Ricardford R. Connor, MPH* ; MAJ Michael R. Boivin, MC USA*; Elizabeth R. Packnett, MPH* ; Christine F. Toolin, MS* ; David N.

MILITARY MEDICINE, 181, 11/12:e1532, 2016 The Relationship Between Deployment Frequency and Cumulative Duration, and Discharge for Disability Retirement Among Enlisted Active Duty Soldiers and Marines Ricardford R. Connor, MPH* ; MAJ Michael R. Boivin, MC USA*; Elizabeth R. Packnett, MPH* ; Christine F. Toolin, MS* ; David N. Cowan, PhD, MPH* ABSTRACT Background: The frequency and duration of deployments associated with increased morbidity is a significant concern for force health protection within the military population. Understanding the association between deployment and disability may provide a clearer understanding of factors adversely affecting U.S. military force readiness. Methods: A case control analysis was conducted using records on enlisted active duty personnel in the Army and Marine Corps who were evaluated for a musculoskeletal disability and received a final disability disposition between FY 2003 and 2012. The study compared deployment, deployment frequency, and total time deployed in personnel who received musculoskeletal disability retirement to those with a musculoskeletal disability discharge other than retirement. Results: For females and males in either service, any deployment was associated with an increased risk of disability retirement (adjusted odds ratios [aor] [95% confidence intervals (CI)]: males 1.76 [1.65 1.87]; females 1.41 [1.21 1.64]). Furthermore, increasing number of deployments (3+ deployments males aor [95% CI]: 2.21 [1.92 2.53]) and time spent deployed (24+ months Army s aor [95% CI]: 2.07 [1.79 2.40]) significantly increased the odds for disability retirement. Conclusion: Increasing frequency and duration of military deployments has an increased risk of disability retirement in service members with a musculoskeletal disability. Further research on this relationship is needed in a more representative sample of the U.S. military population. INTRODUCTION With military operations in Afghanistan (Operation Enduring Freedom) and Iraq (Operation Iraqi Freedom/New Dawn) in the last decade, there has been concern over adverse health effects that may be associated with the increased frequency and duration of military deployments. 1 3 During this time period, more than two million U.S. service members were deployed; of those, approximately 50% were deployed more than once. 4 Deployments generally range from 4 to 15 months depending on service and year of deployment. 4,5 Several characteristics of deployments may influence the health of the deployed population, including the total duration, intensity, location, and type of deployment (i.e., combat-related, peace keeping, or disaster relief). 2,6 Deployment activities and experiences are also unique to each military service and to each theater. Air Force and Navy service members historically have had fewer deployments and are deployed for shorter durations. Army Soldiers are usually deployed for periods up to a year and Marines are usually rotated after periods of 6months. 4 Members of the Army and Marine Corps usually *Preventive Medicine Branch, Walter Reed Army Institute of Research, 503 Robert Grant Avenue, Silver Spring, MD 20910. ManTech International Corporation, ManTech Health, 13755 Sunrise Valley Drive, Suite 500, Herndon, VA 20171. Material has been reviewed by the Walter Reed Army Institute of Research. There is no objection to its publication. The opinions or assertions contained herein are the private views of the authors, and are not to be construed as official, or as reflecting true views of the Department of the Army or the Department of Defense. This manuscript has not been previously published or presented, either in whole or in part. doi: 10.7205/MILMED-D-16-00016 experience more combat exposure and are deployed more frequently than personnel in the other services. 4,7 Deployment can expose service members to many physical stressors, such as temperature extremes, lifting and/or carrying heavy loads, and patrolling for lengthy periods. 8 These stressors may induce many physical strains on the deployed population. 9 11 Musculoskeletal conditions are the leading cause of medical evacuation from theater and are among the most commonly reported diagnoses for military personnel returning from deployment. 