unknown sports types. Only children answering at least 80% of the SMS questions were included in

the final analyses. The risk factors were presented with a one-year follow-up of injury; 2011-2012, 2012-

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2013 and 2013-2014.

A multinomial logistic regression with robust standard errors and taking into account the clustering of

children in classes was used to model the dependent nominal outcome variables, extended to the

longitudinal setting (43). Sex, age, height, BMI, GJH, previous knee injury, participation and type were

introduced as independent variables to explore if any of the factors could be a significant risk factor of knee

injury. The competing risks of no injury, a traumatic/overuse injury or a lower extremity injury other than a

knee injury, depending on the outcome injury type, was taken into account in the analyses, treating

traumatic or overuse injury as categorical under the assumption that the levels of knee injury have no

natural ordering. Interaction between sex and age, and sex and sports type was tested, but did not present

with statistical significance. The results are presented as Odds Ratios (OR). Assumptions of the

multinomial logistic regression model were verified, as the data are case specific; that is, each independent

variable has a single value for each case. Also, the dependent variable cannot be perfectly predicted from

the independent variables for any case.

Odds ratios (OR) were calculated for traumatic and overuse injuries stratified by numbers of sports

participations where appropriate. Interaction between sex and age, and sex and sports type was tested, but

did not present with statistical significance. If the specific sports type by stratification comprised too few

injuries, only numbers of injuries were reported, not OR.

All statistical analyses were performed using STATA (version 13.0: Statacorp, College Station, Texas,

USA) with the pre-specified level of significance being 0.05.

Results

For the SMS survey, 1326 children volunteered to participate in this longitudinal, open cohort study at

baseline in August 2011, 229 children and their parents declined to participate, and 333 children and their

parents did not respond. The response rate for the children participating in the SMS survey was 97% for the

entire period of 128 weeks, excluding summer and Christmas holiday periods (Table 1).

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During the three year study period, the children was represented by 1% children being 8 year old, 8% in 9

year-olds, 18% in 10 year-olds, 22% in 11 year-olds, 22% in 12 year-olds, 17% in 13 year-olds, 10% in 14

year-olds and 2% in 15 year-olds.

Extent and distribution of traumatic and overuse knee injuries

Totally, 2127 lower extremity injuries including knee injuries were diagnosed in the present study. From

these, the prevalence of knee injuries during the study period was 952; hereby 15 % (146) traumatic knee

injuries and 85% (806) overuse knee injuries. There was a peak frequency of overuse knee injuries in 11

and 12 years old (Figure 1).

Traumatic knee injuries consisted mainly of sprains and contusions; while for overuse knee injures it were

primarily traction apophysitis (Morbus Sinding-Larsen-Johansson, Osgood-Schlatter) (Table 2).

Period prevalence per 1000 sport participations

The period prevalence per 1000 sport participations for traumatic and overuse knee injuries was 0.8 and

5.4, respectively. On average, the children had 1.7 times sport participations per week (range 0–8) in

organised sport. Highest period prevalence of traumatic injuries was seen in tumbling gymnastics and

handball, while for overuse injuries it was handball and rhythmic gymnastics (Table 3).

Risk factors for traumatic knee injuries

The only significant extrinsic risk factor for traumatic knee injuries was tumbling gymnastics (OR 2.14, CI

1.01-4.57). There were no increased odds for the intrinsic risk factors (Table 4).

Risk factors for overuse knee injuries

The intrinsic risk factors for overuse injuries were girls with increased odds of 1.38 (CI 1.10-1.74)

compared to boys. Also, having a previous knee injury increased the odds of 1.78 (CI 1.37-2.33) (Table 4).

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For the extrinsic risk factors of overuse injuries, children participating in specific sports like soccer,

handball, basket, rhythmic and tumbling gymnastics had significantly higher odds of sustaining an overuse

knee injury compared to children not participating in sports (Table 4).

Risk factors for traumatic knee injuries by sports type and sports participation

In the bi-variate analyses, higher odds were found for traumatic knee injuries for children participating in

the most popular sports soccer and handball four times per week (OR 2.40 resp. 2.33) and tumbling

gymnastics twice or more per week (OR 3.16). For children participating in handball only once a week,

odds were more than three times higher (OR 3.61) for sustaining a traumatic knee injury (Table 5). Basket

and volley were excluded as sports types from analyses due to few injuries by stratification.

Risk factors for overuse knee injuries by sports type and sports participation

In the bi-variate analyses, children participating more than two times per week in specific sports as soccer,

handball, dance, tumbling and rhythmic gymnastics had increased odds for overuse knee injuries. However,

the odds of sustaining an overuse knee injury were higher for children reporting to participate in soccer and

handball on all levels. Basket and volley were excluded as sports types from analyses due to few injuries by

stratification.

