Knee Injuries

Fig. 10.1

Rates of knee injury in active duty Army, 2000–2005: stratified by prior knee injury status (Rate per 1000 soldiers may fall in more than one category per year) [6]

These include age, rank, military occupation (MOS), gender, and those with category IV Armed Forces Qualification Test Score (AFQTS) [6]. In addition, a history of prior knee injury within 2 years increases the risk of subsequent knee injury tenfold [6]. Soldiers allowed into the military with a medical waiver for a knee injury were 8.0 times more likely to be hospitalized for a knee condition and 14.0 times more likely to be medically discharged for a knee-related condition [13].

Over a recent 5-year period, 148,951 orthopedic surgical procedures were performed on 132,731 active duty soldiers to treat musculoskeletal DNBI, including over 60,000 knee procedures [5]. Unfortunately, not all of these surgeries resulted in return to duty. We analyzed the reoperation rates for many common orthopedic procedures and found the knee to be among the highest to receive revision operation of the same type, with an overall rate of 9 %. Menisectomy cases (medial or lateral) underwent a revision surgery of the same type 14 % of the time, and chondroplasty and anterior cruciate ligament (ACL) had repeated ratios of 12 and 10 %, respectively [5]. The reoccurrence of injury is a matter of concern and is likely due to a number of causes including the physically demanding lifestyle, and the pressure for an accelerated return to one’s unit. Decreased mortality in the field seen in recent years has also led to increased demand on the armed forces rehabilitation assets, perhaps contributing to less emphasis placed on DNBI-related injuries [14].

Increased age has been shown to be related to increased risk of knee injury [6]. Soldiers less than 20 years of age have an incidence of 15–19 per 1000, whereas those of 20–30 years display an incidence of 20–25 per 1000 [6]. Active duty members who are greater than 30 years have an incidence of 24–28 per 1000 [6]. This increased incidence with age may be related to accumulation of prior knee injuries or the increasing pressure for a quick return for those in leadership positions. Related to age, length of service is also associated with an increase in the rate of knee injuries, with soldiers serving less than 1 year having the lowest rates of knee injury, and those serving greater than 10 years having the highest rates [6]. Rank is also a factor with enlisted troops demonstrating knee injuries at a rate of 22–26 per 1000, whereas officers are slightly lower at 20–23 per 1000 [6]. There are many potential explanations for these differences. Enlisted troops include those going through basic training, who have shown to have higher rates of acute traumatic musculoskeletal injuries [15], and officers may be underrepresented in certain more physical duty designations.

No difference has been reported with regard to ethnicity or education. However, there is a difference among soldiers taking the AFQTS [6]. The AFQTS is inversely proportional to the rate of knee injury. Those scoring in group I (the highest group) have the lowest rate of knee injury, 18–24 per 1000 person-years [6]. Those scoring in group IV (the lowest group) have an injury rate of 24–30 per 1000 person-years and were 20–62 % higher than group I every year [6].

Acute Meniscal Injury

Meniscal injuries and their treatment are common in both the general and military population. The meniscus’ primary function is to distribute compressive forces during dynamic joint movements and static loading [16]. Injuries occur secondary to sports as well as during everyday living. Symptoms such as pain, catching, and locking often need to be treated with surgery. Arthroscopic treatment of meniscal tears is very common, with many centers reporting a rate of 10–20 % of all surgical procedures and totalling approximately 1 million surgeries in the USA annually [17, 18]. In the military, arthroscopic treatment of meniscal tears is the most common knee procedure performed with more than 5700 cases annually [5], with nearly 90 % being debridement.

The incidence of acute meniscal injury is higher in the military compared to the general population and has several associated risk factors such as gender, age, rank, branch of service, and ethnicity. The mean incidence of meniscal tears in the general population is 0.33–0.61 per 1000 person-years [19, 20]. The military population has a greater than tenfold increase over the general population, with an incidence of 8.27 per 1000 person-years [21]. These findings are consistent with increased incidence also seen in other knee pathology.

Injury to the meniscus in the general population is more common in males than females with a 2.5–4:1 ratio [22]. Similarly, military men were 20 % more likely than their female counterparts to have an acute meniscal injury [21].

One military study reports that increased age and a higher incidence of meniscal injury are associated; however, this is not consistent with prior data in civilian populations that show peak incidence in men 21–30 years old [21, 23]. In the military, soldiers greater than 40 years old are four times more likely to have a meniscal tear when compared to soldiers less than 20. The difference in age of occurrence between the general population and active duty may be attributed to civilian personnel becoming more sedentary as they age, whereas active duty has daily duty requirements that require continued athletic activity leading to meniscal injuries.

