Incidence of Posterior Shoulder Instability in the United States Military: Demographic Considerations From a High-Risk Population

Background: Relatively little is known about the true incidence of posterior shoulder instability in the United States.

Purpose/hypothesis: The purpose of this study was to characterize the incidence of posterior shoulder instability in the US military population and to examine which characteristics place these patients at highest risk. We hypothesized that the rate of posterior instability in this cohort of patients would be higher than previously reported.

Study design: Descriptive epidemiology study.

Methods: All initial cases of posterior shoulder dislocation and subluxation in the Defense Medical Epidemiology Database were screened between 2016 and 2018 using the relevant International Classification of Diseases-Tenth Revision-Clinical Modification codes. Incidences were calculated, and multivariate Poisson regression was used to calculate adjusted rate ratios for the effects of sex, race, age, military service branch, rank, and year.

Results: The overall unadjusted incidence was 0.032 per 1000 person-years for posterior dislocations, 0.064 per 1000 person-years for posterior subluxations, and 0.096 per 1000 person-years for all cases of posterior shoulder instability. The total incidence of all shoulder instability was 1.84 per 1000 person-years, with posterior shoulder instability accounting for 5.2% of total cases. For posterior subluxation, significant differences between groups were found in the unadjusted and adjusted rate ratio (ARR) for sex (ARR, 3.31; 95% CI, 1.85-5.93 for men), race (ARR, 0.458; 95% CI, 0.294-0.714 for Black servicemembers and ARR, 0.632; 95% CI, 0.421-0.948 for servicemembers of other race), age (ARR, 3.69; 95% CI, 1.56-8.70 for patients aged 30-34 years), and military service branch (ARR, 0.663; 95% CI, 0.460-0.955 for Air Force servicemembers). For posterior shoulder dislocations, a significant difference was found only between men and women (ARR, 4.55; 95% CI, 1.85-11.2 for men).

Conclusion: The incidence of posterior shoulder instability among US military personnel is higher than previously reported in the general US population, with a majority of cases due to subluxation. This increased incidence is likely reflective of a young and highly active population along with improvements in awareness and diagnosis of posterior instability in recent years.