11 Bell et al 12 described the trends in disability within the U.S. Army from 1981 to 2005 and found a rising rate of service-related musculoskeletal disability discharge. Cross et al 10 in a sample of the Army disability population described 84% of Soldiers evacuated from combat theater having at least one musculoskeletal/orthopedic-related disabling condition with more than 50% of evacuations solely attributed to musculoskeletal injury. In similar research, Gubata et al 13 reviewed the trends of disability evaluations and retirements in the Army, Navy, and Marine Corps from 2005 to 2011 and found that, although rates of disability evaluations decreased, severity of disabilities increased, with elevated rates of combat-related disability evaluations and retirements for musculoskeletal disabilities. Many studies have examined combat deployments and subsequent disability and/or retirement, although studies of disability in the U.S. military have frequently examined musculoskeletal disability conditions, 9,10,14,15 research of morbidity following deployment have primarily focused on psychiatric conditions. 16 20 Deployment has been identified as a risk factor for disability in some studies, 14,17,20,21 although in others it has been shown as a protective factor in that e1532

individuals with pre-existing health conditions are less likely to be deployed. 9,22,23 Specific deployment characteristics and their effects on disability and/or retirement within the wider U.S. military population have not been well documented. A study of personnel in the British Armed forces (Royal Navy, Marines, Army, and Air Force) deployed to Afghanistan and Iraq determined that personnel with two or more deployments were 25% more likely than those with one deployment to suffer from multiple physical symptoms and those deployed for greater than 13 months were 45% more likely to suffer from similar physical symptoms compared to those deployed for shorter periods. 6 The primary objective of this case control study of Soldiers and Marines was to evaluate deployment, deployment frequency, and total time deployed in personnel who received musculoskeletal disability. Musculoskeletal disability retirement cases were compared to those with a nonretirement musculoskeletal disability discharge. A secondary aim of this analysis was to examine the inter-relationship between comorbidity and deployment history as a predictor of disability retirement and to show the extent to which these relationships are modified by sex. METHODS The disability evaluation process in the military is guided by both Department of Defense instructions 1332.38 24 and public law U.S. Code Title 10. 25 In the case of severe illness and/or injury, personnel in both the Army and Marine Corps enter the Disability Evaluation System when referred by a physician to the Medical Examination Board. At the Medical Examination Board, it is determined whether the military member meets medical retention standards; those who meet medical retention standards are returned to duty. If it is determined that the person no longer meets medical retention requirements, the case is sent to the Physical Examination Board where a determination on whether the member is fit orunfit is made. If fit, the member is returned to service, if unfit he/she is medically discharged. 24 Study Population Enlisted active duty personnel, in the Army and Marine Corps, who were evaluated for a musculoskeletal disability and received a final disability disposition between FY 2003 and 2012 were eligible for inclusion in this study. Officers and Reserve Component members were excluded from the study because of incomplete and missing data. Subjects placed on the temporary disability retirement list (TDRL) were excluded if their musculoskeletal injury was no longer considered disabling at their most recent evaluation (1,374 subjects excluded). Most individuals placed on the TDRL are permanently retired within 56 months of receiving their disposition. 26 Personnel with deployment begin dates that occurred after the disability evaluation, those with a deployment in a service different to the disability evaluation service and those who received a final disposition of separated without benefit or fit were excluded from the study population. Measures Disability conditions were determined using Veterans Affairs Schedule for Rating Disabilities (VASRD) codes. VASRD codes were developed to rate and compensate disability conditions; however, they are not considered diagnosis codes. 27 Specific VASRD codes were categorized into condition groups (i.e., musculoskeletal and other) utilizing the definitions outlined in the U.S.Code Title 10. 