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. Author manuscript; available in PMC: 2021 Jun 1.
Published in final edited form as: Sports Med. 2020 Jun;50(6):1203–1210. doi: 10.1007/s40279-020-01262-3

Effect of a concussion on anterior cruciate ligament injury risk in a general population

April L McPherson 1,2, Matthew B Shirley 2, Nathan D Schilaty 2,3,4, Dirk R Larson 5, Timothy E Hewett 6
PMCID: PMC7242116  NIHMSID: NIHMS1551491  PMID: 31970718

Abstract

Background:

Recent studies indicate concussion increases risk of musculoskeletal injury in specific groups of patients. The purpose of this study was to determine the odds of anterior cruciate ligament (ACL) injury after concussion in a population-based cohort.

Methods:

International Classification of Diseases, 9th and 10th Revision (ICD-9, ICD-10) codes relevant to the diagnosis and treatment of a concussion and ACL tear were utilized to search the Rochester Epidemiology Project (REP) between 2000 and 2017. A total of 1,653 unique patients with acute, isolated ACL tears were identified. Medical records for cases were reviewed to confirm ACL tear diagnosis and to determine history of concussion within 3 years prior to the ACL injury. Cases were matched by age, sex, and REP availability date to patients without an ACL tear (1:3 match), resulting in 4,959 controls. The medical records of the matched control patients were reviewed to determine history of concussion.

Results:

39 patients with a concussion suffered an ACL injury up to 3 years after the concussion. The rate of prior concussion was higher in ACL-injured cases (2.4%) compared to matched controls with no ACL injury (1.5%). This corresponds to an odds ratio of 1.6 (95% CI 1.1-2.4; p=0.015).

Conclusions:

Although activity level could not be assessed, there are increased odds of ACL injury after concussion in a general population. Based on the evidence of increased odds of musculoskeletal injury after concussion, standard clinical assessments should consider concussion symptom resolution as well as assessment of neuromuscular factors associated with risk of injuries.

1. Introduction

An estimated 1.7 million people sustain a traumatic brain injury (TBI) annually in the United States and are the cause for an estimated 2.8 million emergency department visits.[1, 2] In particular, mild traumatic brain injuries (mTBI) are thought to be underreported and undercounted.[2] For example, it is estimated that between 1.6 million and 3.8 million sports-related concussions, a form of mTBI, occur each year in the United States.[3] Among athletes who sustain a concussion, a recent meta-analysis showed over two times greater odds of sustaining a lower extremity musculoskeletal injury than athletes without concussion.[4] A recent matched-cohort study among active duty soldiers showed an increased hazard of acute lower extremity injury after sustaining a concussion.[5] In further confirmation of these findings, another recent meta-analysis that included both military and athletic populations also confirmed over 2.5 times increased odds of all injuries after a concussion.[6] Therefore, with the high incidence of mTBI and concussion in both the general population and specific groups of athletes and soldiers, it is important to understand the sequelae after a concussion.

The recent data that relates concussion to an increased risk of musculoskeletal injury is in line with prior studies that point to neurologic risk factors in anterior cruciate ligament (ACL) injuries.[7-15] Of particular relevance, noncontact ACL injuries have been associated with lower neurocognitive function in non-concussed athletes, which suggested that neurocognitive differences may be associated with the loss of neuromuscular control and coordination errors leading to increased risk of ACL injury.[14] The incidence and rate of ACL injuries continues to increase, and results in approximately a $2 billion economic burden for the associated treatment and rehabilitation in the United States alone.[16, 17] Thus, it is important to identify potential factors that increase risk of an ACL injury, such as a previous injury like concussion. Previous injury is a known risk factor for ACL injury[12], but to date there is limited evidence that shows if history of a concussion increases risk for an ACL injury.