Stratification in incidence of meniscal injury is seen through the ranks. Junior enlisted claim the highest incidence of meniscal injuries followed by senior enlisted and senior officers. Junior officers have the lowest rate of injury [21]. Junior enlisted may be at increased risk because those soldiers are undergoing basic training, which has been shown to have inherent increased risk of musculoskeletal and knee injuries [15] .

In addition to age and gender, branch of service can affect the incidence of injury. Active duty members in the Army or Marine Corps have higher rates of acute meniscal injuries than the Navy and Air Force [21].

Analysis of race in the active duty population demonstrates association between ethnicity and injury. Three ethnic categories are defined as whites, blacks, and others. Whites and blacks have similar rates of meniscal injury, which is 25 % lower than other classified ethnicities [21].

In published civilian data, the medial meniscus is injured two to four times more frequently than the lateral meniscus [19, 24–26]. Jones et al. reported similar findings in the active duty population breaking the injuries into three groups: 50.3 % of the injuries occur to the medial meniscus, 22.4 % to the lateral, and 27.3 % are not specified [21]. Anatomical analysis explains these differences. The lateral meniscus is more mobile than the medial meniscus that is attached firmly to the joint capsule, leading to higher incidence of injury in the medial meniscus [18, 27].

The menisci play an important role in knee joint stabilization, lubrication, and proprioception [28–30]. Injuries have been associated with long-term changes to include joint dysfunction, degenerative changes, and osteoarthritis [31, 32]. Significant increase in incidence of meniscus injury in soldiers is a unique problem for this population, as osteoarthritis is the single most common cause of disability in the US DOD .

Treatment of meniscal tears is particularly challenging for the military surgeon. It is not uncommon for the soldier to be delayed in having access to an orthopedic surgeon, and thus it is a common perception that many patients are managed conservatively with these injuries for an inordinate amount of time. Such management combined with the aggressive physical requirements of a military member as well as the cultural pressure to keep up with daily physical training often results in severe and irreparable meniscal damage at the time of surgery. Meniscal repair makes up around only 10 % of meniscal type surgery, reflecting the severity of the encountered pathology. Further, the cultural pressure of a quick return to duty makes prolonged rehabilitation, such as that required of a meniscal repair, quite challenging.

Cartilage

Chondral injuries can occur as an isolated entity or in combination with other knee pathology. Approximately 3650 chondroplasties are performed every year in the military, which accounts for the second most common knee surgery in this population [5]. Injury to the cartilage has been reported in 9 % of soldiers with acute ACL injuries and the incidence increases with a delay in surgical treatment [33]. Treatment options for chondral injury range from minimally invasive arthroscopic debridements, to more in-depth chondral transplants. Each approach has its proponents with trade-offs in cost, durability, and time to recovery. Very little literature is available to contrast the different forms of treatment within a military population, but some data do compare these techniques in young athletic populations which may be extrapolated to active duty service members. Gudas et al. performed a randomized clinical trial comparing microfracture to an osteochondral autograft transfer system (OATS) procedure in young athletes and noticed that while OATS patients returned to their previous level of sports at a rate of 93 %, microfracture patients only returned to this level 52 % of the time [34]. A follow-up study by the same authors at 10 years showed that the OATS group maintained their activity in 75 % of patients compared to just 37 % in the microfracture group [35]. There are challenges in translating civilian data to the military experience, however. The treatment of larger chondral lesions with an allograft OATS, for example, has not yielded analogous results. Two studies have evaluated the allograft OATS as it relates to return to activity [26, 36] in civilian populations. Both studies reported high rates of patient satisfaction and nearly 80 % return to sport. In contrast, Shaha et al. studied the same procedure in a military population and found that only 29 % returned to full duty. Further, only 5 % of patients in that study claimed to return to their previous level of sport [37].

The disparity in these results may be partially explained by certain military-specific factors that make chondral injuries more difficult to treat in active duty. The Marine Corps, for example, does not allow for any modification of activity on a long-term basis, and therefore, any Marine who undergoes chondral treatment must return to full duty without limitation or is medically discharged. In addition, the Marine Corps gives a limited duty or “LimDu” for a single 6-month period, and may grant a second in rare circumstances. If the Marine has not returned to duty by the end of the LimDu, he or she is boarded out of the Corps. Another possibility lies in the definition of “return to activity.” In most civilian populations, patients can achieve return and still self-limit activity, whereas a military population generally has daily physical mandatory formations that do not accommodate self-limitation. Thus, with the limited time available for recovery, and the rigorous daily physical requirements required upon return, chondral pathology is a very sobering diagnosis in a military population.

Ligament Injuries

Anterior Cruciate Ligament (ACL)

Injury to the ACL is a common injury in any young active population. Given the soldier’s daily physical demands, it is not surprising that reported rates of ACL injuries in the military are ten times that of the civilian population [16]. The overall incidence of ACL injuries in US active duty servicemen and women is 2.96–3.65 per 1000 [16]. These rates are significantly higher than what is seen in the civilian population that has rates between 0.31 and 0.38 per 1000 (107,108). Over 3000 ACL reconstructive surgeries are performed annually in the military medical system and are the third most common knee procedure performed [5].

