Craig J. McKinnon

Rhabdomyolysis in the Us Active Duty Army, 2004–2006

PURPOSE Rhabdomyolysis (RM) is a skeletal muscle disorder resulting in severe cellular injury caused by vigorous physical activity and other systemic etiologies. RM is associated with significant morbidity, such as acute renal failure, and can be fatal. RM that occurs in the US Active Duty Army (ADA) results in time lost from training, deployment, and combat. We sought to systemically describe the epidemiology of ADA clinical RM by quantifying RM in terms of absolute numbers, examine rate trends, and identify soldiers at elevated risk. METHODS We used data from the Total Army Injury and Health Outcomes Database to calculate yearly RM rates in the overall ADA, as well as adjusted RM rates within soldier subpopulations for 2003-2006. RESULTS During this period, the absolute numbers of clinically diagnosed ADA RM ranged between 382 and 419 cases per year. Annual rates were 7-8 per 10,000, which is 300%-400% higher than the estimated US civilian population (2 per 10,000). In soldiers with a history of a prior heat injury, RM rates climbed to 52-86 per 10,000, a 7- to 11-fold increase. Increased RM rates were seen in soldiers who are male, African American, younger, less educated, and with a shorter length of service. Approximately 8% of yearly ADA RM cases resulted in acute renal failure, an estimate lower than that for the US civilian population. CONCLUSIONS Our findings suggest that rates of RM are higher in the ADA than in the US civilian population. Rates remained fairly stable; however, relative to other ADA soldiers, those with prior heat injury, who are African American, or who have a length of service of less than 90 d are at the highest risk for RM development.

Incidence of Shoulder Dislocations and the Rate of Recurrent Instability in Soldiers.

PURPOSE Shoulder dislocations present a potentially debilitating injury for soldiers and other groups of physically active adults. The purpose of this study was to determine the 10-yr incidence rate of shoulder dislocations in soldiers, the percentage with recurrent instability, and risk factors for these injuries. METHODS This retrospective cohort study used medical encounter data from U.S. Army soldiers to calculate the 10-yr incidence rate for shoulder dislocations and the percentage of chronic or recurrent injuries >3 months and ≤2 yr after the initial diagnosis. A Cox proportional hazards model was constructed using demographic variables (age, race, education level, marital status, and sex) to determine incidence rate ratios for risk factors related to shoulder dislocation. Logistic regression was used to calculate odds ratio for risk factors for recurrent injury, including concurrent diagnoses (brachial plexus or peripheral nerve injuries and fractures of the scapula or proximal humerus). RESULTS There were 15,426 incident shoulder dislocations, with a 10-yr incidence rate of 3.13 per 1000 person-year. Soldiers ≤40 yr old showed greater risk for injury compared with those older than 40 yr. The incidence rate ratio for males compared with females was 1.64, 95% confidence interval = 1.55-1.74. Recurrent injury occurred in 28.7% of cases. Concurrent axillary nerve injury (odds ratio = 3.64, 95% confidence interval = 1.56-8.46) and age ≤35 yr were associated with greater risk of recurrence. CONCLUSION Within the active duty U.S. Army, men and younger individuals showed greater risk for shoulder dislocations. Over one-quarter of incident cases became recurrent. Axillary nerve injuries and younger age increased the odds of recurrent injury.

Rates of Knee Injury in the U.S. Active Duty Army, 2000–2005

We sought to summarize knee injuries (KI) in the U.S. Active Duty Army (ADA) in terms of absolute numbers, examine current rate trends, and identify ADA who were at increased risk for experiencing a KI. We used the Total Army Injury and Health Outcomes Database (TAIHOD) to compute unadjusted and adjusted rates of KI, categorized by the Barell Matrix, within the ADA for the years 2000-2005. During this period, 21 to 25 per 1,000 ADA suffered from KI. The highest yearly rates were observed for knee dislocation and sprains/strains (31 per 1,000 ADA). In ADA with a history of a KI (within 2 years), rates increased nearly tenfold. Elevated KI rates were also seen in ADA with prior upper or lower leg injuries, those > 30 years of age, and those with a category IV Armed Forces Qualification Test score (lowest admissible in Army). ADA KI rates remained fairly stable throughout the study period. Relative to other ADA Soldiers, those with prior knee, upper leg, or lower leg injuries are at increased risk for subsequent KI.