25 Retirement status was determined by the final disability percent rating; a disability percent rating greater than or equal to 30% was considered retired and a disability percent less than 30% was considered not-retired. Personnel who received a musculoskeletal disability retirement were defined as cases, and controls were defined as personnel with a musculoskeletal disability discharge who did not receive retirement. Individuals who do not receive retirement are not as severely disabled as those who receive retirement and also have a lower level disability rating. 27 Comorbidity was defined as having a musculoskeletal disability as well as one or more separately disabling conditions. Demographic characteristics at the time of disability evaluation and information pertaining to the disability evaluation (including disposition rating and the conditions by which deemed unfit for continued service) were collected from each service s disability evaluation agency. For service members placed on TDRL with more than one evaluation, demographic information was taken from their first record and data such as disposition, rating, and disability condition were taken from their final evaluation. Primary Service Occupation Codes from the Department of Defense were used to determine the military occupational specialty (MOS) of Soldiers at the time of his/her disability. Soldiers were classified as an MOS according to communication, health care, infantry, technical, or other. Service members were also classified into three groups on the basis of military rank (E1 E4, E5 E6, and E7 E9). Rank coincided with military experience: junior (E1 E4), mid-level (E5 E6), and senior (E7 E9). Deployment information was provided by the Defense Manpower Data Center Contingency Tracking System. Deployment exposure categories were defined as ever deployed (Yes/No), number of deployments (1, 2, and 3, or more), and total months deployed (<6 months, 6 11 months, 12 24 months, and 24+ months). Total months deployed was calculated as a summation of time elapsed from the begin date and end date of all deployments. Statistical Analysis The 10 most common musculoskeletal and comorbid conditions were identified and are listed in order of decreasing prevalence. Unadjusted ORs and 95% CIs were calculated to compare the comorbid conditions among cases and controls. Since VASRD e1533

codes assigned are not mutually exclusive, totals do not sum to 100%. Demographic characteristics of the study population are described by frequency tables stratified by sex. Bivariate and multivariate logistic regression models were used to calculate crude ORs and aors with 95% CIs to evaluate the relationship between deployment and disability retirement. Models, with the exposure groups for ever deployed and number of deployments, in addition to being stratified by sex were separated by individuals with only musculoskeletal disabilities compared to those with comorbidities. The models were controlled for age, race, rank, MOS, and missing MOS to adjust for the absence of MOS codes for personnel in the Marine Corps. As a result of differences in duration of deployments between the Army and Marine Corps, the models reported with the exposure group total months deployed were separated by service and each service was stratified by comorbidity status and sex. The adjusted models reported for the Army were controlled for age, race, rank, and MOS, although adjusted models reported for the Marine Corps were controlled for age, race, rank, and missing MOS. All statistical analyses were performed using SAS version 9.3 (SAS Institute, Cary, North Carolina). RESULTS Descriptive Analyses The study population included 60,389 Soldiers and Marines who were discharged for a disability resulting from a musculoskeletal condition; 30% (17,951) were identified as cases (retired) and 70% (42,438) were identified as controls (discharged and not-retired). The population was mostly white TABLE I. (overall 75%), ages 20 to 29, and included more males (82%) than females (18%). Leading VASRD codes for musculoskeletal disabilities included dorsopathies, arthritis, and limitation of motion (arthropathies); most notably among the musculoskeletal disabilities, cases were significantly more likely than controls to have amputations (OR: 17.94, 95% CI: 15.43, 20.87) and joint replacements (OR: 54.51, 95% CI: 25.57, 116.22). Leading nonmusculoskeletal disability conditions (comorbidities) were post-traumatic stress disorder (PTSD), paralysis, and residuals of traumatic brain injury (TBI); on the basis