Background: Redundancy of the capsule has been considered to be the main pathologic condition responsible for atraumatic posteroinferior multidirectional shoulder instability; however, there is a paucity of measurements providing quantitative diagnosis.
Purpose: To determine the significance of superior-capsular elongation and its relevance to atraumatic posteroinferior multidirectional shoulder instability at magnetic resonance (MR) arthrography.
Material and Methods: MR arthrography was performed in 21 patients with atraumatic posteroinferior multidirectional shoulder instability and 21 patients without shoulder instability. One observer made the measurements in duplicate and was blinded to the two groups. The superior-capsular measurements (linear distance and cross-sectional area) under the supraspinatus tendon, and the rotator interval were determined on MR arthrography and evaluated for each of the two groups.
Results: For the superior-capsular measurements, the linear distance under the supraspinatus tendon was significantly longer in patients with atraumatic posteroinferior multidirectional shoulder instability than in control subjects (P<0.001). The cross-sectional area under the supraspinatus tendon, and the rotator interval were significantly increased in patients with atraumatic posteroinferior multidirectional shoulder instability compared to control subjects (P<0.001 and P=0.01, respectively). Linear distance greater than 1.6 mm under the supraspinatus tendon had a specificity of 95% and a sensitivity of 90% for diagnosing atraumatic posteroinferior multidirectional shoulder instability. Cross-sectional area under the supraspinatus tendon greater than 0.3 cm2, or an area under the rotator interval greater than 1.4 cm2 had a specificity of more than 80% and a sensitivity of 90%.
Conclusion: The superior-capsular elongation as well as its diagnostic criteria of measurements by MR arthrography revealed in the present study could serve as references for diagnosing atraumatic posteroinferior shoulder instability and offer insight into the spectrum of imaging findings corresponding to the pathologies encountered at clinical presentation.
