Poster 106: Association Between Glenoacromial Spacing and the Risk for Shoulder Instability

OBJECTIVES: Biomechanical studies have shown how the muscular dynamics about the shoulder are affected by the relative glenoid and acromion positions. Some believe these dynamics may play a role in predisposing shoulders to develop either osteoarthritis or rotator cuff disease. It is unclear if these dynamics also place individuals at increased risk for shoulder instability. To address these gaps in the literature, our objectives were to establish normative values for shoulder morphology in young physically active military service members and determine the influence of shoulder morphology, specifically the critical shoulder angle (CSA) and acromial index (AI) on rotator cuff strength and risk of shoulder instability. METHODS: A subset of 160 US Service Academy cadets (25 Female, 18.81 ± 0.95 years, 177.01 ± 8.14 cm, and 72.52 ± 12.12 kg) were analyzed from a larger prospective cohort study, including all participants who sustained a shoulder instability injury during the 4-year follow-up period (n=36, all anterior). The remaining participants served as healthy controls. Upon entry to the primary study, baseline bilateral shoulder magnetic resonance imaging (MRI) and rotator cuff strength data were collected. Baseline MRIs were used to assess critical shoulder angle (CSA) defined as the angle formed between the glenoid plane at the inferior most glenoid to the lateral most aspect of the acromion and acromial index (AI) defined as glenoacromial distance (GA) divided by the glenohumeral distance (GH). Additionally, isometric internal rotation (IR) and external rotation (ER) strength was assessed at both 0° and 45° of glenohumeral abduction. Normative values for healthy shoulder morphology were reported by limb dominance. Multivariate linear regression models were used to estimate the association between shoulder morphology and strength. Finally, odds ratios (OR) were calculated using multivariate logistic regression models to estimate the likelihood of shoulder instability based on shoulder morphological outcomes. All multivariable models statistically controlled for the influence of potential confounding variables, including sex, height, weight, and limb dominance. RESULTS: Normative baseline values for CSA, AI, glenohumeral distance and glenoacromial distance were established in young physically active military service members. In multivariate modeling, a positive relationship was detected between GH and ER strength at 45° abduction (Coefficient = 0.35, 95% CI: 0.04, 0.67), and between GA and IR strength at both 0° abduction (Coefficient = 4.29, 95% CI: 0.01, 8.56) and 45° abduction (Coefficient = 4.48, 95% CI: 0.81, 8.14). A positive relationship was detected between shoulder morphology and the likelihood of sustaining a shoulder instability injury (CSA: OR = 1.29, 95% CI: 0.97, 1.72; GA: OR = 14.63, 95% CI: 1.03, 206.98). These results indicate every 1° increase in CSA was associated with being 29% more likely to suffer a shoulder instability injury, while every 1cm increase in GA was associated with being 14 times more likely to suffer a shoulder instability injury. No other significant relationships were detected. CONCLUSIONS: Shoulder morphology was observed to play a role in both rotator cuff strength and the likelihood of glenohumeral instability injury. Interestingly, glenoacromial distance, specifically, was associated with both ER and IR strength, and the likelihood of shoulder instability. One theory is the larger the glenoacromial distance, the greater the moment arm for the anterior and posterior deltoid to act on the humerus. This increased mechanical advantage and resultant shear at the glenohumeral joint may be what contributes to the increased risk for instability. Another theory is the laterally placed acromion could be providing a fulcrum for the humerus to dislocate. Shoulder instability is a complex problem with many contributing factors and the glenoacromial scapular morphology may be yet another risk factor contributing to relative rotator cuff weakness and risk of shoulder instability in young active- duty military service members.