of unadjusted ORs and 95% CIs, cases were significantly more likely than controls to have each of the nonmusculoskeletal comorbidities (Table I). Over 75% of male cases were deployed sometime throughout their military career, compared to 47% of controls. cases were deployed more (46%) than controls (22%). More than half of all evaluated cases (disability retired) male or female had comorbidities, although fewer than 7% of male or female controls were affected by nonmusculoskeletal disability conditions (Table II). Regression Analyses Since estimates from both the crude and adjusted models were similar, only data from the adjusted models were reported. For males and females in either service, any deployment was associated with an increased risk of musculoskeletal disability retirement with aor of 1.76 (95% CI: 1.65 1.87) and 1.41 (95% CI: 1.21 1.64), respectively. Risk of musculoskeletal disability retirement increased with number of deployments (Table III). s without comorbidities and deployed three or Unadjusted Odds Ratios (OR) and 95% Confidence Intervals (CI) for 10 Leading Disabilities Among and n % n % OR 95% CI Musculoskeletal Conditions Dorsopathies 9,555 22.5 15,318 30.5 2.01 1.94, 2.09 Arthritis 4,376 10.3 16,785 33.5 0.49 0.47, 0.51 Limitation of Muscles (Arthropathies) 5,605 13.2 7,317 14.6 2.18 2.09, 2.27 Joint Disorders of Inflammation 1,461 3.4 2,420 4.8 1.47 1.37, 1.57 Skeletal and Joint Deformities 1,303 3.1 2,300 4.6 1.37 1.27, 1.47 Limitation of Motion of Muscles 1,128 2.7 1,640 3.3 1.67 1.54, 1.80 Amputations 1,373 3.2 195 0.4 17.94 15.43, 20.87 Osteopathies 233 0.6 1,315 2.6 0.41 0.36, 0.47 Joint Replacement 160 0.4 7 0 54.51 25.57, 116.22 Diseases and Injuries of the Muscles 28 0.1 17 0 3.90 2.13, 7.12 Other Comorbid Conditions Post-Traumatic Stress Disorder 5,411 12.8 376 0.8 48.27 43.40, 53.69 Paralysis 1,839 4.3 426 0.9 11.26 10.11, 12.53 Traumatic Brain Injury 1,618 3.8 121 0.2 34.65 28.78, 41.71 Mood Disorder 1,285 3 237 0.5 13.73 11.94, 15.79 Migraine 1,047 2.5 171 0.3 15.31 13.01, 18.01 Neuralgia 563 1.3 369 0.7 3.69 3.23, 4.21 Anxiety Disorders 679 1.6 91 0.2 18.29 14.69, 22.78 Neuritis 592 1 137 0.3 10.53 8.74, 12.69 Scars 425 1 72 0.1 14.27 11.11, 18.33 Asthma 398 0.9 92 0.2 10.44 8.31, 13.10 e1534

TABLE II. Characteristics of Musculoskeletal Disability and at Disability Evaluation by Sex n % n % n % n % Comorbidity No 5,233 33.4 31,498 93.7 933 40.6 8,388 94.9 Yes 10,419 66.6 2,104 6.3 1,366 59.4 448 5.1 Ever Deployed No 3,742 23.9 17,889 53.2 1,250 54.4 6,866 77.7 Yes 11,910 76.1 15,713 46.8 1,049 45.6 1,970 22.3 Age <20 158 1 1,352 4 41 1.8 968 11 20 29 8,748 55.9 23,866 71 1,194 51.9 6,076 68.8 30 39 5,063 32.4 7,496 22.3 743 32.3 1,546 17.5 40 1,683 10.8 888 2.6 321 14 246 2.8 Race White 12,131 77.5 26,147 77.8 1,324 57.6 5,930 67.1 Black 1,871 12 4,307 12.8 727 31.6 2,064 23.4 Other 1,641 10.5 3,122 9.3 243 10.6 835 9.5 Missing 9 0.1 26 0.1 5 0.2 7 0.1 Rank E1 E4 8,122 51.9 24,132 71.8 1,365 59.4 7,194 81.4 E5 E6 6,259 40 9,021 26.9 783 34.1 1,567 17.7 E7 E9 1,259 8 437 1.3 150 6.5 68 0.8 Missing 12 0.1 12 <0.1 1 0 7 0.1 Occupation Type Communications 1,293 8.3 2,683 8 43 1.9 540 6.1 Health Care 1,035 6.6 1,678 5 329 14.3 1,362 15.4 Infantry 4,741 30.3 8,175 24.3 25 1.1 181 2.1 Technical 2,656 17 7,424 22.1 520 22.6 1,777 20.1 Other 3,152 20.1 7,073 21.1 1,152 50.1 3,979 45 Missing a 2,775 17.7 6,569 19.6 230 10 997 11.3 a Marine Corps Occupation Type was not reported in the disability records before FY 2011. more times had twice the odds for musculoskeletal disability retirement than those who did not deploy. s with comorbidities who had three or more deployments were more than four times more likely than those never deployed to be medically retired for a musculoskeletal condition. Similar but less pronounced associations were also observed among females within these respective groups (Table III). s without comorbidities and deployed three or more times were more than 1.8 times more likely to receive musculoskeletal disability retirement, although females with comorbidities and three or more deployments were 3.5 times more likely to receive musculoskeletal disability retirement compared to females without a history of deployment. Among both male Soldiers and Marines, increasing months of deployment were associated with an increased likelihood of musculoskeletal disability retirement. This was true for those with and without comorbid conditions. Soldiers without comorbid disabilities and deployed for a total of 24+ months had twice the odds