Thoracic and Lumbar Spine Injuries

Fig. 12.1

Seated position of pilot during flight

Operating aircraft controls often requires frequent trunk rotation and lateral bending and maintenance of pelvic retroversion with notable differences between specific aircrafts (Fig 12.1).

In addition to ergonomic factors, these individuals are subjected to long periods of exposure to whole body vibration (WBV). WBV has been implicated as a potential contributing factor in the development of back pain in multiple studies [18–20], in particular when combined with awkward posture [21]. Vibratory frequency of rotary aircraft resembles the spinal resonant frequency [22, 23], which theoretically increases its potential harm. Despite this exposure, the role of WBV in the pathogenesis of back pain in this population is unclear as it is difficult to isolate WBV exposure from the exposure of flight or other aircraft-related factors. Shanahan et al. performed a study utilizing a helicopter cockpit with and without vibration exposure. They found no difference in back pain rates or intensity after prolonged flying with or without vibration exposure in this simulation [24].

When considering the above factors, it is not surprising that this population experiences an extremely high rate of lower back pain with rates ranging from 50 to 92 % [23, 25, 26]. Back pain is also associated with significant disability and compromised mission readiness. Among pilots with back pain, approximately ½ reported interference with concentration [26] and compromised performance secondary to their pain [27]. Additionally, between 16 and 28 % of pilots admit to rushing flights because of back pain [23, 26].

The pain associated with this occupation is generally felt to be directly related to the operation of the aircraft itself. Back pain typically begins during flight and in the majority of cases resolves within hours after flight is completed. Pain is more common during operations that require flying that is more dependent on manual control including precision and instrument flying. This is often more frequent in pilots greater than 71 in., which results in a more hunched-forward posture during aircraft operation. Orsello et al. demonstrated a 9 % increase in incidence for every 1-in. increase in height [28].

While fixed-wing pilots also experience high rates of back pain and injury, they have been shown to have much lower rates compared to rotary-wing aircrew with a 50 % lower rate reported in the Norwegian military [27]. Fighter pilots have been shown to have high rates of thoracic spine pain, with rates among the highest in the military [29].

Fixed-wing pilots are also subjected to prolonged periods of sitting in a poor ergonomic environment. Military aircraft seats are typically angled in a forward-flexed position, which places the spine in a poor position during flight. In addition, these individuals are subjected to WBV and often experience high G-forces which may also play a significant role in spinal pain or injury. While the majority of research has focused on the cervical spine in these individuals, there are significant consequences on the thoracic and lumbar spine in this population .

An additional risk, which fixed-wing aircrew are uniquely exposed to is aircraft ejection. Ejection from an aircraft subjects the spine to incredibly high forces with injury rates up to 69 % [30] and vertebral fracture rates between 26.2 and 35.2 % [31]. The location of injury during ejection is characteristically the thoracolumbar junction [32] with fractures occurring primarily at T12 and L1. Such fractures typically arise from a combination of axial load and forward flexion, which occurs during the ejection process [33]. This typically results in a compression-type injury, however more severe spinal fractures can occur.

It should be noticed that while the majority of research focuses on the pilots of the aircraft, the entire aircrew are at risk of back pain and injury. Flight crew, in particular the flight engineers, also encounter frequent awkward positions, which have the potential to result in back pain or injury [23, 34]. In addition, Simon-Arndt et al. demonstrated the flight engineers to have rates of diagnosed back problems higher than the pilots of the same aircraft [35].

Drivers

Similar to aviators , drivers have also been implicated to be at increased risk of lower back disorders. Occupational driving has been implicated in development of low back pain in multiple studies in civilian populations with anywhere from 15 to 300 % increase in incidence compared to nondrivers [36–38]. Despite a high rate in such occupations, studies are limited by the diverse nature of different driving occupations including different vehicles and driving duration. Also, occupational drivers frequently perform tasks in addition to driving such heavy lifting or loading of vehicles, which would also place the back at increased risk of injury. Military vehicles, in particular, play a potential role with prolonged exposure. Such vehicles often have poor ergonomic design with significant WBV exposure [39]. Despite this, few studies have evaluated the role of occupational driving in the military.