Trends in overweight and obesity in soldiers entering the US Army, 1989-2012.

The US Army recruits new soldiers from an increasingly obese civilian population. The change in weight status at entry into the Army between 1989 and 2012 and the demographic characteristics associated with overweight/obesity at entry were examined.

Risk of Stress Fracture Varies by Race/Ethnic Origin in a Cohort Study of 1.3 Million U.S. Army Soldiers†

Stress fractures (SF) are common and costly injuries in military personnel. Risk for SF has been shown to vary with race/ethnicity. Previous studies report increased SF risk in white and Hispanic Soldiers compared with black Soldiers. However, these studies did not account for the large ethnic diversity in the US military. We aimed to identify differences in SF risk among racial/ethnic groups within the US Army. A retrospective cohort study was conducted using data from the Total Army Injury and Health Outcomes Database from 2001 until 2011. SF diagnoses were identified from ICD‐9 codes. We used Cox‐proportional hazard models to calculate time to SF by racial/ethnic group after adjusting for age, education, and body mass index. We performed a sex‐stratified analysis to determine whether the ethnic variation in SF risk depends on sex. We identified 21,549 SF cases in 1,299,332 Soldiers (more than 5,228,525 person‐years of risk), revealing an overall incidence rate of 4.12 per 1000 person‐years (7.47 and 2.05 per 1000 person‐years in women and men, respectively). Using non‐Hispanic blacks as the referent group, non‐Hispanic white women had the highest risk of SF, with a 92% higher risk of SF than non‐Hispanic black women (1.92 [1.81–2.03]), followed by American Indian/Native Alaskan women (1.72 [1.44–1.79]), Hispanic women (1.65 [1.53–1.79]), and Asian women (1.32 [1.16–1.49]). Similarly, non‐Hispanic white men had the highest risk of SF, with a 59% higher risk of SF than non‐Hispanic black men (1.59 [1.50–1.68]), followed by Hispanic men (1.19 [1.10–1.29]). When examining the total US Army population, we found substantial differences in the risk of stress fracture among racial/ethnic groups, with non‐Hispanic white Soldiers at greatest risk and Hispanic, American Indian/Native Alaskan, and Asian Soldiers at an intermediate risk. Additional studies are needed to determine the factors underlying these race‐ and ethnic‐related differences in stress fracture risk.

BMI and Lower Extremity Injury in U.S. Army Soldiers, 2001–2011

INTRODUCTION Little data exist regarding the long-term impact of excess weight on lower extremity musculoskeletal injury/disorder (MID) in U.S. Army Soldiers. This prospective analysis examines the association between BMI of Soldiers at accession and risk of MID. METHODS A total of 736,608 Soldiers were followed from accession into the Army, 2001-2011. Data were analyzed January through March 2015. MID was categorized as any first incident lower extremity musculoskeletal injury/disorder, and secondarily, as first incident injury/disorder at a specific site (i.e., hips, upper legs/thighs, knees, lower legs/ankles, feet/toes). Multivariable-adjusted proportional hazards models estimated associations between BMI category at accession and MID risk. RESULTS During 15,678,743 person-months of follow-up, 411,413 cases of any first MID were documented (70,578 hip, 77,050 upper leg, 162,041 knee, 338,080 lower leg, and 100,935 foot injuries in secondary analyses). The overall MID rate was 2.62 per 100 person-months. Relative to Soldiers with normal BMI (18.5 to <25 kg/m(2)) at accession, those who were underweight (<18.5); overweight (25 to <30); or obese (≥30) had 7%, 11%, and 33% higher risk of MID, respectively, after adjustment. Risks were highest in Soldiers who were obese at accession, and lowest in those with a BMI of 21-23 kg/m(2). CONCLUSIONS Soldier BMI at accession has important implications for MID. A BMI of 21-23 kg/m(2) in newly accessing Soldiers was associated with the lowest risk of incident MID, suggesting that accession be limited to people within this range to reduce overall incidence of MID among service personnel.