for musculoskeletal disability retirement, although male Soldiers with comorbidities deployed for the same were more than 4 times more likely to receive musculoskeletal disability retirement than those deployed for a total of less than 6 months (Table IV). Marines without comorbidities and deployed for 12+ months were 1.5 times more likely to receive retirement for a musculoskeletal condition and male Marines with comorbidities and deployed 12+ months had twice the odds for musculoskeletal disability retirement than those deployed for less than 6 months. Similar patterns were observed among female Soldiers, but not Marines. Like male Soldiers, female Soldiers deployed for 24+ months had twice the odds for musculoskeletal disability retirement than female Soldiers deployed for less than 6 months; female Soldiers with comorbidities like their male counterparts were also more than 4 times more likely than those deployed less than 6 months to receive the same outcome. There were no significant associations found between duration of deployment and musculoskeletal disability retirement among female Marines. DISCUSSION This study provides evidence that Soldiers and Marines who deployed and those who experienced multiple deployments or longer duration deployments were more likely to receive musculoskeletal disability retirement compared to their disability discharged counterparts who had never deployed. These relationships were true for both sexes and for those e1535

TABLE III. Adjusted Odds Ratios (aor) With 95% Confidence Intervals (CI) for Disability Retirement by Total Deployments, Stratified by Comorbidity and Sex, Army Soldiers and Marines Combined Musculoskeletal Disability (%) (%) (%) (%) n: 5,233 n: 31,498 aor a 95% CI n: 933 n: 8,388 aor a 95% CI Ever Deployed No 37 54.2 1 65.6 78.3 1 Yes 63 45.8 1.76 1.65 1.87 34.4 21.7 1.41 1.21 1.64 Deployments None 37 54.2 1 65.6 78.3 1 1 37.5 32 1.63 1.52 1.75 24.4 17.3 1.33 1.12 1.58 2 18.6 10.8 1.97 1.8 2.16 7.8 3.7 1.61 1.22 2.14 3+ 6.9 3 2.21 1.92 2.53 2.1 0.7 1.88 1.09 3.23 Musculoskeletal Disability Plus One or More Comorbid Conditions (%) (%) (%) (%) n: 10,419 n: 2,104 aor a 95% CI n: 1,366 n: 448 aor a 95% CI Ever Deployed No 17.3 38.8 1 46.7 67.2 1 Yes 82.7 61.2 3.07 2.76 3.4 53.3 32.8 2.13 1.68 2.69 Deployments None 17.3 38.8 1 46.7 67.2 1 1 42.9 38.9 2.64 2.36 2.96 35.6 25 1.93 1.5 2.49 2 27.3 16.2 3.76 3.25 4.34 12.8 6.3 2.61 1.69 4.02 3+ 12.5 6.1 4.22 3.44 5.18 4.9 1.6 3.5 1.57 7.83 a Model adjusted for age, race, rank, and Military Occupational Specialty. with and without comorbid conditions. Physical Examination Board reviews of musculoskeletal conditions that result in disability retirement are a general indication that the conditions are more serious or severe than reviews that result in a lesser degree of disability. Prior studies on the trends of disability in the Army and Marine Corps have noted a substantial increase in disability retirement as a result of a rise in combat-related disability. 13,28 This study extends the findings of prior research by providing a review of the impact of deployment frequency and total time spent deployed on the likelihood of receiving musculoskeletal disability retirement among the Army and Marine Corps disabled population. In this study, there was a clear positive linear association with increasing numbers of deployments and greater time spent deployed with the odds for receiving musculoskeletal disability retirement. The odds for musculoskeletal disability retirement were more pronounced among personnel identified with comorbidities compared to those without. Study results also indicated that a high proportion of cases with comorbidities were affected by PTSD and TBI. In 2008, Department of Defense policy established new guidelines to ensure that a person with a mental disorder received the appropriate rating and disposition for his/her condition. 29 Service members who developed a mental disorder severe enough to warrant release from active military service were assigned a disability rating no less than 50% and were scheduled for an examination within 6 months of their discharge. 29 These individuals were either given permanent disability retirement if they had an additional disabling condition rated 80% or greater, not related to the mental disorder diagnosis, or were placed on TDRL and revaluated within an additional 6-month period. 29 More than three quarters of service members placed on the TDRL become permanent disability cases once their conditions become stable. 