While the evidence for the relationship between concussion and musculoskeletal injury risk continues to grow, most studies to date have focused on specialized cohorts such as competitive athletes or soldiers. As such, their results may not be generalizable to a larger population. It is important to determine if the increased odds of injury after concussion is also present in a general population as concussion and/or musculoskeletal injuries can occur outside of elite athletic environments.[18] However, geographically-based epidemiological data are lacking to support previous findings of increased odds of musculoskeletal injury after concussion. Also, most studies have included any injury, or only limited the injuries to the lower extremity.[19-28] Given the increasing number of children who participate in athletics, and the increasing population of fitness enthusiasts, researchers should examine the general population-based odds of ACL injury after concussion.[16] Therefore, the current study sought to perform a population-based cohort study that relates concussion to a specific musculoskeletal injury known to be modulated by neuromuscular control. The general population is from a single geographic region that is generalizable to the larger population.[29] It was hypothesized that patients with a concussion have increased odds of ACL injury within three years after the concussion compared to patients without history of concussion. It is important to identify this relationship in order to make clinicians aware of potential sub-clinical threshold changes after concussion that lead to increased risk of musculoskeletal injury upon return to normal activities of daily living in the general population.

2. Methods

The Rochester Epidemiology Project (REP) is a medical record linkage system that provides access to complete medical records for all residents of Olmsted County, Minnesota, USA, regardless of the medical facility in which the healthcare was delivered.[29-32] The REP database has included over 6.1 million health records since its creation. This unique population-based data infrastructure allows for the complete determination and follow-up of all clinical diagnoses in a geographically defined community.[30] Moreover, REP epidemiological data have demonstrated generalizability to larger populations.[29] Institutional review board approval was obtained from both the Mayo Clinic and the Olmsted Medical Center. All patients provided general research authorization for use of their medical records at the time of medical care.

The REP database was queried for all occurrences of mTBIs, concussions, and ACL tears (based on International Classification of Diseases, 9th Revision [ICD-9], 10th Revision [ICD-10] diagnosis codes) from 2000 to 2017. Moderate and severe TBIs were excluded. While it would be preferable to match cases to controls by activity level, activity level is not consistently documented in the medical record; therefore, age was utilized as a surrogate for activity level (Table 1).[33] Records were included if evidence of an ACL tear (inclusive of grade I-grade III) existed based on clinical orthopedic examinations, arthroscopic examinations, magnetic resonance imaging (MRI), or operative reports of an ACL reconstruction.[33] The results yielded 1,653 patients in the defined population with an ACL injury (Figure 1). Roughly 81% of these patients had an ACL reconstruction, which aligns with previously published studies on this cohort.[34] These 1,653 patients were matched to Olmsted County residents identified in the REP database without an ACL diagnostic code in their medical record using a computerized matching algorithm. Each case was matched to three controls based on age (± 1 year, >99%), sex (exact, 100%), and event date (± 1 year, 100%), where the event date is the date of ACL injury for the cases or the REP availability date for the matched controls. A 1:3 case-control matching was performed to achieve sufficient statistical power,[35] which resulted in 1,653 ACL injury cases and 4,959 matched controls.[35]

Table 1:

Demographic comparison between ACL cases and matched controls with no ACL injury.

ACL Control
n (M/F) 1,653 (924/729) 4,959 (2,772/2,187)
Number with concussions 39 73
Age in years (SD) 30.8 (13.1) 30.7 (13.0)
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ACL, anterior cruciate ligament; M, male; F, female; SD, standard deviation.

Figure 1:

Figure 1:

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Flow chart to identify ACL injury cases in the REP database, then identification of matched controls subjects. Both case and control medical records were checked for diagnosis of an mTBI or concussion within the three years prior to the event date. REP, Rochester Epidemiology Project.

