Since 2001, our military overseas contingency operations (i.e., Iraq and Afghanistan) have resulted in 58,000 wounded and nearly 74,000 medical evacuations [13–15]. Several publications have elucidated the types of injuries incurred by those wounded in action indicating that musculoskeletal injuries are the most common and result in the greatest treatment resource utilization [16–18]. Over 50 % of injuries occur to the extremities and each injured service member sustains an average of four wounded body regions [17]. Seventy percentage of disability following combat injury is orthopedic in nature: 48 % of musculoskeletal injuries resulting in disabling conditions within the cohort were to the spine or appendicular joints [5]. As evidenced by this study, the burden of orthopedic injuries and resultant disability sustained in our recent conflicts is nearly half composed of injuries that place the joints at risk for permanent disability. This is consistent with reports from other historical conflicts where musculoskeletal injuries were common and intra-articular fractures comprised 71 % of combat-sustained fractures [19]. To further specifically define the impact of post-traumatic arthritis disability, Rivera et al. examined the same aforementioned cohort finding that 28 % of combat-wounded individuals were granted disability for post-traumatic arthritis conditions [20]. This is dramatically higher than the estimate in the civilian trauma population where the prevalence of post-traumatic arthritis among all arthritis is reported to be 12 % [21].

Rates of post-traumatic arthritis in the civilian trauma population are difficult to delineate: heterogeneous populations of patients with both post-traumatic and degenerative cause of arthritis are often studied together making conclusions about post-traumatic arthritis problematic. Furthermore, studies on how to define arthritis as an end point are variable, some studies using radiographic criteria, some using clinical symptoms, and others using arthroplasty rates as indicators of arthritis. However, the fracture literature for the lower extremity suggests that poor outcomes including post-traumatic arthritis are lower in the civilian trauma population compared to those in the military who are combat wounded. Following intra-articular fractures of the tibial plateau, rates of post-traumatic arthritis in the civilian trauma literature range from 23 to 44 % while 100 % of combat knee injury resulted in an arthritis-related disability [20, 22, 23]. Fractures of the tibial plafond result in post-traumatic arthritis in up to 74 % of civilian trauma patients while combat ankle injury leads to post-traumatic arthritis in 91 % of subjects [20, 24, 25]. Arthritis outcomes for fractures of the hip and acetabulum, however, appear to be similar in military and civilian populations, with reports of rates of 21 % and 24 %, respectively [20, 26] (Fig. 3.2, Table 3.1).

The differences seen in the rates of arthritis development may be contributed to the fact that mechanisms of combat injury are usually different from mechanisms experienced in the civilian trauma population. Reports of all injuries sustained in recent war indicated that explosions account for the vast majority of injuries, whereas over 90 % of civilian trauma is from blunt mechanisms [5, 12, 18]. Of joint injuries, 81 % are due to explosive mechanisms which can result in fractures, soft tissue injury, and/or penetrating injury from projectiles that result from the explosion [20]. Explosions cause multiple categories of injury including increases in stress and shear due to blast energy, fragmentary wounds, blunt and crush injury from the body being propelled by a blast, and injury from heat or other environmental exposure [27]. This inherent difference in injury mechanism may be the cause of differential outcomes following combat joint injury and post-traumatic arthritis outcomes reported in the civilian literature.

Low back pain and spondylosis have historically been and continue to be major causes of medical discharge from the military [5, 9]. However, specific studies on arthritis of the spine in military populations are lacking. However, military service places individuals at risk for spinal disease as well. Spine injuries are more common in combat compared to non-battle injury [28]. Among spine injuries in medically evacuated battle-injured service members, 92 % are fractures [29]. In a cohort of 450 medically evacuated combat casualties who were eventually medically discharged, the rate of post-traumatic arthritis per spine injury was 31 % [20]. Only 25 % of these cases of spinal post-traumatic arthritis could be attributed to pre-deployment conditions.

The development of post-traumatic arthritis following combat appendicular joint injury is rapid and predictable. The military’s disability system’s disposition time indicating the presence of post-traumatic arthritis occurs on average 19 months following injury [20]. The sequelae of multiple injuries affect the demands placed on uninjured extremities as well. Biomechanical study of intact limb loading during transtibial and transfemoral prosthetic ambulation indicates that the intact limb experiences high mean and peak ground reaction forces, potentially placing the intact limb at risk of joint microtrauma [30]. Despite severe injuries, especially to the lower extremities, current reconstructive technology has allowed the successful limb salvage of multiple severely injured lower limbs [31, 32]. A salvaged limb, however, is also source of substantial disabilities to include deficits of nerve, volumetric muscle loss, and post-traumatic arthritis [33]. As the science supporting new and more advanced limb salvage options grows, additional research for these residual deficits is paramount to maximizing positive outcomes for retained severely injured limbs.

One success story for post-traumatic arthritis research includes an integrated rehabilitation protocol for patients with hind foot and ankle post-traumatic arthritis. While post-traumatic arthritis currently cannot be prevented, the physicians, therapists, and orthotists of the Center for the Intrepid’s Return to Run Clinical Pathway at San Antonio Military Medical Center (Joint Base San Antonio, Fort Sam Houston, TX) have improved functional outcomes despite foot and ankle post-traumatic arthritis. These outcomes are the result of a specific therapy regimen that begins early in the patient’s course of limb salvage and the use in appropriate patients of the Intrepid Dynamic Exoskeletal Orthosis (IDEO), a custom carbon fiber, energy-storing ankle foot orthotic fabricated at the Center for the Intrepid [34, 35].

