The Incidence of Posterior and Combined AP Shoulder Instability Treatment with Surgical Stabilization Is Higher in an Active Military Population than in the General Population: Findings from the US Naval Academy

Abstract

Background

Anterior instability has consistently been shown to be the most common type of glenohumeral instability. Recent studies have demonstrated a higher percentage of posterior and combined (anterior and posterior) instability than had previously been reported; however, this work has not been replicated recently in a particularly young military population, which may be representative of an especially athletic or high-demand group.

Question/purpose

What proportion of arthroscopic shoulder stabilization procedures are performed to address isolated anterior instability, isolated posterior instability, and combined instability in a young, military population?

Methods

Between August 2009 and January 2020, two sports medicine fellowship–trained surgeons performed arthroscopic shoulder surgery on 543 patients at a single institution. During that time, the indication to be treated with arthroscopic stabilization surgery was symptomatic glenohumeral instability, as diagnosed by the operative surgeon, that restricted patients from carrying out their military duties. Of those, 82% (443 of 543) could be evaluated in this retrospective study, while 18% (100 of 543) were excluded due to either incomplete data or because the procedure performed was not to address instability. No patient underwent an open stabilization procedure during this period. Of the 443 patients investigated, the mean age was 22 ± 4 years, and 88% (392 of 443 patients) were men. Instability type was characterized as isolated anterior, isolated posterior, or combined (anterior and posterior) according to the physician’s diagnosis as listed in the patient’s clinical records and operative reports after the particular capsulolabral pathology was identified and addressed.

Results

Isolated anterior instability occurred in 47% of patients (210 of 443). Isolated posterior instability happened in 18% of patients (80 of 443), while combined anteroposterior instability occurred in 35% of patients (153 of 443).

Conclusion

Shoulder instability is common in the military population. Although anterior instability occurred most frequently, these findings demonstrate higher proportions of posterior and combined instability than have been previously reported. Surgeons should have a heightened suspicion for posterior and combined anteroposterior labral pathology when performing arthroscopic stabilization procedures to ensure that these instability patterns are recognized and treated appropriately. The current investigation examines a unique cohort of young and active individuals who are at particularly high risk for instability and whose findings may represent a good surrogate for other active populations that a surgeon may encounter.

Level of Evidence Level III; therapeutic study.

Introduction

As one of the most mobile joints in the body, the glenohumeral joint is vulnerable to dislocation and subluxation, especially when it is placed in compromised positions or engaged in high levels of activities or contact sports [12]. The glenoid labrum and its associated capsuloligamentous attachments provide direct restraint to subluxation and dislocation of the humeral head. An anterior shoulder dislocation involves tearing of the inferior capsuloligamentous complex and labrum from the anterior glenoid 97% of the time [18]. Similarly, posterior instability occurs when there is damage to the posterior capsuloligamentous structures and labrum, glenoid retroversion, or loss of dynamic subscapularis function [4]. Damage to these structures, with or without an associated osseous fragment, may warrant operative intervention to restore the proper anatomy and stability [3, 11]. With various surgical techniques indicated based on the anatomic location of the labral injury, proper recognition of tear patterns is critical for surgical preparation and patient success [16, 19]. Anterior instability has been reported to occur in as many as 90% of patients with shoulder instability [6, 14]. Posterior instability occurs less frequently, having traditionally been reported to occur in 2% to 10% of all patients with instability; however, more recent studies have reported proportions up to 18% [2, 7, 15].

Instability is a common cause of shoulder pain and dysfunction among young, active individuals. Glenohumeral dislocation rates in the United States are estimated at 0.24 per 1000 person-years, with higher rates of dislocations (1.69 to 4.35 per 1000 person-years) and subluxations (28.3 per 1000 person-years) observed among military service members than among the general population [8, 12, 14, 20]. Younger age has also been associated with higher rates of shoulder instability, particularly among men aged 20 to 29 years [5, 9, 10, 13]. In general, the military population is comprised of physically active people who engage in exercise and physical activity on a more routine basis than the general population, leading them to be more susceptible to activity-related injuries. This is especially the case at the military service academies, which comprise the youngest cohort within the active-duty population. Although anterior instability remains the most common instability type, studies across various population types have demonstrated higher proportions of posterior and combined instability than was previously reported [1, 8, 18, 20]. The current study seeks to evaluate the occurrence of instability type in a unique population that is at particularly high risk for instability. Although similar studies have looked at the occurrence of instability types among various populations, this work has not been replicated in a military population of this age group.

We therefore asked: What proportion of arthroscopic shoulder stabilization procedures are performed to address isolated anterior instability, isolated posterior instability, and combined instability in a young, military population?