30 The higher odds for musculoskeletal disability retirement among military personnel with comorbidities compared to those without observed in this study may be attributable to policy changes and the subsequent increase in screenings for psychiatric disability cases that result in permanent disability retirement. Many reports have documented higher rates of overall disability retirement as well as disability retirement for musculoskeletal conditions among women than men. 9,31,32 Contrary to previous studies, findings from this study found the odds for musculoskeletal disability retirement in relation to frequency of deployments to be similar among both men and women. Similar associations among men and women were also observed when looking at increasing duration of deployments and disability retirement in the Army. This discrepancy may be attributable to the fact that the association between sex and musculoskeletal disability retirement in this study is underpowered by the small population of deployed females. In addition, the population of this study was restricted to disabled e1536

TABLE IV. Adjusted Odds Ratios (aor) With 95% Confidence Intervals (CI) for Disability Retirement by Total Months Deployed, by Comorbidity and Sex, by Branch of Service Army (%) service members. The association between sex and disability severity in disabled population may be different than the association between sex and disability risk. Further research is necessary to determine the role of sex as a risk factor for musculoskeletal disability and severity in a population that includes both disabled and nondisabled service members. This study extends previous studies by exploring the inter-relationship between comorbidity and deployment history as predictors of disability retirement resulting from musculoskeletal conditions, and determining the extent to which those relationships were modified by sex within the Army and Marine Corps. Strengths of this study include the relatively large sample size, and its comprehensive data capture including demographic, service-related, and disability characteristics of service members. Thus, the results of this study consider various exposures throughout the life cycle of disabled service members. However, this study was limited by the use of VASRD codes to classify disability type and the fact that the population was restricted to only disabled service members. VASRD codes were the only source of information about (%) aor c 95% CI (%) (%) aor c 95% CI Musculoskeletal Condition n: 2,606 n: 12,382 n: 290 n: 1,712 Total Months Deployed a <6 Months 17 19.9 1 17.9 26.5 1 6 11 Months 33.5 41 1.38 1.26 1.51 40 43.9 1.33 1.07 1.67 12 24 Months 37.7 32.1 1.85 1.69 2.02 33.8 25.3 1.78 1.39 2.28 24+ Months 11.9 7 2.07 1.79 2.4 8.3 4.3 1.99 1.21 3.26 Musculoskeletal Condition Plus at Least One n: 7,613 n: 1,189 n: 702 n: 144 Comorbid Condition Total Months Deployed a <6 Months 12.6 20.2 1 14.4 34 1 6 11 Months 28.7 39.3 1.83 1.61 2.07 41.5 39.6 2.32 1.69 3.19 12 24 Months 41.1 32 3.08 2.7 3.52 35.6 22.9 3.31 2.24 4.9 24+ Months 17.6 8.5 4.68 3.76 5.83 8.6 3.5 4.92 1.94 12.5 Marine Corps b (%) (%) aor d 95% CI (%) (%) aor d 95% CI Musculoskeletal Condition n: 691 n: 2,038 n: 31 n: 111 Total Months Deployed a <6 Months 34 29.6 1 22.6 36.9 1 6 11 Months 39.1 46.8 1.3 1.11 1.52 51.6 43.2 1.67 0.89 3.13 12+ Months 26.9 23.6 1.53 1.26 1.86 25.8 19.9 1.63 0.68 3.91 Musculoskeletal Condition Plus at Least One n: 998 n: 99 n: 26 n: 3 Comorbid Condition Total Months Deployed a <6 Months 33.7 34.3 1 30.8 66.7 1 6 11 Months 37.7 40.4 2 1.38 2.91 46.2 33.3 4 0.47 33.9 12+ Months 28.6 25.3 2.41 1.52 3.81 23 a The population sample used for total month deployed categories was restricted to personnel with deployment experience. b Because of overall differences in deployment duration by service, Marine Corps categories were limited to three groups. c Model adjusted for age, race, rank, and Military Occupational Specialty (MOS). d Model adjusted for age, race, rank, and missing MOS. the medical condition that precipitated the disability discharge in this population. Musculoskeletal VASRD codes in particular are considered nonspecific and are not directly linked to International Classification of Diseases, 9th Revision, Clinical Modification diagnoses. 