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Epidemiology of Shoulder Instability

Incidence, Risk Factors, and Prevention of Instability in the Athlete

David J. Tennent, MD; Matthew A. Posner, MD; and Kenneth L. Cameron, PhD, MPH, ATC

Acute traumatic glenohumeral joint instability, subluxations and dislocations, is a pervasive problem in young and physically active individuals.^1–8 In the US population, the incidence of shoulder dislocation is approximately 23.9 per 100,000 person-years, with nearly half of the glenohumeral dislocations occurring during sporting activities and in patients between age 15 to 29 years.¹ These injuries typically occur more commonly in male than female individuals at a ratio of 2.64:1 male:female.¹ Similar trends in the age- and sex-based association for glenohumeral instability have been reported in Norwegian, Swedish, Canadian, Polish, and British population-based studies.^3,5–9

These injuries can be particularly problematic in young, high-risk male patients because individuals with a history of a glenohumeral instability event, especially at a young age, have a substantially increased risk of subsequent recurrent events.^8,10–14 These recurrent injuries can result in further damage to the soft-tissue structures of the capsule, labrum, ligaments, and rotator cuff. Furthermore, recurrent instability events can lead to progressively increasing levels of osteochondral loss from the humerus and glenoid that can result in an additional risk of future glenohumeral instability.^15–18 Persistent glenohumeral joint instability also can contribute to the initiation and progression of osteoarthritis in the shoulder that can cause a substantial degree of disability, socioeconomic cost, and health care cost.^19–22 Consequently, many of these injuries require surgical stabilization to definitively manage the loss of the stabilizing effects of the attenuated surrounding osseous and soft-tissue structures that can lead to persistent glenohumeral instability and eventual glenohumeral arthropathy.^4,22–25 Though surgical stabilization can restore joint stability and improve function, whether surgical repair can mitigate the long-term consequences of the initial instability event remains unclear.

INCIDENCE AND PREVALENCE OF GLENOHUMERAL INSTABILITY

Multiple epidemiologic and population-based studies have evaluated the incidence of glenohumeral instability within the general population and in young, active, high-risk populations such as athletes, military service members, and cadets (Table 2-1). The majority of these studies have focused on anterior glenohumeral dislocations as reported by various large database registries.^{1,6,7,9,11,26–30} In sum, these studies suggest that young men are at the highest risk of sustaining a traumatic glenohumeral dislocation event. These studies have shown that the nationwide incidence of glenohumeral dislocation in North American and Western European countries is between 23.1 to 56.3 cases per 100,000 person-years.^1,5 In men, the incidence was reported to be between 34.9 to 40.4 cases per 100,000 person-years, and in women incidence was much lower at 11.8 to 15.5 cases per 100,000 person-years. Furthermore, these studies show that 60% to 91.3% of all dislocations occur in male patients, with a peak incidence rate in men younger than 30 years ranging between 47.8 to 204.3 cases per 100,000 person-years.

Several studies have evaluated glenohumeral instability in athletes. When looking at high school–age athletes, Robinson et al found an overall incidence rate for dislocations to be 2.15 per 10,000 athlete exposures.³⁴ More recently, using online reporting databases, Kraeutler and colleagues found the overall incidence rate for shoulder dislocation to be 2.04 per 100,000 athletic exposures (AEs) in high school athletes and 2.58 per 100,000 AEs in collegiate athletes.26 These rates were highest in collegiate men’s ice hockey (7.42 per 100,000 AEs), wrestling (5.05 per 100,000 AEs), lacrosse (3.59 per 100,000 AEs), football (3.29 per 100,000 AEs), and women’s basketball (3.32 per 100,000 AEs).²⁶ Although lower, these rates were similarly high between college and high school athletics except in football, which showed a lower incidence rate in collegiate football compared to high school football (3.29 vs 6.34 per 100,000).²⁶ This corresponds to previous studies looking at all collegiate glenohumeral instability episodes that showed the highest-risk sports consisting of football, wrestling, and hockey.³⁵ Furthermore, these injuries most commonly occur during practices and are frequently secondary to direct contact with another player in male athletes and with an object in female athletes.³⁵

