Shoulder Position During MRA Affects Diagnosis of Instability

Differences in humeral rotation during shoulder magnetic resonance arthrogram can have a big impact on capsular measurements as well as the accuracy of diagnosis of instability, researchers say.

They wrote that they sought “to determine whether change in shoulder position between internal rotation (IR) and external rotation (ER) during magnetic resonance arthrography (MRA) affects previously defined capsular measurements and to determine the utility of rotation in the diagnosis of instability.”

The findings of their study, “Shoulder Position During Magnetic Resonance Arthrogram Significantly Affects Capsular Measurement,” published in the January 2021 issue of the journal Arthroscopy showed that capsular length, capsular area, and humeral subluxations were greater with external rotation during the magnetic resonance arthrogram than with internal rotation.

Patients with instability had greater external rotation capsular length (p = .0006) and external rotation capsular area (p = .005) than the controls, according to the findings.

For the retrospective study, the researchers collected data on patients who had received a shoulder MRA with humeral internal and external rotation views. Then they identified the patients with an arthroscopically confirmed diagnosis of instability, those with clinically stable shoulders, no history of instability and no MRA findings of instability.

Humeral rotation, glenoid retroversion, humeral head subluxation, capsular length, and capsular area using axial sequences of internal rotation and external rotation views were all recorded and compared.

The study included 31 patients with diagnosed shoulder instability and 28 controls. Analysis showed that age, weight, sex, external rotation capsular length, and retroversion were all significant predictors of instability.

External rotation capsular length was particularly predictive with 86% sensitivity and 84% specificity.

The researchers wrote, “Variance in humeral rotation during shoulder MRA significantly affects capsular measurements. Rotational views increase the utility of capsular measurements when assessing for instability, particularly 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.”