For most of the past decade, the US military has been engaged in two extended conflicts—Operation Iraqi Freedom (OIF) in Iraq and Operation Enduring Freedom (OEF) in Afghanistan. These two wars combined have resulted in thousands of deaths and injuries in tens of thousands, which is the greatest number of combat casualties since the Vietnam Conflict. Early in the course of these conflicts, there was an anecdotal appreciation for the significant burden of musculoskeletal combat wounds in this population; and eventually large-scale studies evaluating data from the Joint Theater Trauma Registry (JTTR) emerged confirming this trend [2, 3]. Not only did extremity battle wounds comprise 54 % of all wounds but a cost-utilization analysis found that musculoskeletal wounds also consumed 65 % of all inpatient care costs as well as 64 % of all disability costs and resulted in 64 % of all hospital readmissions [4, 5]. See Chap. 2 of this text for a more detailed description of the burden of combat-related musculoskeletal injuries in the military.

As a follow-up to the JTTR database studies, a large prospective cohort study followed a Brigade Combat Team (BCT) during a 15-month deployment to Iraq, allowing a comprehensive picture of mortality and wounding risk from the perspective of an individual unit [6]. While the combat battle injury data were consistent with previous studies from the JTTR [7], this study design allowed for an accurate evaluation of the non-battle injuries as well—those that are sustained in a deployed environment but not as the result of direct enemy contact. In previous wars, disease and non-battle injuries (DNBI) have been as much or more of a burden on the military health-care system than true battle injuries. Similar to experiences in previous wars, the non-battle injuries resulted in far more medical evacuations from theater than the battle injuries. Furthermore, musculoskeletal injuries and conditions represented the greatest number and proportion of non-battle injuries and medical evacuations. Musculoskeletal injuries comprised 50 % of the DNBI casualties and 43 % of the DNBI casualties requiring evacuation [8]. Additionally, many service members sustained non-emergent musculoskeletal injuries, which are treated conservatively in theater and ultimately require surgery following their combat tour [8, 9]. The anterior cruciate ligament disruption and first-time shoulder dislocation incidence rates from non-battle injuries are nearly five times greater than that of the civilian population and similar to the endemic rates found in the non-deployed military population [10], and this is indicative of the daily rigors of the combat environment. A detailed description of the impact of noncombat musculoskeletal injuries and conditions during deployment is presented in Chap. 3 of this book .

As noted above, there is a great burden due to musculoskeletal injuries and conditions from combat wounds, as well as due to non-battle injuries in military populations. Non-battle musculoskeletal injuries and conditions are not only the greatest threat to combat readiness among soldiers deployed to Iraq and Afghanistan, but they also place a significant burden on the entire military population. Musculoskeletal injuries are endemic within military populations, and they pose the greatest public health problem facing military service members during both peacetime and combat operations. Musculoskeletal injuries and conditions are also the greatest threat to military readiness [11]. It is these injuries that occur during normal peacetime activity that were the focus of a series of studies conducted in order to better document the burden of musculoskeletal injury and disease as well as to help focus and plan prevention strategies.

Over the past decade, a series of population-based epidemiological studies utilizing data from the Defense Medical Surveillance System was performed to systematically evaluate the burden of musculoskeletal injuries and disorders affecting military service members [10, 12–29]. The initial focus was on acute traumatic joint injuries that typically lead to significant time loss from duty, morbidity, and the need for surgical intervention. The incidence rates for several specific musculoskeletal injuries were documented among active duty military service members, in addition to the demographic and occupational risk factors associated with these conditions. These data have been critical in defining the scope of the injury problem within the military, identifying the groups at highest risk for certain injuries, and have been helpful in targeting high-risk populations for injury prevention interventions.

The majority of musculoskeletal injuries seen in the military population are joint sprains and muscle strains and injury patterns that are similar to those observed in athletic populations [30]. The anterior cruciate ligament in the knee has been shown to have an incidence rate that is an order of magnitude greater than the general population [10]. Similar endemic rates have been shown for ankle sprain [20] as well as shoulder instability [22]. These major joint sprains have significant impact on the soldiers’ readiness, often resulting in recurrent sprain syndromes with deleterious effects on articular cartilage and joint homeostasis. The endemic rates make the study of joint sprain in this high-risk population extremely appealing in order to help determine both preventive measures and optimal treatment modalities [31]. The resultant posttraumatic osteoarthritis (OA) as well as the surveillance systems that are available also makes this an attractive population in which to study the outcomes of joint injury [32].

There is substantial evidence, in both animal and human studies, to support the link between traumatic joint injury and the subsequent occurrence of degenerative joint disease and OA. Because of the higher rates of joint injury observed among military populations and the significant occupational and physical training demands in this population, similar studies were conducted using data from the Defense Medical Surveillance System and the JTTR to examine the incidence rate and burden of OA among military service members. These reports found that the incidence rate for OA was significantly higher in every age group among military service members and that the disparity between military service members and the general population increased with increasing age [13].

In a separate study, the incidence of hip OA among active duty military service members was examined [17]. The overall incidence rate for males was 35 cases per 100,000 person-years, with rates ranging from 32 cases per 100,000 person-years among males to 54 cases per 100,000 person-years among females. While incidence rates for hip OA were lower than previously reported in the literature, this is likely because the majority of published studies have focused on the incidence of OA in much older study populations. The observed incidence rates for hip OA in this relatively young and healthy population are disconcerting and combined with the overall rates for OA in comparison to the general population raise concerns about the burden of OA in load-bearing joints following military service.

Cross et al. [33] reviewed physical evaluation board records for disability discharge among military service members and noted that orthopaedic and musculoskeletal injuries resulted in the majority of long-term disabilities in this study cohort. They also reported that degenerative arthritis was the leading cause of disability discharge from military service in this population and was the third most significant in terms of impact (e.g., frequency × average percent disability). The advanced rate of posttraumatic OA was alarming with many service members progressing to OA within a couple of years following acute traumatic injury. The long-term burden in terms of health-care costs and disability-adjusted life years in this cohort will be significant into the future, and concerted efforts are needed to improve the quality of care and life for these veterans.