There have been multiple studies investigating the rate of ACL injury in men compared to women in a military population. Examining a select population at the US Naval Academy, Gwinn was able to demonstrate increased risk of ACL injuries in female midshipmen [38]. This cohort may not represent the entire US military; however, as Owens et al. evaluated the larger military population, controlling for age and race, no difference was found in the incidence of ACL tears between men and woman active duty members [16].

Return to duty status after ACL reconstruction is reported in level III studies. Return to duty for all military personnel has been reported in as high as 92 % of patients [39]. This data should be interpreted carefully, however, as many patients who undergo ACL reconstruction remain on physical limitations for a protracted period of time. Recent data demonstrate that three fourths of military patients who underwent an isolated ACL reconstruction are still on duty limitations (“profile”) at 3 months out from surgery, and one fourth remain there at 9 months out from surgery [1].

Revision surgery resulted in longer recoveries and longer time on a limited duty status compared to those patients undergoing a primary repair [39].

ACL injuries can occur as an isolated injury or as a combination of injuries to the structures about the knee. Individuals with ACL injuries will often have concurrent injuries of the cartilage and menisci. In the active duty population, 33.3 % of soldiers who have an acute ACL tear also sustain a medial meniscus injury, while 40 % suffer a lateral meniscus injury [33]. Patients who have subacute or chronic ACL injuries without restrictions of their activities have an increase and change in incidence of a meniscal injury . In the subacute group, 44 % suffer injury to the medial meniscus , while 51.7 % have an injured lateral meniscus [33]. Those with chronic ACL injuries have the highest rates of meniscal injury, with the medial meniscus injured in 79.5 % and the lateral meniscus injured in 61.5 % of patients [33]. Anatomically, the medial meniscus acts as a secondary joint stabilizer and therefore is placed at a higher risk for injury in patients whose primary stabilizer, the ACL, has been injured for a long period of time. The relative risk of injury to the medial and lateral menisci in soldiers with chronic ACL injuries is 7.75 and 2.4, respectively [33]. Data from the military population are similar to that of the general population where the prevalence of a meniscal injury with an acute ACL tear is estimated to be between 41 and 82 % and with a chronic injury 58–100 % [33]. Increase in age is not associated with increase in risk for concurrent meniscal injury with an ACL tear [33]. Incomplete tears are more common in the lateral meniscus compared to the medial meniscus [33]. Meniscal tears associated with ACL ruptures are generally complete, longitudinal, and localized to the posterior meniscus close to the meniscocapuslar junction [33].

Chondral lesions are also commonly associated with ACL injury. In the general population, chondral lesions are found to increase from 19 % at the time of ACL injury to as high as 70 % in patients having chronic injuries. In the active duty population, chondral injuries are found in about 9 % of patients with acute ACL injuries [33]. This number increases to 26 % in those with subacute injuries and as high as 70 % in patients with a chronic ACL tear. Active duty soldiers who have chronic untreated ACL injuries are 23 times more likely to have a chondral injury than soldiers with acute injuries [33]. In the active duty population, 55 % of patients with an ACL injury have a least two associated lesions to the cartilage or menisci, and 79 % of soldiers in the chronic ACL insufficient group have two lesions, almost 15 times higher than the acute group (20 %) [33]. Consistent with the above data, 24 % of patients more than 30 years old have both meniscus and cartilage lesions, which is significantly higher than the soldiers less than 30 years of age [33].

Treatment of chondral lesions in the setting of ACL deficiency has been recently evaluated. In patients undergoing ACL reconstruction with an associated chondral lesion, Gudas et al. compared debridement, microfracture, and an OATS procedure in a randomized clinical trial. At 3-year follow-up, the authors found that all forms of chondral treatment were inferior to isolated ACL reconstructions, but the OATS group significantly outperformed the microfracture and debridement groups in subjective patient satisfaction, whereas microfracture and debridement were not statistically different [40]. These data suggest that every effort should be made to restore native anatomy as close as possible for optimal return to athletic activities.

Multiligamentous Knee Injury and Knee Dislocation

Multiligamentous knee injuries and knee dislocations can be caused by high-energy trauma, sports injuries, or even a low-energy fall. While the true incidence within the military population is unknown, it is not an uncommon injury, and several reports exist describing the pathology and prognosis associated with these devastating injuries.