Rozali et al. demonstrated a 73 % 12-month prevalence of low back pain among Malaysian armored vehicle drivers, with rates reaching nearly 82 % in drivers of tracked vehicles [39]. This study also demonstrated low back pain to be correlated with driving in a forward-flexed posture as well as WBV exposure in the x-axis. Knox et al. performed a US military-wide study to compare low back pain incidence rates between service members employed as drivers compared to matched controls. This study revealed a 15 % increased risk of new onset low back pain among occupational drivers compared to controls, however they identified a much higher risk effect in female drivers who experienced a 45 % increased risk compared to females in nondriving occupations [40].

Parachuting

Soldiers involved in military parachuting activities are another group worth special mention. Such activities subject the spinal column to significant and place these individuals at increased risk of spinal injury. Spine injuries represent the second most common type of injury after parachute jumps, and comprise 15 % of acute injuries after both training and combat jumps [41]. Injuries primarily occur during landing and are related to axial load force, which often results after a hard landing on the buttocks. Traumatic vertebral fractures from such landings are typically compression fractures and occur primarily about the thoracolumbar junction [42]. In addition, the spine is subjected to deceleration forces during parachute opening.

In addition to acute traumatic events, persons engaged in repetitive parachuting activities are at relatively high risk of chronic thoracic or lumbar conditions. Murray-Leslie et al. reported on the rate of lumbar spine symptomatology as well as radiographic degeneration in ex-military parachutists. Fifty six percent of these individuals reported either current or prior lower back pain with nearly 24 % having lost work time due to back pain. Additionally, 84.8 % had radiographic degeneration of the lumbar spine and 21.7 % had evidence of prior vertebral fractures. Interestingly, 80 % of individuals with prior spine fracture were unaware of the presence of such an injury at the time of the study [42].

Military Factors

Branch of service

Due to differences in occupational demands as well as training regimens, differences in spine injury and disability is expected between the branches of service. Few studies have evaluated these differences as the majority of research focuses on particular groups of individuals and few have military-wide study samples. One study that evaluated a military-wide sample demonstrated significant differences in low back pain rates. In this study, the Army carried the highest incidence with a greater than twofold increased risk compared to the Navy and Marine Corps. The Navy and Marine Corps demonstrated the lowest rates with minimal differences between the two services, whereas the Air Force had an intermediate risk with approximately 50 % greater risk than the Navy [6].

Rank

Rank is an important consideration in back injuries and has significant implications in the incidence of back disorders in military populations. The primary reason this is a consideration is that as individuals advance in rank, they often acquire more supervisory roles with potentially less rigorous activities required on a regular basis. Additionally, more senior service members often have the capability of self-modifying their training environment. This allows them to stop or decrease certain activities that are creating discomfort, whereas the more junior ranking individuals may be required to continue these activities despite the beginnings of a significant injury.

An important factor to be considered is level of education amongst these individuals. Lower level of education is a contributing factor in rates of low back pain. Individuals with an education level of bachelor’s degree or higher experience have lower rates of back pain compared to those with lower levels of education [43]. As higher rank is associated with higher levels of education, this plays a potential role in the different incidence rates between ranks.

This difference in incidence between different ranks has been shown in multiple studies. In a military-wide sample, Knox et al. demonstrated significantly increased rates of low back pain in the more junior-ranking service members. This was consistent across all age groups with the highest rates in junior enlisted (E1–E4) and the lowest rate in senior officers (O4–O9) with a nearly twofold difference in incidence between these groups. MacGregor et al. also showed lower ranking Marines to have significantly higher rates of back pain after deployment to Afghanistan [15].

Basic training

The basic training environment represents a period of significant stress to the spinal column. Recruits are subjected to rigorous daily physical training including prolonged running, grenade throwing, marching, often with heavy combat load. In addition, recruits/conscripts are often physically deconditioned and many had led primarily sedentary lifestyles prior to this period. Wang et al. demonstrated that only 10 % of the Chinese conscripts were engaged in regular physical activity or heavy labor prior to entry into basic training [13]. As such, many such individuals lack the physical conditioning and core strength necessary to protect the spine from injury.

Because of these numerous factors, the lumbar spine represents a very common site of injury in the basic training environment [44, 45] armies. Glomsaker et al. reported that low back pain represented 18.6 % of all injuries in their population with an incidence of 23.9 per 1000 conscript months. Additionally, 0.7 % of trainees sustained disc herniation with a rate of 0.9 per 1000 conscript months [44]. Taanila et al. evaluated a group of Finnish conscripts over a 6-month period of training. During this period, 16 % developed low back pain with an incidence rate of 1.2 per 1000 person days of training [46]. Similar rates have been shown in other studies as well [47–50].

