Shoulder Position during Magnetic Resonance Arthrogram Significantly Affects Capsular Measurements

Purpose: The purpose of this study was to determine if change in shoulder position between internal rotation (IR) and external rotation (ER) during MRA affects previously defined capsular measurements, and determine the utility of rotation in the diagnosis of instability.

Methods: A retrospective study of patients who received a shoulder MRA with humeral internal (IR) and external rotation (ER) views was conducted. Patients with an arthroscopically confirmed diagnosis of instability and those with clinically stable shoulders, no history of instability, and no MRA findings of instability were identified and compared. Humeral rotation, glenoid retroversion, humeral head subluxation, capsular length, and capsular area using axial sequences of IR and ER views were recorded. Analysis compared IR, ER, and Delta (Δ) capsular measurements between groups using independent t-tests, univariate and multivariate regression.

Results: Thirty-one subjects who were diagnosed with instability were included along with 28 control subjects. Capsular length, capsular area, and humeral subluxations were significantly greater with ER compared to IR views (p < 0.001, p < 0.001, p < 0.001). Patients with instability displayed greater ER capsular length (p = 0.0006) and ER capsular area (p = 0.005) relative to controls. Multivariate logistic regression identified age, weight, gender, ER capsular length and retroversion to be significant predictors of instability. ER capsular length independently predicts instability with 86% sensitivity and 84% specificity. Inter-observer reliability using the intra-class correlation coefficient were rated good or excellent on all measurements.

Conclusion: Variance in humeral rotation during shoulder MRA significantly affects capsular measurements. Rotational views increase the utility of capsular measurements when assessing for instability, particularly those of capsular length and capsular area. The implementation of ER positioning enhances the ability to examine capsular changes of the shoulder joint and can assist in the diagnosis of instability.