Owens et al. performed one such study and noted that 100 % of the patients with a knee dislocation had disruption of the ACL and posterior cruciate ligament (PCL), with an additional 86 % demonstrating disruption of the posterior lateral corner and 93 % sustaining an injury to the lateral collateral ligament [41]. In addition, there was a 3.5 % rate of vascular injury, and the peroneal nerve was injured 75 % of the time. While 67 % of patients with neurologic injury experienced a full recovery, the prognosis of nerve recovery was related to the severity of the injury, as complete nerve injuries generally did not recover meaningful function [41].

Return to duty after treatment of a multiligamentous knee injury in a military population is challenging. Ross et al. reported a return to duty of 54 %, with 46 % of soldiers eventually undergoing medical discharge for their injury [42]. In that study, there was no correlation between the soldier’s MOS (job title) and medical discharge. On the other hand, there was a positive correlation between higher rank and greater percentage return to military duty after surgery [42]. In general, soldiers reported their knees to feel stable, but reported that after surgery and rehab they were able to perform sports at “half speed” and had some limitations in daily living functional scores [42]. Those with associated injuries outside the knee had higher rates of medical discharge, with 67 % of soldiers undergoing a medical discharge [42].

The most common reported complication with treatment of the multiligamentous knee injury is arthrofibrosis. Owens et al. noted that 18 % of active duty soldiers sustaining a knee dislocation required a second operation for arthrofibrosis, while another 13 % had significant stiffness without requirement for a second procedure. The average arc of motion after reconstruction was 119°, with a 1.9° loss of extension and 10.2° loss of flexion [41]. Severe injury to multiple ligaments of the knee to include knee dislocation is a difficult injury to treat with a high rate of associated injury and complications .

Patellofemoral Joint

Dislocations

Acute traumatic patellar dislocation accounts for approximately 3 % of all injuries to the knee [43–45] and is caused by a valgus force in flexion in up to 93 % of cases [46]. In the civilian population, patellar dislocation has been reported between 0.029 and 0.070 per 1000 person-years, with 61 % of these dislocations secondary to an athletic injury [47–49]. Participation in a sport and/or physical activity is associated with patellar dislocation [46, 47, 50]. The rate of patellar dislocation among military personnel is significantly higher than the civilian population, with a rate of 0.69 per 1000 person-years [51]. This finding is echoed in military populations outside the USA [46]. The incidence of patellar dislocation varies by age, gender, military service, rank, and race [51] (Table 10.1).

Table 10.1

Comparison of previous population-based studies calculating incidence rates for patellar dislocation injuries [51]

Study (duration)	Population	Injuries	Population (person-years)	Age (years)	Incidence rate (per 1000 person-years)
Atkin et al. [52] (3 y)^a	Civilian, urban	74	1,102,005	11–56	0.067
Fithian et al. [20] (2–5 y)^a	Civilian, urban	125	1,944,000	10–3 +	0.058
Sillanpaa et al. [15] (5 y)	Military, Finnish	73	96,200	17–30	0.774
Current (10 years)	Military, USA	9299	13,443,448	17–40 +	0.692

^aResults for short- and long-term follow-up within the same cohort

Patellar dislocation results in injury to the medial patellofemoral ligament, medial retinaculum, and a hemarthrosis in almost all patients [46]. Nearly 25 % will have an osteochondral fracture [45], but the clinical significance of this is not well studied. The rate of dislocation is inversely proportional to age, with a higher number occurring in younger age groups. Active duty members less than 20 years are 84 % more likely to sustain a dislocation than those who are greater than 40 years old. Similar trends have been demonstrated in civilian populations [47, 50].

Active duty females are 61 % more likely to sustain a patellar dislocation when compared to men with an incidence rate of 0.63 per 1000 person-years compared to active duty men with an incidence rate of 0.39 per 1000 person-years [51]. Civilian literature demonstrates no difference between men and women in acute primary dislocation rate [49].

Other military -specific factors demonstrate increased risk of patellar dislocation. Higher rates of dislocation are seen in the Marine Corps, Army, and Air Force compared to the Navy, as a marine is 50 % more likely to sustain a dislocation compared to a sailor [51]. Junior officers and enlisted sustain the highest rate of dislocation when comparing ranks, whereas senior officers sustain the least. Senior enlisted suffer more dislocations than junior officers, which suggests rank does play a greater role than just age itself [51].

Recovery and return to duty after a dislocation is slow and significant. Over the first 6 months following injury, up to 50 % of patients report a decline in sports activity and pain associated with cutting, jumping, kneeling, and squatting [47]. Twenty one percent of patients experience functional limitations that prevent them from returning to active duty military service [46]. Recurrent instability and dislocations can be disabling. Some studies report recurrent instability and dislocations in 50 % of patients managed nonoperatively [53, 54]. Patients with two or more dislocations are 6.5 times more likely to experience another episode of instability [50]. Posttraumatic osteoarthritis is common regardless of recurrent instability [55].

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