REP database query results for patients with both concussion and ACL diagnostic codes were searched for confirmation of an mTBI or concussion diagnosis. Two authors (ALM, MBS) reviewed all medical records to confirm the concussion diagnosis. A patient was included if their medical record contained documentation of an mTBI or concussion diagnosis by a doctor within three years prior to the ACL injury. Clinical diagnosis followed standard consensus statements for the respective time (e.g. Consensus Statement on Concussion in Sport: The 5th International Conference on Concussion in Sport, Berlin, October 2016).[36] Patients were excluded from analysis if the concussion and ACL injury event dates were the same or greater than three years apart to ensure all cases near the two year time point were captured. Previous studies have follow-up time frames that range from 90 days to two years post-concussion and have reported increased risk of injury after concussion within this timeframe.[19, 20, 23] In addition, a three year timeframe was selected to extend the current literature and to determine if increased risk of injury is longer than what previous studies have captured. The records of the matched controls were cross-referenced to verify whether an mTBI or concussion diagnosis was recorded in their medical record within three years prior to their match date. Matched controls were excluded if a moderate or severe TBI was documented within three years prior to their match date.

The data were summarized using means and standard deviations for continuous variables, and counts and percentages for categorical variables. The study was designed as a matched case-control study in which the outcome was defined as ACL injury and the exposure was defined as a concussion in the prior three years. The association of concussion and ACL injury was evaluated using logistic regression. The model was adjusted for age, sex, and year of ACL injury or control availability. Additional logistic regression models were generated for males and females separately, as well as by age groups. The results were summarized using odds ratios (OR) and reported with 95% confidence intervals. The association was considered significant if p ≤ 0.05 and the 95% confidence interval (CI) for the odds ratio did not contain 1. Assumptions were checked and model diagnostics were examined to verify model validity. All analyses were conducted using JMP 14 and SAS version 9.4 (SAS Institute, Inc., Cary, NC).

3. Results

Each ACL case (n = 1,653) was matched to three healthy control subjects (n = 4,959) from the REP population database (Table 1). 56% of the ACL cases (n = 924) and matched control subjects (n = 2,772) were male. Within the time frame queried, there were 1,653 ACL injuries, of which 39 cases of ACL injury after a concussion were confirmed (Figure 2). 54% of ACL case concussions were attributed to a sporting mechanism compared to 48% of the control concussions (Figure 3). The average time between concussion and ACL Injury in the current cohort (n = 39) was 1.43 (± 0.9) years. The rate of prior concussion was higher in patients with an ACL injury (2.4%) compared to matched controls with no ACL injury (1.5%). This corresponds to an odds ratio of 1.6 (95% CI 1.1-2.4), p = 0.015, which indicates a significant association between concussion and subsequent ACL injury (Tables 2-3).

Figure 2:

Figure 2:

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Flow chart of ACL injury case identification and exclusion.

Figure 3:

Figure 3:

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Distribution of concussion mechanism between ACL cases and matched control subjects with no ACL injury. Undetermined was selected if the medical record gave no indication of how the concussion injury was sustained. Non-sporting mechanism included, but was not limited to, motor vehicle accidents, assault, and falls.

Table 2:

Adjusted logistic regression models for the overall group, and for sex-specific models.

Adjusted logistic regression models
Group Model Effects Odds Ratio (95% CI) p-value
Overall Concussion 1.6 (1.10, 2.43) 0.014
Age 1.0 (1.00, 1.01) 0.588
Sex 1.0 (0.89, 1.12) 0.999
Calendar year 1.0 (0.99, 1.01) 0.820
Males Concussion 2.0 (1.19, 3.25) 0.0 08
Age 1.0 (1.00, 1.01) 0.551
Calendar year 1.0 (0.98, 1.01) 0.809
Females Concussion 1.2 (0.64, 2.36) 0.535
Age 1.0 (1.00, 1.01) 0.836
Calendar year 1.0 (0.98, 1.02) 0.946
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Table 3.

Concussion and ACL injury.

No concussion Concussion Odds Ratio (95% CI) p-value
ACL Injury (n=1653) 1,614 (97.6%) 39 (2.4%) 1.6 (1.1, 2.4) 0.014
Control (n=4959) 4,886 (98.5%) 73 (1.5%) Ref
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ACL, anterior cruciate ligament; CI, confidence interval.