Discussion

As anticipated, in school-aged children, overuse knee injuries were 5.5 times more frequent than traumatic

knee injuries, the latter being mainly sprains and contusions, while for overuse knee injures it was primarily

traction apophysitis. The highest period prevalence for traumatic injuries was found in tumbling gymnastics

and handball, and for overuse injuries in handball and rhythmic gymnastics.

The extrinsic factor of children participating in tumbling gymnastics was the only significant risk factor for

traumatic injuries. For overuse knee injuries, the intrinsic risk factors were sex (being a girl), and previous

knee injury, and the extrinsic factors were participation in specific sports like soccer, handball, basket,

rhythmic and tumbling gymnastics.

The extrinsic factor of sport participation above two times per week in specific sports as soccer, handball,

dance, rhythmic and tumbling gymnastics increased odds for both injury types.

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Traumatic injuries

The overall period prevalence for traumatic injuries being 0.8/1000 sport participations is comparable to

similar studies (6, 48). The current sports specific traumatic period prevalence were generally highest in

tumbling gymnastics and handball and lowest in rhythmic gymnastics and swimming, consistent with

findings of other studies, although these rates vary considerably (11, 15). Injury rates may be greatly

influenced by the study design, population, culture and definition of injury used (6, 48).

The only significant risk factor for traumatic knee injuries was children participating in tumbling

gymnastics, a term covering tumbling, power tumbling and TeamGym. These are all popular Danish

gymnastic disciplines, with TeamGym being a relative new competition team sport from Scandinavia

including three disciplines as floor exercise routine, tumbling on Trampette (mini trampoline) and on a

fiber track. Team Gym and power tumbling demands force, power and motor control skills to make the

body move fast and powerful in three planes on and off the ground, landing with high impact. During

competition, the lower extremities account for 70% of the injuries, primarily in the ankle and the knee, with

joint compression and joint rotation as the primary traumatic injury mechanisms (20, 28). In the current

study, knowledge of the injury situation, being training or competition was not obtained, but the odds of

having a traumatic knee injury increased by increasing participation level, as the odds ratio was remarkably

high, when participating in tumbling twice or more times a week. This increase could plausibly be

explained by more time spent during the sport meaning more exposure to injuries, or by children

participating more times a week having a high competitive level, with more intensive training, tumbling

skills and demands. Ankle and knee injuries are also recognised as the most frequently injuries in paediatric

gymnastics with tumbling (9), with special emphasis on the landing phase, as this phase seems critical to

traumatic injuries (20).

Controlling the knee during landing, in both tumbling and other sports, may be further compromised by

increased flexibility or laxity, like in individuals with GJH (24, 26, 37, 40). In the current study, no higher

odds for traumatic injuries were found for the intrinsic risk factor of having GJH. On the contrary, a

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positive association between GJH and knee injuries was reported in a recent meta-analysis, where sport

participants (9-39 years) with GJH (BT cut-point ≥4/9) had five times the risk of knee injuries, especially

during contact sport activities (37). Sports-specific studies and studies of older adolescents, especially with

female participants, may differ from the present study by typically revealing a higher number of traumatic

knee injuries, as participants are more exposed to high-risk injury situations like landing, pivoting and side-

cutting (32, 33, 36, 41). Still, risk factors are depending on the population studied, the study design and the

data collection methods.

Another risk factor suggested is the amount of sport participation, with more exposure time implying

higher risk of injury. In the bivariate analyses, higher odds were found for this extrinsic factor for traumatic

knee injuries when participating in soccer and handball four times a week, typically with three times of

training and one match at the weekend. Both soccer and handball are high-load contact sports, involving

player-to-player contact with tackling and cutting, jumping, pivoting and landing, all being potential

traumatic knee injury risk situations. For handball, higher odds were also seen with participation only once

a week, possibly due to a lack of basic training leading to injuries in low-skilled players.

In several studies of children and youth, the volume and intensity of training are positively correlated with

the risk of overuse injury, plausibly due to repetitive traction of the apophyses, exceeding workload

thresholds (8). Hence, activity volume, intensity and recovery time between training and competition bouts

must be considered to avoid fatiguing conditions, plausibly leading to overuse injury in populations of

maturing children (27).