Patients and Methods

Study Design and Setting

Following institutional review board approval, we performed a retrospective study of clinical records and operative reports on all patients with shoulder instability who underwent arthroscopic shoulder stabilization surgery performed at the United States Naval Academy between August 2009 and January 2020. Two sports medicine fellowship–trained surgeons (JPHR, LEL) performed all the surgical procedures included in this study.

The United States Naval Academy is a university attended by college-aged United States Navy service members who take academic classes to earn a bachelor of science degree while training to become commissioned officers in the United States Navy or Marine Corps over the course of a 4-year enrollment. The training involved to become a commissioned officer in the United States Navy and Marine Corps often consists of rigorous physical activity and exercise.

Patients

We reviewed clinical records and operative reports to obtain patient demographic data and to confirm the type of instability (isolated anterior, isolated posterior, or combined) that was treated. We included all patients who underwent an index arthroscopic shoulder stabilization procedure for capsulolabral injuries with or without the presence of a superior labral lesion by one of the two senior surgeons (JPHR, LEL). Final diagnosis of instability type was based on the physician diagnosis in the operative report. We excluded patients undergoing arthroscopic procedures without a diagnosis of instability and those undergoing a revision stabilization procedure.

Between August 2009 and January 2020, two sports medicine fellowship–trained surgeons (JPHR, LEL) performed arthroscopic shoulder stabilization surgery on 543 patients at a single institution. During that time, the indication to be treated with an arthroscopic stabilization surgery was symptomatic glenohumeral instability that prevented patients from carrying out their military duties. No open stabilization procedures were performed in the primary setting. Of those, 82% (443 of 543) could be evaluated in this retrospective study, while 18% (100 of 543) were excluded due to either incomplete data or because the procedure performed intended to treat another shoulder pathology. Of the 443 patients investigated, the mean age was 22 ± 4 years, and 88% (392 of 443 patients) were men.

Surgical Techniques

All arthroscopic procedures were performed with the patient in the lateral decubitus position. In all procedures, patients underwent an examination under anesthesia and an arthroscopic diagnostic evaluation to determine the location of the pathology needing repair. Stabilization was performed with suture anchor constructs to secure the involved capsulabral tissue onto the glenoid. If a superior labral lesion was noted, it was addressed with either a suture-based repair or débridement with or without biceps tenodesis. The anterior instability group included patients in whom the anterior or anteroinferior labral complex was primarily stabilized. The posterior instability group included patients in whom the posterior or posteroinferior labral complex was primarily stabilized. The combined instability group included those with a diagnosis of either panlabral tears, anterior plus posterior instability, multidirectional instability, and instability with extension of labral pathologic features into the superior labrum.

Data Collection

All clinical records were reviewed to ensure that the patient history, physical examination, and clinical diagnoses were consistent with the type of instability addressed at the time of surgery. Final categorization of instability type was based on the operative surgeon’s diagnosis in the operative dictation, and it was recorded on a master data spreadsheet as was each patient’s demographic data.

Statistical Analyses

Study size was determined by the total number of arthroscopic shoulder procedures performed by two surgeons between August 2009 and January 2020. Descriptive statistics were used to analyze patient age, sex, and location of the repaired labrum.

Results

Arthroscopic anterior stabilization occurred in 47% (210 of 443) of patients. Arthroscopic stabilization to address combined instability occurred in 35% (153 of 443) of patients. Posterior instability was addressed in 18% (80 of 443) of the patients undergoing posterior stabilization (Table 1).

Table 1.

Patient demographics based on the type of instability (n = 443)

Factor	Anterior	Posterior	Combined	Total
Percentage who underwent  stabilization, % (n)	47 (210)	18 (80)	35 (153)	433
Mean age in years ± SD	22 ± 3	22 ± 4	23 ± 5	22 ± 4
Men, % (n)	85 (178)	93 (74)	92 (140)	88 (392)

Discussion

Shoulder instability can be debilitating. Although anterior instability remains the most common type, recent studies [7, 17] have shown that posterior and combined instability occur more frequently than previously thought [1, 14]. Young and active populations present the greatest risk for experiencing instability, which may result in unique injury patterns. A better understanding of the common injury patterns seen in this population allow the provider to make a more complete diagnosis and provide the appropriate treatment plan to address the patient’s pathology. We found posterior and combined instability occurred more frequently than what has been traditionally reported. Maintaining a degree of high degree of suspicion of posterior and combined capsulabral injury in these patients may prevent surgeons from inadequately treating the full extent of the patient’s injury and contribute to better outcomes.