Despite its frequency, the majority of back pain among basic trainees is self-limiting and 65 % of cases will resolve by the end of the basic training period [47]. George et al. also showed that rates of low back pain with demonstrate a gradual decrease with increasing time in military service. The highest rates were seen in soldiers with less than 5 months of service, who reported back pain rates of over 55 %, however this rate dropped to only 19.1 % after 1 year [51]. This is likely reflective of the strenuous physical training that is encountered during basic training and the physical adaptations that occur .

Thoracic back pain is much less common, representing only 2.1 % of injuries in conscripts with an incidence of only 2.7 per 1000 conscript months [44].

Deployment

Deployment also is a period that represents a period at high risk of developing or exacerbating lower back pain or injury. The back is the most common site of injury during deployments, representing 17.4 % of musculoskeletal injuries in a series of 593 soldiers deployed to Afghanistan [52]. Lower back pain was experienced by as many as 77 % of soldiers deployed to Afghanistan and 22 % reported pain rated as moderate or higher [53]. Spine pain/injuries are also a common cause for evacuation representing 7.2 % of evacuations from Operation Iraqi Freedom (OIF) and Operation Enduring Freedom (OEF).

Service members in the deployed environment are subjected to long periods of wearing body armor and combat gear, have increased duration and intensity of work, and are subjected to significant psychosocial stressors. Mydlarz showed that 68 % of individuals with preexisting degenerative disc disorders experienced an exacerbation during deployment [4]. Deployment-related exacerbations were more common in the Army compared to other services, particularly those in the armor/motor transport occupational group where the risk of exacerbation approached 100 %. Males and service members in the youngest (17–19 years) and the oldest (> 40 year) age groups were also more affected. Patients with preexisting disorders are also at nearly twice the risk of all-cause evacuation from theater (Odds Ratio (OR) 1.98), however less than 2 % of these individuals were evacuated for lower back conditions [4].

Roy et al. evaluated the variables associated with increased rate of low back pain in deployed service members and reported body armor wear, lifting activities, walking patrols, and heavy equipment weight to be statistically significant variables associated with increased risk [53]. The most common activities resulting in spine injury resulting in evacuation has been reported to be lifting (15 %), falls (11 %), and driving (8 %) [54]. Work shifts have been implicated in this as well. Nevin et al. demonstrated that helicopter pilots with increased work hours during deployment have significantly higher increases in rates of lower and mid back pain compared to those who maintained the same schedules.

Another significant factor is the load carried by service members. Military forces have seen a dramatic increase in the combat loads with modern combat loads including up to 68 kg of gear depending on the individual’s combat role and the mission being performed. Roy et al. demonstrated an average carrying load during deployment of 16.1 % of body weight for females (maximum 32.8 %) and 26.4 % for males (maximum 46.5 %) [52].

Carrying such loads has been shown to have numerous deleterious effects on the spine. Rodriguez-Soto et al. used upright MRI to evaluate kinematic changes in active duty Marines wearing such loads. Their study demonstrated a loss of lumbar lordosis at L4–5 and L5–S1 with an associated loss of anterior intervertebral disc height. More superior levels, however showed an increased lordosis [55]. Roy et al. [53] demonstrated significantly increased rates of low back pain with increased duration of body armor wear in deployed soldiers. In this study, wearing body armor greater than 6 h per day was associated with greater than fivefold rate of low back pain compared to those who did not wear body armor [53]. Also, increased equipment weight directly increased the incidence of back pain in this cohort with a linear increase in risk with higher weights [53]. Additionally, between 29 [56] and 41 % [53] of soldiers who develop back pain during deployment attribute their pain to wearing combat gear [56].

Service members are also subjected to increased psychosocial stressors, which has been shown to be important in the development of lower back pain and conversion to chronic or recurrent pain. Shaw et al. demonstrated that coexisting anxiety disorders, PTSD, or depression significantly increases the risk of acute low back pain becoming chronic .

Due to dramatic differences in job-related activities, significant differences in injury rates are expected between occupations. MacGregor et al. reported on the rates of post-deployment lower back pain in active duty Marines. They reported the highest rates in the service/supply occupational group (OR 1.3) with Marines involved in construction-related occupations demonstrating the highest rates (8.6 %). In contrast, Marine infantrymen had one of the lowest rates of lower back pain (3.3 %) [15].