Overuse injuries

In the current study, overuse knee injuries were reported more than fivefold to traumatic injuries,

confirming the phenomenon of traumatic injuries being only the ´tip of the iceberg´ in children and

adolescents (3, 21, 45). The overuse injuries consisted mainly of traction apophysitis, such as Morbus

Sinding-Larsen-Johansson and Morbus Osgood-Schlatter, which are two common growth-related overuse

knee injuries. No previous study has reported Morbus Sinding-Larsen-Johansson as more frequent than

Morbus Osgood-Schlatter, which has no consequences for treatment, however, in respect to diagnostics,

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knowledge of the prevalence of this condition in the maturing child could be important. Although the

consequences of overuse injuries were not reported in the current study in terms of time loss to sport or

medical assistance, substantial time loss in sports and physical activity has previously been reported for

overuse injuries (7). Consistently, the current study showed a large effect of previous knee injury of any

kind for developing an overuse knee injury in the current study, as reported previously (7, 15, 44).

Sex was identified as an intrinsic risk factor for overuse knee injuries, with girls having significantly higher

odds. This is in line with some age matched school child cohort studies (6, 44, 48), but in contrast to a

study of school children below 12 years of age (21). The difference between sexes may be explained by

maturation, especially growth spurt, which makes the current girls at 11-13 years of age more susceptible to

growth-related overuse injuries compared to the boys, maturing later than girls.

In the current study, the intrinsic risk factor BMI was borderline significant with a small effect on the odds

for overuse knee injuries. Conflicting evidence regarding the effect of increased BMI for lower extremity

injuries is seen in other studies (6, 15) and appears to be injury and sports specific.

Period prevalence was ranging from highest in children playing handball to lowest for children

participating in horse riding. Comparison to other studies is complicated by the small number of available

studies on risk factors specific for overuse knee injuries in school-aged children and the different reporting

methods. Not surprisingly, higher odds for reporting an overuse knee injury was seen for the extrinsic

factor of sport participation more than two times a week, suggesting that increased sports participation may

have some negative drawbacks as growth-related overuse injuries. Still, the relative low severity of these

injuries as well as the many positive effects of sport participation must be included in injury prevention

conclusions and recommendations.

The odds for overuse knee injury were remarkably higher for children regarding the

extrinsic factor of children participating in soccer and handball on all participation levels,

which partly may be explained by forceful and repeatedly quadriceps muscle contractions

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during running and kicking, stressing the apophyses by traction of the tendons at the

inferior patella pole and the tuberositas tibiae. As apophyseal injuries are associated with

growth, these injuries are self-limiting by nature (29), however, the short and long-term

consequences are not known.

Weaknesses

The weaknesses of the current study are the sparse information of the short and long term consequences of

both traumatic and overuse knee injuries. The current study has presented injury types, including

information of severity of injuries expressed by ICD-10 diagnoses, but could also have been more

informative including data of time loss, clinical outcome and economic cost. Also, children with multiple

knee injuries may be presented in the analyses multiple times, not taking recurrent events into account,

plausibly oversampling the number of risk factors within those children. Another concern of the current

study is the frequent data collection on basis of the reporting source being parents, plausibly causing over

and under estimation of reporting. Still, weekly SMS survey has so far proven valid, reliable, feasible, and

user-friendly, with high compliance rates, capturing more overuse injuries than standard injury surveillance

methods (21, 32).

Strengths

The strengths of the current study are a relatively large sample size in this longitudinal, prospective cohort

study for a period of 128 weeks, with a high weekly response rate of 97% for the SMS survey. This method

minimizes recall bias and provides a solid basis for collecting information of weekly exposure to sport, and

knee injuries being either traumatic or overuse, using an internationally accepted definition of injury

classification (18). Such injury classification definition allows for registration of knee injuries not resulting

in absence from school or sports, or injuries not requiring medical treatment, which is assumed not to lead

to an underestimation of the actual number of overuse injuries.

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Conclusion

Overuse knee injuries, presented by traction apophysitis, were the main injury type. Traumatic knee injuries

were primarily sprains and contusions, with tumbling gymnastics as the only significant extrinsic risk

factor. Intrinsic risk factors for overuse knee injuries were being a girl and having had a previous knee

injury. The extrinsic factors for overuse knee injuries were children participating in soccer, handball,

basket, rhythmic and tumbling gymnastic. Further, sport participation above two times/week were extrinsic

risk factors for both types of injury.

The results of this study indicate that growth-related overuse knee injuries affect a major part of children.

Although these knee injuries are a self-limiting condition, the short and long term consequences of growth-

related overuse knee injuries are not well documented. Still, considerations of pain level, training

programmes, skill levels, frequency of training and the amount of sport participation are suggested for as

well the parents, trainers, sports clubs and society. Future research should target healing time for knee

apophysitis, and determine whether this condition is a recurrent problem that could predict future knee

problems.