Within the highest-risk cohort of male collegiate football players participating in the National Football League Combine, 14.9% of athletes showed magnetic resonance imaging (MRI) evidence of a labrum tear indicative of some degree of glenohumeral instability.36 These rates were highest in linemen, with 19.2% of athletes displaying labrum pathology. Furthermore, the labrum pathology was found to be nearly equally distributed between anterior (30.4%), posterior (34.7%), and combined anterior and posterior pathology (34.7%).³⁶

Studies evaluating the incidence of glenohumeral instability in military populations have routinely demonstrated that this population is at high risk for acute traumatic and recurrent instability. When looking at the general US military population, Owens et al reported that 92.5% of glenohumeral dislocation events occurred in men, with an overall incidence rate of 169 cases per 100,000 person-years.²⁹ Similarly, Kardouni and colleagues found an overall 10-year incidence rate of 31.3 cases per 100,000 in US Army military personnel, with the highest risk groups in young, male soldiers.¹¹ This incidence rate increased to 435 cases per 100,000 when looking at isolated collegiate-age men and women matriculating at the US Military Academy at West Point who sustained any instability event.³⁰

Traumatic glenohumeral subluxations and dislocations can have a substantial negative effect on an individual’s ability to perform at a high level. Acute glenohumeral instability injuries can result in up to 8% of high school athletes being unable to return to sport during the same competitive season.³⁴ Although the majority of these are able to return to sport during the same season, 30% of high school collision athletes require more than 10 days to return to play following an acute glenohumeral instability event.^26,34,37 When compared to high school athletes, collegiate athletes have shown a longer recovery time required following an acute glenohumeral instability event, with approximately 45% of collegiate athletes requiring more than 10 days to return to sport.³⁸ Furthermore, only 73% of collegiate contact athletes are able to return to their sport during the same season, with the vast majority of these athletes continuing to experience symptomatic instability during the same season before undergoing stabilization procedures.³⁸ Professional athletes have shown the fastest return to sport among collision athletes; however, the sequela of acute or chronic glenohumeral instability can drastically affect the future careers of those athletes because of decreased playing time and later draft selection in those athletes participating in the National Football League Combine.^39–41

CHARACTERIZATION OF INSTABILITY

The overwhelming majority of glenohumeral subluxations and dislocations in the general population occur as a traumatic anterior instability injury or event.^2,15,29,42 It is further important to differentiate between a true glenohumeral dislocation event, which can be defined as a complete dissociation of the glenohumeral joint that often requires a reduction maneuver to restore joint congruity, and a subluxation event, in which some degree of glenohumeral articular contact remains. Subluxation episodes account for up to 85.6% of all glenohumeral instability events, and approximately 90% of all glenohumeral instability events occur secondary to an acute traumatic event.28 Furthermore, between 48% and 60% of dislocations are related to sporting activities.^1,28 Although anterior instability events are most commonly reported, combined anterior and posterior labrum pathology is frequently encountered in up to 37% of cases on MRI and during arthroscopic surgery in young, active patients.^36,43,44 This is critical to recognize intraoperatively to prevent treatment failures due to residual pathology and continued pathologic glenohumeral motion, because MRI can often incompletely characterize the full extent of the injury.^44,45 Whereas the majority of surgically managed glenohumeral instability is primary anterior instability, recent literature has shown that up to 25% of surgically treated glenohumeral instability cases showed evidence of anterior and posterior labrum pathology that required operative intervention at the time of arthroscopy.⁴⁴ Isolated posterior instability accounts for up to 17% to 19% of all glenohumeral events in some studies.^2,44 Although a true posterior dislocation event requiring a manual reduction is uncommon, a subluxation event is reported in 54% of cases with symptomatic shoulder pain during activities that load the posterior labrum, such as pushups and bench-press, reported in 42% of cases.²