Limitations

This study has several limitations. The study was limited to patients who underwent arthroscopic shoulder stabilization, presenting a selection bias. Excluding patients with a diagnosis of instability managed nonsurgically or those treated with open stabilization fails to fully capture the true occurrence of instability types. The young, military patient population investigated in this study mostly consisted of men, which may also make the findings less generalizable to a broader patient population. This population is at higher risk for instability than other populations, leading to unique injury patterns potentially not seen elsewhere. The indication for arthroscopic stabilization was instability that was functionally limiting. The daily physical demands on this cohort make nonoperative management of shoulder instability less likely, resulting in more patients proceeding with surgical stabilization than would be seen in less active individuals.

Types of Instability

Among the patients included in this study, isolated anterior instability occurred most frequently, followed by combined and posterior instability, respectively. The proportions of combined and posterior instability are the most compelling aspect of our study. Although anterior instability has been consistently shown to be the most common type to occur, newer studies have demonstrated higher frequency of combined and posterior instability than what was previously considered [1, 7, 14, 17]. The present study supports these recent findings, while demonstrating even higher frequencies of combined and posterior instability (Table 2). The present study observes these trends in a uniquely high-risk population of younger individuals than what was observed in prior studies, which showed increased rates of combined and posterior instability. Providers caring for patients who are at particularly high risk of instability should be aware of the different instability patterns that are more prevalent than previously thought. Heightened awareness of the increased prevalence of the posterior and combined capsulolabral pathology in these patients may lead to more accurate diagnoses and corresponding operative plans that more comprehensively address the patient’s condition.

Table 2.

Comparison of our results with those of previous studies

Study	Anterior, % (n)	Posterior, % (n)	Combined, % (n)	Mean age in years	Total
Owens et al. [14]	80 (94)	10 (12)	9 (11)	—a	117
Blomquist et al. [1]	83 (336)	10 (40)	7 (28)	—a	404
Song et al. [17]	57 (132)	24 (56)	19 (43)	26	231
Javed et al. [7]	53 (234)	16 (72)	31 (136)	26	442
Current study	47 (210)	18 (80)	35 (153)	22	443

^aThe mean age of all patients was not reported.

In a prospective evaluation of military personnel at the United States Military Academy, Owens et al. [14] reported on the characteristics of shoulder instability observed during a 1-year period. Within the observed cohort, they found anterior instability to be the most common type (Table 2). Although this study evaluated a similar population of young military personnel, it also included patients with shoulder instability that was managed nonoperatively. Another study of active-duty military personnel retrospectively evaluated a series of 231 patients who underwent surgical stabilization [17]. Although anterior instability remained the most common type observed in this study, higher proportions of posterior and combined instability were observed (Table 2). The study population was similar to the present study in that it consisted of active-duty military personnel who were mostly men. However, fewer patients were observed, and with a higher mean age of 26 years, this may be a patient population that is, albeit active, at slightly lower risk for instability than those in the present study.

Before the current study, Javed et al. [7] published the largest retrospective study investigating the incidence of shoulder instability types; they reviewed 442 shoulders that underwent surgical stabilization over a 3-year period. The authors observed a heterogenous population of both active (“sporting”) and less active (“nonsporting”) individuals with a mean age of 26 years. Of all patients, the proportion of combined and posterior instability was found to be 30.8% and 16.3%, respectively, with anterior instability remaining the most common type. Further comparison between “sporting” and “nonsporting” groups showed higher proportions of posterior and combined instability in the “sporting” group (17.5% and 34.7%, respectively) than in the “nonsporting” group (12.6% and 18.9%, respectively). These findings are consistent with the increased occurrence of combined instability found in our investigation. Although the observed patients were older, the increased occurrence of combined instability in the “sporting” group is consistent with our current findings in highly active patients when compared with less active populations [1]. The increased occurrence of posterior and combined instability seen in the current study further emphasizes the importance of investigating for further capsulolabral pathology when evaluating young and active individuals.

Conclusion

Instability is a common cause of functional shoulder limitation. Although anterior labral tears remain the most common injury pattern, this study further corroborates findings [7, 17] that posterior and combined glenohumeral instability occur more frequently than previously reported [1, 14]. Furthermore, the higher frequency seen in this young, military population suggests that younger patients may be more likely to experience combined instability compared with older patients reported on in other reports [7, 17]. In addition to age, further investigation of a patient’s activity level can help the provider identify those at greater risk for posterior and combined instability. Performing a thorough history, physical examination, and interpretation of diagnostic imaging is important to accurately diagnose capsulolabral pathologic conditions and to determine the best treatment plan. When evaluating young and active patients presenting with instability, the provider should have a heightened suspicion for posterior or combined capsulolabral pathology to develop an appropriate treatment plan. Further studies are needed to delineate patient-specific risk factors that place young and active individuals at risk for the various types of instability, while incorporating radiographic parameters to gain a more comprehensive understanding of the type and severity of instability seen in this patient population.