Acknowledgements

This project requires a substantial amount of manpower, especially when collecting data. Therefore, thanks

are extended for the great interest and contribution from so many people, especially physiotherapy students,

sport, chiropractor and nurse students. A special thanks is given to all the children participating in the

project as well as to their parents.

The authors gratefully acknowledge the following for funding individual researchers and for funding the

CHAMPS Study Denmark part II: The Nordea Foundation, The TRYG Foundation, The IMK Foundation,

The Region of Southern Denmark, The Egmont Foundation, The A.J. Andersen Foundation, The Danish

Rheumatism Association, Østifternes Foundation, Brd. Hartmanns Foundation and TEAM Denmark,

University College Lillebaelt Department of Physiotherapy, University of Southern Denmark, The Danish

Chiropractic Research Foundation, and the Nordic Institute of Chiropractic and Clinical Biomechanics and

Research in Childhood Health for providing office space, The Svendborg Project by Sport Study

Svendborg as well as The Municipality of Svendborg.

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Conflict of interest

There were no potential conflicts of interest and/or sources of funding for all authors involved in the

writing of this manuscript. The results of the present study do not constitute endorsement by ACSM.

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ACCEPTED


Figure captions

Figure 1. Distribution of the relative amount of traumatic and overuse knee injuries by age.

ACCEPTED


Table 1. Demographics of children in the three test rounds of 2011, 2012 and 2013. Values are mean (range), unless

otherwise indicated.

2011

(n=1327)

2012

(n=1242)

2013

(n=1084)

Sex, % boys 47 49 49

Age, years 10 (8-14) 11 (9-15) 12 (10-15)

Height, cm 147.1(121.5-189) 151.9 (126-186) 157.7 (131-192.5)

Body mass, kg 38.8 (21.1-81.1) 42.3 (22.9-97.7) 46.6 (24.8-95.2)

BMI 17.5 (12.7-29.6) 18.1 (12.4-31.5) 18.5 (13.1-34.1)

GJH, % (no.) 13% (149) 9% (108) 11% (121)

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Table 2. Distribution of knee injuries by type, traumatic or overuse, and distribution of injury type by ICD 10 diagnoses

in percentage.

Traumatic knee injuries

S83.4 Distorsion 57% (83)

S80.0 Contusion 34% (49)

S83.5R ACL injury 2% (3)

Other diagnoses 7% (11)

Totally 146

Overuse knee injuries

M92.4 Mb. Sinding Larsen-Johansson 28% (228)

M92.5A Mb. Osgood-Schlatter 23% (184)

M22.2 Patellofemoral disorders 15% (117)

M76.5 Patellaris Tendinitis 5% (44)

M76.8 Other Specified Enthesopathies of lower limb 4% (34)

M70.9 Unspecified Soft Tissue Disorder 4% (31)

Other diagnoses 21% (169)

Totally 806

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Table 3. Period prevalence of traumatic and overuse injuries per 1000 sports participations, distributed by sports type.

Sport

participation

units

Number of

traumatic

knee

injuries

Traumatic injury

period prevalence per 1000

sports participation units

(95% CI)

Number

of overuse

knee

injuries

Overuse injury

period prevalence per

1000 sports participation

units

(95% CI)

Sports

Soccer 37613 36 0.96 (0.64-1.27) 212 5.64 (4.88-6.40)

Handball 19064 25 1.31 (0.80-1.83) 142 7.45 (6.22-8.67)

Basketball 2061 2 0.97 (0.80-1.83) 13 6.31 (2.88-9.74)

Volleyball 2006 3 1.50 (-0.20-3.19) 8 3.99 (1.22-6.75)

Rhythmic gymnastics 4129 2 0.48 (-0.19-1.16) 30 7.27 (4.67-9.87)

Tumbling gymnastics 6574 12 1.83 (0.79-2.86) 39 5.93 (4.07-7.79)

Swimming 7071 4 0.57 (0.01-1.12) 29 4.10 (2.61-5.59)

Horse riding 3844 0 - 13 3.38 (1.54-5.22)

Dance 6586 7 1.06 (0.28-1.85) 23 3.49 (2.07-4.92)

Others 16859 3 0.18 (-0.02-0.38) 66 3.91 (2.97-4.86)

Totally 105807 94 0.89 (0.71-1.07) 575 5.43 (4.99-5.88)

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Table 4. Odds Ratio (OR) estimates of the multinomial logistic regression by injury type for the explored intrinsic

factors; sex, age, height, BMI, GJH, previous knee injury, and the extrinsic factors; sport participation and type.