CONCOMITANT INJURIES

Although the anterior inferior labrum is most commonly injured following a glenohumeral instability event, these injuries are often accompanied by various concomitant pathologic entities that can complicate appropriate management. Approximately 23% of patients have combined superior labrum and anterior or posterior labrum pathology when undergoing surgery.²⁸ Furthermore, when looking at intra-articular pathology seen at the time of arthroscopy, Yiannakopoulas et al reported that patients undergoing surgery for glenohumeral instability had an anterior labroligamentous periosteal sleeve avulsion lesion in 10.23% of cases, humeral avulsion of the glenohumeral ligament lesions (HAGL) in 1.57% of cases, and a Hill-Sachs lesion in 88.1% of cases.⁴⁶ Interesting, HAGL lesions have been seen at a much higher incidence in female patients, with as many as 25% of female athletes having evidence of HAGL lesions at the time of arthroscopy.⁴⁷ Concomitant rotator cuff pathology in glenohumeral instability is rare in young patients, with several small series reporting rates between 3% and 6%; however, these injuries become more common with increasing age following a traumatic dislocation event.^27,48–52 Axillary nerve injury can occur in 13.5% to 48% of glenohumeral dislocation events, and this is also correlated with increased age at the time of injury.^27,53,54 Articular cartilage injuries are commonly seen in approximately 18% of primary stabilization procedures and in nearly half of those individuals undergoing revision glenohumeral stabilization procedures.⁵⁵ Glenoid bone loss in relation to humeral bone loss must also be carefully assessed for appropriate management because first-time and recurrent glenohumeral instability events can result in increasing levels of clinically relevant bone loss that can result in failed management.^56–58

Identifying modifiable and nonmodifiable risk factors for acute traumatic glenohumeral instability is important because they represent targets for injury-prevention intervention and define subpopulations at the greatest risk for injury who could benefit from injury-prevention interventions, respectively. A summary of potential risk factors for glenohumeral instability is presented in Table 2-2. Although these risk factors are not all actionable for preventive measures, understanding who is at risk for glenohumeral instability is critical for appropriate surveillance and implementing injury-prevention interventions. Furthermore, appropriate identification of those individuals with modifiable risk factors may allow preventive interventions to be most effective.

Modifiable Risk Factors

Possible modifiable risk factors for glenohumeral instability can be classified into 2 categories: strength modulation and activity modification. The dynamic stabilizing effect of the rotator cuff has been investigated as a potential modifiable risk factor for glenohumeral instability with unclear results.^59–61 Weak rotator cuff musculature has been associated with glenohumeral instability retrospectively; however, in a large, prospective cohort study specifically looking at external and internal rotation strength, no differences were seen between those individuals who had a subsequent glenohumeral instability event and those who did not.^59,60 Consequently, although a weakened rotator cuff may result following a glenohumeral instability event, evidence does not support increasing rotator cuff strength as a primary prevention intervention for glenohumeral instability. Strengthening the rotator cuff following incident injury, however, may be an important secondary prevention intervention in terms of mitigating recurrent instability.60 The proprioceptive and sensorimotor function of the capsulolabral soft tissues may represent a target for secondary injury–prevention interventions as well, because the mechanoreceptor activity has been found to be disrupted and subsequently restored following surgical stabilization after a glenohumeral instability event.^62–64 However, it remains unclear whether sensorimotor deficits before initial injury exist in those who go on to experience an incident glenohumeral instability event in contrast to those who do not. Exercises specifically focused on improving the proprioceptive activity and dynamic stability of the glenohumeral joint may help prevent further instability events after initial injury and should be considered important secondary injury–prevention interventions.^64,65

Because the majority of glenohumeral instability events occur during contact sports and high-velocity activities, activity modification and avoidance of high-risk activities can greatly reduce an individual’s risk for glenohumeral instability. In particular, those individuals participating in contact sports and in military occupations have shown significantly higher risks for glenohumeral instability because of contact forces and/or occupational hazards. Although limiting the number of exposures to these activities may reduce the risk of glenohumeral joint instability, it is often not reasonable to modify these activities because of personal desire and/or occupational requirements. However, limiting exposure through free-time and recreational activities may be warranted.