Sex-specific analysis revealed that the rate of ACL injury was higher in concussed males (2.8%) than non-concussed males (1.5%), which corresponds to an odds ratio of 2.0 (95% CI 1.2-3.3; p = 0.008) (Table 2, Figure 4). The association between concussion and subsequent ACL injury was not significant in females, with an odds ratio of 1.2 (95% CI 0.6-2.4; p = 0.535). For age-group analysis, the association between concussion and subsequent ACL injury was significant only for the 16 years old and younger age group (OR = 2.9, 95% CI 1.5-5.9; p = 0.002) (Figure 4).

Figure 4:

Figure 4:

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Forest plot shows the odds ratio for the adjusted logistic regression models for the overall group, each sex-specific, and each age-group analyses.

4. Discussion

Previous studies have identified increased risk of lower extremity musculoskeletal injury in athletic and active military populations, but not specifically for ACL injuries, and not in a general population.[4, 5, 19-28] The current study confirms the hypothesis that there are increased odds of ACL injury after a concussion in a population-based cohort. The odds ratio of sustaining an ACL injury within three years after a concussion observed in this study (OR = 1.6, 95% CI 1.1-2.4) is consistent with prior reported statistics of any lower extremity injury after a concussion.[19-28] Given the substantial body of evidence that shows the majority of ACL injuries occur by a non-contact mechanism[37, 38] and that ACL rupture has been linked with altered neuromuscular control[10, 39], it follows that after a concussion the odds of an ACL injury are increased. The resultant neurocognitive and neuromuscular dysregulation from the concussion may contribute to the increased risk of lower extremity injury previously established[4, 40], and now specifically increased odds of ACL injury.

A three year window was selected for inclusion criteria for ACL injury after a concussion in the current study to extend the window of time previous studies have utilized to investigate the association between concussion and injury. Previous studies have follow-up time frames that range from 90 days to two years post-concussion and have reported increased risk of injury after concussion within this time frame.[19, 20, 23] The average time between concussion and ACL Injury in the current cohort (n = 39) was 1.43 (± 0.9) years, which falls within the range of the previous studies’ follow-up window. This suggests that the average time from concussion to lower extremity injury, and specifically ACL injury, is between six months and two years; accordingly, this is the time period that should be monitored closely.

As the research community and the media have focused on the effects of concussions, clinical emphasis has also focused on assessment of individuals for appropriateness for return to activity.[11, 12, 41-47] Players, parents, coaches, and physicians appropriately assess the risk of long-term neurologic deficits in consideration of return to sport after a concussion. Clinical (e.g. Standardized Assessment for Concussion [SAC], Sport Concussion Assessment Tool [SCAT]) and computer assessments (e.g. ImPACT, Cogstate) for concussion diagnosis and monitoring of concussion symptoms have been developed, but usually only assess mental status as well as cognitive and physical performance.[48-51] In addition, computerized neurocognitive testing can reveal deficits in mental function and processing (such as processing speed and reaction time); however, they may have limited utility if a patient-specific baseline score has not been established prior to injury.[48, 51]

Clinical neurological assessment is recommended to also include measures of sensorimotor, function, coordination, gait, and balance.[36] In a review on neuromuscular control deficits after concussion, divided attention was concluded to be a likely contributing factor to risk of future injury.[40] Alternatively, Eagle et al. postulate a direct perception theory, in which concussion dysregulates a “continuous perception-action coupling loop.”[52] This theory states that a concussion prevents an athlete (or in the current study, a member from the general population) from appropriately perceiving opportunities from physical action in the dynamic environment around them, thus increasing their risk of a new injury.[52] Accordingly, it was recommended that multifactorial assessments are used to assess patients’ readiness for return to play after a concussion.[40]

Concussion symptoms and severity vary between individuals, which confounds diagnosis and determination of physiological recovery.[53] However, it is clear from this study and others[4] that all concussed individuals are at an increased risk of lower extremity injuries, specifically injury to the ACL. Thus, consideration and assessment of neuromuscular control should be incorporated in addition to concussion symptom resolution for all patients, not specifically just athletes, who desire to return to activity. For example, clinical evaluations could incorporate a divided-attention component to a task (e.g. gait analysis, drop vertical jump) to assess neuromuscular control after a concussion.[40, 54, 55]