Body Mass Index at Accession and Incident Cardiometabolic Risk Factors in US Army Soldiers, 2001-2011.

Individuals entering US Army service are generally young and healthy, but many are overweight, which may impact cardiometabolic risk despite physical activity and fitness requirements. This analysis examines the association between Soldiers’ BMI at accession and incident cardiometabolic risk factors (CRF) using longitudinal data from 731,014 Soldiers (17.0% female; age: 21.6 [3.9] years; BMI: 24.7 [3.8] kg/m2) who were assessed at Army accession, 2001–2011. CRF were defined as incident diagnoses through 2011, by ICD-9 code, of metabolic syndrome, glucose/insulin disorder, hypertension, dyslipidemia, or overweight/obesity (in those not initially overweight/obese). Multivariable-adjusted proportional hazards models were used to estimate hazard ratios (HR) and 95% confidence intervals (CI) between BMI categories at accession and CRF. Initially underweight (BMI<18.5 kg/m2) were 2.4% of Soldiers, 53.5% were normal weight (18.5−<25), 34.2% were overweight (25−<30), and 10.0% were obese (≥30). Mean age range at CRF diagnosis was 24–29 years old, with generally low CRF incidence: 228 with metabolic syndrome, 3,880 with a glucose/insulin disorder, 26,373 with hypertension, and 13,404 with dyslipidemia. Of the Soldiers who were not overweight or obese at accession, 5,361 were eventually diagnosed as overweight or obese. Relative to Soldiers who were normal weight at accession, those who were overweight or obese, respectively, had significantly higher risk of developing each CRF after multivariable adjustment (HR [95% CI]: metabolic syndrome: 4.13 [2.87–5.94], 13.36 [9.00–19.83]; glucose/insulin disorder: 1.39 [1.30–1.50], 2.76 [2.52–3.04]; hypertension: 1.85 [1.80–1.90], 3.31 [3.20–3.42]; dyslipidemia: 1.81 [1.75–1.89], 3.19 [3.04–3.35]). Risk of hypertension, dyslipidemia, and overweight/obesity in initially underweight Soldiers was 40%, 31%, and 79% lower, respectively, versus normal-weight Soldiers. BMI in early adulthood has important implications for cardiometabolic health, even within young, physically active populations.

Risk Factors for Rhabdomyolysis in the U.S. Army

The standardized mortality rate of rhabdomyolysis (RM) in Active Duty U.S. Army Soldiers is considerably higher than in the civilian population. RM occurs when large amounts of intracellular contents from damaged skeletal muscle escape into circulation, leading to serious sequelae (e.g., acute renal failure, hyperkalemia, compartment syndrome). Extended physical exertion, especially in hot environments, and trauma can precipitate RM. The aim of this study was to identify RM risk factors among U.S. Active Duty Army (ADA) Soldiers. METHODS This nested case-control study used data from the Total Army Injury and Health Outcomes Database (years 2004-2006) to examine RM among ADA male Soldiers. Demographic and occupational variables were identified as potential risk factors. Each RM case was age and date-matched to 4 controls. Adjusted odds ratios (OR) were computed using conditional logistic regression analyses. RESULTS From years 2004 to 2006, 1,086 Soldiers (0.19%) met the study criteria for clinically diagnosed RM. Three variables were found to increase the odds of acquiring RM: (1) prior heat stroke, OR 4.95 (95% confidence interval [CI] 1.1-21.7); (2) self-reported Black race, OR 2.56 (95% CI 2.2-3.0); and (3) length of service (0-90 days), OR 2.05 (95% CI 1.6-2.7). CONCLUSION There is a substantially greater likelihood for male U.S. Army Soldiers to develop RM who: (1) have had a prior heat injury, (2) self-report in the Black racial category, and (3) who are within the initial 90 days of service. Greater awareness of the risk factors associated with RM may improve force health protection and readiness through targeted mitigation strategies.