Midback pain represents a much less common entity experienced during deployment. While low back pain represents 75.6 % of spine area pain, mid back pain represented only 3.3 % in a recent study by Carragee et al. [57].

Reserves

Another population that deserves special mention is that of the reservist. While active duty service members are involved in regular physical conditioning and preparation for their role in their military occupation , reserve service members often lead relatively sedentary lifestyles with significantly different occupations than those performed during their active duty obligation. As such, it could be presumed that these individuals may be at increased risk of injury or development of overuse injuries. Warr et al. reported that back injuries represented 17 % of musculoskeletal injuries in deployed National Guardsmen [58]. Additionally, low back pain rates in deployed National Guardsmen (NG) was lower than active duty service members with those in active duty experiencing a 1.45-fold increased risk vs. NG. Similarly, George et al. revealed an increased rate of low back pain in active duty service members compared to reservists with a similar effect size (OR 1.441) [51].

Individual Factors

Gender

Gender has been implicated as a factor associated with higher rates of lower back pain in both civilian [59–64] and military service members [6, 8, 11, 15, 51, 65, 66]. Knox et al. revealed an odds ratio for females to males of low back pain resulting in a visit to a health-care provider of 1.45 compared to matched controls [6]. Strowbridge et al. revealed a much higher effect with female soldiers experiencing between 2.71 and 4.97-fold risk compared to males [8, 65]. They also reported that female soldiers more frequently attributed their low back pain to military activities, work, and off-duty activities compared to their male counterparts [8]. Gemmell demonstrated the incidence among female recruits to be significantly correlated to the training regimen. In their series, female recruits engaged in “gender fair” training with separate standards for men and women sustained 4.8-fold rate of back-related medical discharges during basic training compared to male recruits. After implementation of uniform training across genders, this rate increased to a 9.7-fold [67]. In addition to increased incidence rates, George et al. demonstrated a shorter duration to onset of low back pain in female service members [51].

Age

Increased age has been associated with increased prevalence of low back pain in numerous studies in civilian populations [60, 61, 63, 68]. This is due to both increased cumulative exposure to potentially injurious activities as well as age-related degenerative changes. Studies in the military setting have also shown age-related differences in back pain rates. An important consideration in the military setting is potential confounding between age and rank. As age and rank are often linked, it is important for studies to control for this to isolate the effects of age.

The age-related differences in low back pain incidence was evaluated by Knox et al., who demonstrated a bimodal distribution of back pain in this population with the highest rates in those over 40 years old as well as those less than 20⁶. This was shown after adjusting for other potential confounders including gender, branch of service, and rank. MacGregor et al. also reported higher rates of low back pain among Marines over 25 years old compared to those younger than this age in a post-deployment sample [15].

Race

Race is another important consideration in the epidemiology of back pain and back injury. Multiple studies on civilian populations have demonstrated significant differences in prevalence rates between racial groups. Knox et al. reviewed and evaluated the incidence of low back pain resulting in a visit to a health-care provider between different racial groups of active duty service members [69]. In their series, the lowest incidence was seen in Asian/Pacific Islanders win an incidence rate of 30.7 per 1000 person-years. Conversely, African-Americans had the highest rate of 43.7. These racial differences were present across all age groups and genders, however they showed that the effects of age and race were variable between racial groups.

Fitness

Personal fitness is an important consideration in the risk of back injury and subsequent disability/loss of productivity. This is evident from multiple studies that demonstrate the protective effect of core strengthening against lumbar injury and low back pain. While the military represents a population with a higher overall physical fitness, variation in fitness level has been implicated in differences in low back pain rates.

Morken et al. demonstrated low levels of physical activity to be associated with increased risk of thoracic and lumbar spine injuries among Norwegian sailors [9]. More specifically, Taanila demonstrated a higher rate of acute low back pain in conscripts with lower push-up and sit-up scores on physical fitness testing [46]. In a large study of American soldiers, George et al. found no difference in low back pain rates depending on physical fitness test scores or routine exercise. What this study did show was higher pain intensity and more psychological distress among soldiers with lower physical fitness testing scores [51]. Warr et al. also demonstrated a significant correlation between cardiorespiratory fitness (measured by peak oxygen uptake (VO2 peak)) and the number of visits for back complaints in deployed National Guardsmen [58]. Similarly, Feuerstein et al. reported significantly increased risk of low back pain resulting in lost work time in soldiers who report only rare aerobic exercise [66].

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