Research on athletes has highlighted post-concussive symptoms and changes in gait, balance, and biomechanics in athletic tasks.[40, 54, 56-59] Moreover, the current study will facilitate improved understanding of the epidemiological repercussions of mTBI beyond cognitive performance. Elite athletes, trainers, coaches, physicians and now parents, weekend-warriors, and laborers should take precautions in return-to-activity with the understanding that they are at increased risk for a major lower extremity musculoskeletal injury after concussion. Based on the current and previous evidence of increased risk of musculoskeletal injury after concussion, clinical assessments should consider concussion symptom resolution as well as neuromuscular risk factors associated with lower extremity musculoskeletal injuries and specifically acute ACL injuries. However, future research into appropriate screening and assessment of neuromuscular risk factors for ACL injury following a concussion is necessary to better understand the underlying cause of increased ACL injury risk.

The current study is not without limitations. First and foremost, while the REP database is comprehensive in its coverage of individuals within the defined geographic boundaries, the data is only as good as what is charted. While the diagnosis of ACL either by clinical exam, MRI imaging, or operative reports is reliable within the time frame of our study, the same cannot be said about concussion diagnosis. Awareness of concussion among the public, and the ability and willingness of clinicians to specifically diagnose it within the medical record, is unmeasurable and thus could be severely under reported–especially in cases of mTBI. In addition, concussion diagnosis was not standardized (e.g. a validated checklist). Clinical diagnosis by a doctor followed guidelines recommended from the appropriate consensus statement for concussion in sport.[36] Also, the current study did not match ACL cases by body mass index, activity level, or activity exposure as it was not contained within the medical record. This information could alter the perceived odds of sustaining an ACL injury after concussion, likely with an odds increase for higher body mass index, activity level, and athletic exposures. However, age is a surrogate for activity level as older individuals are less likely to be involved in competitive athletics and are typically more sedentary.[33] In addition, mechanism of ACL injury could not be included in analysis, as it was not reliably documented in the medical records. Consideration of ACL injury mechanism (contact vs. non-contact) may provide additional insight into how concussion affects ACL injury risk. Finally, the study could not account for previous injury or concussion sustained by individuals that was not documented in the REP and medical records.

5. Summary and Conclusions

In conclusion, in a geographical population the current study reported that all individuals have increased odds (1.6) of an ACL injury within three years post-concussion. While activity exposure or activity level after concussion could not be accounted for in the current study, the results of this study with a large sample size support the existing literature and further confirm an increased risk of musculoskeletal injury after a concussion.

Key Points:

  • This is the first study to identify increased odds of specifically an ACL injury within the three years following a concussion in a geographically-representative population. Increased risk of lower extremity musculoskeletal injuries after a concussion has previously been established in specific groups of athletes.

  • Clinicians should incorporate neuromuscular assessments as part of return-to-activity from a concussion for all patients, not just athletes.

Acknowledgments

Funding: Fellowship funding was provided by the Mayo Clinic Graduate School of Biomedical Sciences [ALM]. Funding for this research was received from the National Institute of Arthritis and Musculoskeletal and Skin Diseases (R01AR55563 [TEH] and L30AR070273 [NDS]) and the National Institute of Children and Human Development (K12HD065987 [NDS]). This study was also made possible using the resources of the Rochester Epidemiology Project, which is supported by the National Institute on Aging of the National Institutes of Health under award R01AG034676.

Footnotes

Publisher's Disclaimer: This Author Accepted Manuscript is a PDF file of an unedited peer-reviewed manuscript that has been accepted for publication but has not been copyedited or corrected. The official version of record that is published in the journal is kept up to date and so may therefore differ from this version.

Ethical Approval: Institutional review board approval was obtained from both the Mayo Clinic (IRB# 18-001196) and the Olmsted Medical Center (005-OMC-18) and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. This article does not contain any studies with human participants performed by any of the authors. All patients provided general research authorization for use of their medical records at the time of medical care. All authors have no conflicts of interest to declare.

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