Risk Factors for Training-Related Injuries during U.S. Army Basic Combat Training

Background Injuries during basic combat training (BCT) impact military health and readiness in the U.S. Army. Identifying risk factors is crucial for injury prevention, but few Army-wide studies to identify risk factors for injury during BCT have been completed to date. This study examines associations between individual and training-related characteristics and injuries during Army BCT. Methods Using administrative data from the Total Army Injury and Health Outcomes Database (TAIHOD), we identified individuals who apparently entered BCT for the first time between 1 January 2002 and 30 September 2007, based on review of administrative records. Injuries were identified and categorized based on coded medical encounter data. When combined with dates of medical services, we could count injuries per person, identify unique injuries, and identify the quantity and type of medical care delivered. Regression models produced odds ratios (ORs) and 95% confidence intervals (CIs) to identify risk factors for injury during BCT (yes/no), adjusted for potential confounders. Results Of the 278,045 (83.4%) men and 55,302 (16.6%) women who were apparently first-time trainees, 39.5% (n = 109,760) of men and 60.9% (n = 33,699) of women were injured during training based on over 2 million recorded medical encounters entries. The large cohort yielded statistically significant, small magnitude associations between injury and all individual and training-related covariates for men, and all but medical accession waivers and weight for women. After adjustment, largest magnitude effects among men were due to age > 25 yr vs. 17-18 yr (OR = 1.83, 95% CI: 1.75, 1.91); having been married in the past vs. being single (OR = 1.36, 95% CI: 1.24, 1.49); rank E4-E7 vs. E1 (OR = 0.56, 95% CI: 0.53. 0.59); training at Ft. Jackson (OR = 0.66, 95% CI: 0.64, 0.69), Ft. Leonard Wood (OR = 0.67, 95% CI: 0.65, 0.70), or Ft. Knox (OR = 0.69, 95% CI: 066, 0.72) vs. Ft. Benning. Odds of injury were highest during 2005, 2006, and 2007. After adjustment for weight and body mass index, taller men had higher odds of BCT injury than average height men (OR = 1.08, 95% CI: 1.05, 1.11). Among women, short stature (OR = 1.11; 95% CI: 1.04, 1.19), training at Ft Leonard Wood (OR = 1.10; 95% CI: 1.04, 1.16) and evidence of injury prior to training based on accession waiver (OR = 1.12; 95% CI: 1.00, 1.26) increased injury risk. Conclusions This Army-wide analysis reveals higher BCT-related injury rates for both men and women than prior studies and identifies risk factors for injuries during BCT. The large data set allows adjustment for many covariates, but because statistical analysis may yield significant findings for small differences, results must be interpreted based on minimally important differences determined by military and medical professionals. Results provide information that may be used to adapt training or medical screening and examination procedures for basic trainees.

Risk for Lower Extremity Injury After Concussion: A Matched Cohort Study in Soldiers

• BACKGROUND: Rates of lower extremity musculoskeletal injury are reportedly higher in professional and collegiate athletes following concussions. However, there is a paucity of evidence on this relationship in individuals who are not high‐level athletes. • OBJECTIVES: To examine the risk of acute lower extremity musculoskeletal injury in soldiers within 2 years of an incident concussion, compared to matched nonconcussed soldiers. • METHODS: This was a matched‐cohort study that used the medical encounter and personnel data of active‐duty US Army soldiers from 2005 to 2011. Incident concussions were identified using International Classification of Diseases‐Ninth Revision codes in medical encounter data of all soldiers from 2005 to 2009. One nonconcussed soldier in the US Army during the same month was matched by age, sex, rank, length of service, deployment status, and military career field to each concussed soldier. Hazard ratio (HR) and 95% confidence interval (CI) were calculated for the risk of lower extremity injury within 2 years of the incident concussion. Monthly HRs were compared to identify differences in injury rates between the groups, and an HR for the period of greatest difference was also calculated. • RESULTS: A total of 23 044 individuals (11 522 concussed and 11 522 nonconcussed) were included in the study. Within 2 years of concussion, the hazard of lower extremity injury was 38% greater in concussed compared to nonconcussed soldiers (HR, 1.38; 95% CI: 1.30, 1.46), while the 15‐month hazard was 45% greater (HR, 1.45; 95% CI: 1.36, 1.56). • CONCLUSION: The rate of lower extremity musculoskeletal injury among this population of physically active adults is higher following concussion, and the risk remains elevated for more than a year following injury. • LEVEL OF EVIDENCE: Prognosis, level 2b.