Compared to commonly used, off-the-shelf orthotics, the IDEO allowed significant improvements in agility, power, and speed in a cohort of 18 limb salvage patients [36]. In a small cohort of patients with tibiotalar or subtalar post-traumatic arthritis following lower extremity fractures, 81 % of patients were able to return to running activities, 69 % returned to agility sport, and 44 % continued active duty and were not medically discharged due to their injury [35, 37]. Up to 19 % of individuals who stay on active duty with assistance from the IDEO have been able to deploy with an orthotic, including those who have deployed with special operations forces [38]. The IDEO has allowed patients who desire an amputation to proceed with limb salvage because it often reduces pain and permits a higher level of activity. These results highlight the importance of how a rehabilitation pathway, improved orthotic technologies, and multidisciplinary treatments can improve outcomes for individuals with articular injuries despite post-traumatic arthritis.

As previously mentioned, active duty service members are at risk from more than solely combat operations. The day-to-day activities of active duty individuals include physical training as well as typical sport and recreation. Even in a deployed setting, non-combat-related orthopedic injuries occur more frequently than battlefield injury [39, 40]. These injuries are classified as non-battle injury or illness. Medical evacuations from Operation Iraqi Freedom/Operation New Dawn from January 2003 to December 2011 included evacuations of over 50,000 service members for all causes, where 17.7 % were done so for battle injury, 16.3 % of medical evacuations, and over 8,000 service members were for non-battle musculoskeletal injury [15] (Fig. 3.3). Musculoskeletal non-battle injury compared to all other causes of medical evacuation was the second most common cause of evacuation for both male and female service members: second to battle injury in males and second to mental health disorders in females. A similar trend occurs in military operations in Operation Enduring Freedom between October 2001 and December 2012. In Afghanistan, 9.2 % of medical evacuations were for battle injury while non-battle musculoskeletal conditions resulted in 5.6 % [14]. As in Iraq, musculoskeletal non-battle injury was the second most common cause of evacuation for male service members following battle injury and the third most common cause for female service members following mental disorders and ill-defined conditions.

Following completion of full deployment, non-emergent orthopedic care is required in 19 % of service members returning after deployment [41]. A majority of these consultations are also the result of non-battle injury or exacerbations of conditions present prior to deployment. Among musculoskeletal non-battle injuries, fractures are the most common followed by inflammatory and overuse syndromes, sequelae of joint dislocations, sprains/strains, and other internal joint derangements [40]. The most common anatomical locations affected by non-battle injury are the back, knee, wrist, ankle, and shoulder. The most common mechanisms for these injuries are from sports and physical training.

Rupture of the anterior cruciate ligament (ACL) is among the most concerning sports injuries resulting in increased risk for post-traumatic arthritis [42–45]. Studies in military exclusive populations indicated that military personnel have a ten times higher prevalence of ACL injury compared to the general population: 3.65 per 1,000 person-years for acute injuries and 2.96 per 1,000 person-years for chronic injuries [46] (Table 3.2). Rates for the civilian population are generally reported to be 0.31–0.38 per 1,000 person-years. Among individuals hospitalized for sports or training injury, the knee was the most commonly injured body region and the ACL the most commonly injured structure [51]. As with civilian publications, ACL injuries in the military are associated with other knee pathology such as meniscus tears and chondral lesions [52]. Outcomes following ACL reconstruction are comparable in the military versus civilian cohorts with the exception that allograft graft selection may result in higher early failure rates compared to autograft in this young, active population [53]. The correlation between ACL injury specifically in military populations and post-traumatic arthritis has not been studied; however, the extrapolation from civilian research is likely valid and should be considered given the prevalence of ACL in injury in this young population.

Meniscus injury has also been associated with development of post-traumatic arthritis at rates of 50 % 10–20 years post-injury [44, 54]. Rates of meniscus injury in civilian, active populations range from 0.33 to 0.61 per 1,000 persons. Meniscus injuries in the military treated at both hospital and ambulatory settings however occur at a rate of 7.08 per 1,000 males and 6.02 per 1,000 females [48]. These rates increased with age from 2.99 per 1,000 persons in active duty individuals younger than 20–12.68 per 1,000 persons for those older than 40. Service in the Army or Marines and being of junior or senior enlisted rank are also risk factors for meniscus injury.

Rates of osteochondral lesions of the knee in military populations only are not available, though rates of chondral injury associated with other knee pathologies are likely comparable to civilian studies—incidental finding of articular cartilage lesions is 60 % at the time of arthroscopy [55, 56]. While evidence is available for return to sport outcomes following various cartilage repair surgical options, this data too is sparse for military populations where physical fitness is a requisite for continuing on active duty [57]. A single study demonstrates after osteochondral allograft transplantation that return to full duty rate is 29 % while return to pre-injury level of sport participation was only 5 %. Those in military occupation of combat arms were significantly less likely to return to duty [58, 59].