Traumatic

knee injuries

OR (95% CI)

p-value Overuse

knee injures

OR (95% CI)

p-value

Intrinsic factors

Sex 1.03 (0.64-1.66) 0.882 1.38 (1.10-1.74) 0.005

Age 1.04 (0.81-1.35) 0.719 1.01 (0.92-1.10) 0.821

Height 1.01 (0.97-1.04) 0.566 0.99 (0.98-1.01) 0.619

BMI 1.03 (0.93-1.14) 0.490 1.03 (0.99-1.08) 0.073

GJH 1.69 (0.92-3.09) 0.089 1.12 (0.79-1.59) 0.517

Previous knee injury 1.38 (0.86-2.22) 0.179 1.78 (1.37-2.33) 0.001

Extrinsic factors

Sport participation 1.02 (0.87-1.21) 0.746 0.96 (0.89-1.04) 0.368

Sports type

Soccer 1.14 (0.54-2.40) 0.723 1.64 (1.12-2.39) 0.010

Handball 1.32 (0.56-3.11) 0.523 1.95 (1.31-2.92) 0.001

Basket 0.96 (0.19-4.83) 0.965 2.07 (1.10-3.90) 0.023

Volley 2.01 (0.41-9.79) 0.379 0.69 (0.25-1.87) 0.472

Rhythmic gymnastics 0.64 (0.09-4.38) 0.657 1.98 (1.04-3.76) 0.035

Tumbling gymnastics 2.14 (1.01-4.57) 0.047 1.74 (1.02-2.94) 0.039

Swimming 0.72 (0.08-6.13) 0.770 1.03 (0.60-1.75) 0.907

Horse riding - - 0.90 (0.47-1.72) 0.769

Dancing 1.27 (0.34-4.79) 0.716 1.08 (0.56-2.07) 0.810

Significant odds ratios are shown in bold.

BMI: Body Mass Index, GJH: Generalised Joint Hypermobility

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Table 5. Odds ratios (OR) by injury type for the individual sports type stratified by weekly sports participations.

Traumatic

knee

injuries

Traumatic

knee injuries

OR (95% CI)

p-value Overuse

knee

injuries

Overuse

knee injures

OR (95% CI)

p-value

Soccer

Once 7 1.31 (0.59-2.89) 0.505 35 1.52 (1.06-2.18) 0.021

Twice 8 0.85 (0.41-1.80) 0.679 63 1.57 (1.18-2.08) 0.002

Three 7 0.81 (0.36-1.81) 0.568 62 1.64 (1.23-2.18) 0.001

Four 12 2.40 (1.27-4.52) 0.007 32 1.45 (1.01-2.11) 0.048

Five or more 2 - - 19 1.07 (0.67-1.72) 0.767

Handball

Once 8 3.61 (1.71-7.65) 0.001 34 3.62 (2.51-5.22) 0.001

Twice - - 30 1.46 (0.99-2.15) 0.051

Two-three 9 0.92 (0.45-1.88) 0.831 - -

Three - - 51 2.43 (1.79-3.31) 0.001

Four 7 2.33 (1.05-5.16) 0.037 24 1.82 (1.19-2.78) 0.006

Rhythmic gymnastics

Once - - - 8 1.70 (0.84-3.46) 0.139

Twice - - - 5 1.23 (0.50-3.01) 0.638

Three 2 - - 8 2.78 (1.36-5.66) 0.005

Four or more - - - 9 2.38 (1.22-4.66) 0.011

Tumbling gymnastics

Once - - - 13 1.57 (0.89-2.75) 0.115

Twice - - - 9 1.21 (0.61-2.35) 0.581

Twice or more 12 3.16 (1.68-5.96) 0.001 11 1.94 (1.18-3.19) 0.009

Swimming

Once 1 - - 8 0.85 (0.42-1.73) 0.660

Two-three 2 - - 9 0.98 (0.50-1.92) 0.966

Four or more 1 - - 11 1.38 (0.75-2.53) 0.299

Horseback riding

Once - - - 13 0.84 (0.39-1.79) 0.661

Dance

Once 5 - - 6 0.50 (0.22-1.13) 0.098

Two-three 2 - - 7 0.79 (0.37-1.69) 0.552

Four or more - - - 10 2.64 (1.39-4.99) 0.003

Basketball

Once 2 - - 3 - -

Twice - - - 5 - -

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Three - - - 2 - -

Four or more - - - 3 - -

Volleyball

Once 1 - - 3 - -

Twice - - - 1 - -

Three 2 - - 1 - -

Four or more - - - 1 - -

ACCEPTED


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