Nonmodifiable Risk Factors

Understanding the nonmodifiable risk factors for glenohumeral instability is important to identify those individuals who are at a high baseline risk for glenohumeral instability. Those individuals in high-risk baseline categories should be targeted for further study and the development and evaluation of preventive strategies and counseling to reduce the risk of incident injury. The most important nonmodifiable risk factors for glenohumeral instability and recurrent instability are age and sex. Multiple studies have established that male individuals younger than 30 years have the highest dislocation rates in all populations.^1–8

Although much of the literature has shown that male sex is in itself a risk factor for glenohumeral instability, more recent studies have shown contrasting results in female athletes in sex-matched sports and exposures such as those seen in many contact and collision sports.^35,66 Specifically, no differences were reported in collegiate soccer, basketball, softball/baseball, ice hockey, and rugby.^35,66 The similarities in glenohumeral dislocation rates in these sports are likely due to the similar exposure in these sports between the sexes compared to the general population.

Anatomy has also been recognized as an important non-modifiable risk factor for glenohumeral instability. Glenoid gross anatomy, attritional bone loss, glenoid version, and coracohumeral interval distance have all been implicated in glenohumeral instability. The “inverted pear” glenoid, in which the inferior glenoid is less wide than the superior glenoid, has been shown to correlate with increasing levels of glenoid bone loss and subsequent instability.⁵⁶ In a large, prospective cohort study, participants with tall, narrow glenoids were found to be at higher risk of anterior glenohumeral instability than those with short, wide glenoid morphology.⁴² Those patients with an anatomic chondrolabral cleft were similarly found to have 2.8 times higher risk for anterior glenohumeral instability.⁶⁷ Glenoid version has also been implicated in posterior glenohumeral instability. Those patients with posterior instability were observed to have 5 degrees more retroversion than those without instability and had a 17% increased risk for posterior instability for every 1 degree of increased glenoid retroversion.^68,69 Similarly, Owens et al reported that for every 1 mm of increased coracohumeral distance, individuals were 20% more likely to sustain a glenohumeral instability event.⁴²

Risk of Recurrence

The success of nonoperative treatment of glenohumeral instability is influenced by age at the time of initial dislocation and male sex. In a recent systemic review, recurrent glenohumeral instability developed in 47% of individuals when combining all level-I studies; participants were most likely to have a recurrent event in the first year following injury, male participants were 3 times more likely to experience a recurrent episode, and those younger than 20 years were 13 times more likely to fail nonoperative treatment.¹³ In the general population, 55.7% of patients who experience a first-time anterior dislocation will develop recurrent instability by 2 years.¹² Furthermore, 86.7% of those individuals who experience a recurrent dislocation will sustain this by 2 years.⁷⁰ In 25-year outcome data looking at nonoperative treatment of primary anterior shoulder dislocations sustained in patients age 12 to 40 years, 57% of patients sustained a recurrent anterior glenohumeral instability event with increased failure of nonoperative treatment corresponding with a younger age at the time of initial dislocation.⁷¹ Other risk factors found to be strongly correlated in a meta-analysis with recurrent anterior glenohumeral instability include hyperlaxity and a corresponding greater tuberosity fracture.¹⁴ Weaker evidence has also suggested that a bony Bankart lesion, occupation, participation in physical therapy, and nerve palsy were also risk factors for recurrent instability.¹⁴

In high-risk populations the risk of recurrence following nonoperatively treated glenohumeral instability is even higher. In the military cadet population, individuals with a prior history of nonoperatively treated instability were 5.6 times more likely to have a subsequent anterior glenohumeral event and 4.6 times more likely to sustain a posterior instability event. The risk of recurrent anterior glenohumeral dislocation in high-school and collegiate athletic populations is as much as 9.5 times greater than in those without a history of previous instability.72,73 When instability is further classified as a glenohumeral dislocation or subluxation, subluxationevent patients were shown to return to sport in 89% of cases during the same athletic season without surgery compared to only 26% of true glenohumeral dislocations.⁷⁴ These differences between athletes and the general population are likely due to a higher exposure to an injury event without anatomic restoration of the native stabilizing anatomy.

Related posts:

Team Physician Principles for the Management of Athletes Open Treatment of Anterior Instability Evaluation of Shoulder Instability on the Field and in the Clinic Management of In-Season Athletes With Posterior Glenohumeral Instability Arthroscopic Anterior Shoulder Instability in the Athlete Imaging of Posterior Shoulder Instability

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