7 Therefore, it is difficult to determine which musculoskeletal conditions are specifically associated with frequent or lengthy deployments. Because prior medical history and dates of clinical onset of musculoskeletal disabilities could not be adequately accounted for; the possibility remains that the association between the deployment exposure and disability retirement may be the result of some predeployment risk factors that may have had amoresignificant long-term impact on personnel receiving a disability discharge. 33 In addition, because the number of female service members who deploy and subsequently experience disability, this study may be underpowered to detect an association between deployment frequency or duration and musculoskeletal disability in women. Finally, this study may be limited by the lack of specific deployment location or type of deployment. In previous studies reviewing similar associations, when estimates were adjusted for deployment location e1537

or type of deployment, insignificant findings were reported. 6 It is highly unlikely that the absence of this information would affect the results in this study as models were adjusted for MOS, which indirectly adjusts for similar exposures. However, as a result of insufficient data, MOS categories were only assigned to Army personnel which reduces the accuracy of the associations among the Marine Corps population. Despite the limitations, this study design provides great insight into the causal pathway between deployment and disability retirement. All the study participants received a musculoskeletal-related disability and the review of the deployment histories predated their final disposition date allowing for a clearer distinction between the studied effects and the risk for disability retirement. Using a disabled population does limit any conclusions that can be drawn with respect to the wider Army and Marine Corps population. There also wasnoindicationofa healthy warrior effect where in other studies a protective association is observed among individuals with multiple deployments compared to those without. Such effects usually occur as a result of individuals who experience adverse health outcomes during their first deployment being deemed unfit for subsequent deployments, creating a deployable force that is generally healthier. 9,33,34 Deployment activities and experiences are quite unique to each military service. Soldiers can be deployed for 12+ months at a time, and Marines are usually rotated after periods of approximately 6 months. 2,4 The difference in deployment length makes it difficult to make general assertions across services. In this study, there were significant associations found among female Soldiers deployed for greater periods of time and disability retirement, although none were found among female Marines. The lack of significant findings among female Marines may be explained by the relatively small sample size of disability retirement cases rather than any interservice differences in exposures during time spent deployed. The importance of understanding the physical effects of multiple and longer duration of deployments should be taken into consideration in the event of future long-duration hostilities. Further research is needed to conclusively determine the relationship between deployment, disability retirement, and the inter-relationship with comorbidity status, utilizing a wider subset of the U.S. military population. The results of this unmatched case control analysis, however, indicate that adherence to policy to reduce the number of deployments and cumulative time spent deployed for service members, may have beneficial effects toward reducing the number of disability cases and the associated health costs within the military population. ACKNOWLEDGMENTS The authors thank COL Earl H. Lynch, MC, USAR, and LTC Paul O. Kwon, Director, Walter Reed Army Institute of Research, Preventive Medicine Branch, for their careful review of the manuscript and Ms. Janice Gary, Preventive Medicine Branch, ManTech International Corporation, for her administrative support. This study was reviewed and approved by the Walter Reed Army Institute of Research Institutional Review Board. This research wasfundedbythedefensehealthprogram. REFERENCES 1. Steel-Fisher GK, Zaslavsky AM, Blendon RJ: Health-related impact of deployment extensions on spouses of active duty army personnel. Mil Med 2008; 173(3): 221 9. 2. Adler AB, Huffman AH, Bliese PD, Castro CA: The impact of deployment length and experience on the well-being of male and female Soldiers. J Occup Health Psycol 2005; 10(2): 121 37. 3. Sheppard SC, Malatras JW, Israel AC: The impact of deployment on us military families. Am Psychol 2010; 65(6): 599 609. 4. 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