Ultrasonographic Assessment of Posterior Shoulder Capsule Thickness in Baseball Pitchers: A Validation Study

A thickened posterior shoulder capsule may increase injury risk in baseball pitchers due to maladaptive shoulder biomechanics such as altered arthrokinematics, decreased internal rotation range of motion (ROM) and increased scapular upward rotation. While diagnostic ultrasound offers a reliable method of measuring posterior capsule thickness (PCT), it is currently unclear whether PCT thus measured is valid.

Diagnostic ultrasound is a valid method for measuring PCT compared with magnetic resonance imaging (MRI).

Cross-sectional.

Level 4.

Asymptomatic baseball pitchers recently drafted into professional baseball from a single organization were enrolled during the 2021 to 2024 seasons. Pitchers underwent both diagnostic ultrasound and shoulder MRI on the same day to minimize any acute changes in PCT. All shoulder ultrasounds were performed with a 15-MHz linear transducer. Posterior capsule was identified as the tissue immediately lateral to the tip of the labrum between the humeral head and rotator cuff. PCT was measured on axial MRI scans by a musculoskeletal radiologist blinded to the ultrasound measurements. Agreement between modalities was evaluated through Pearson correlations and Bland-Altman analysis.

Overall, 25 drafted pitchers were included. PCT obtained via diagnostic ultrasound had a mean of 2.4 ± 0.6 mm while PCT obtained via MRI scan had a mean of 2.4 ± 0.8 mm. Ultrasound PCT and MRI PCT were strongly positively correlated (R = 0.945, R² = 0.892, P < 0.001). The Bland-Altman plot demonstrated 95% limits of agreement of 0.55 mm between diagnostic ultrasound and MRI measurements of PCT.

Measuring PCT with diagnostic ultrasound is a valid technique compared with the gold standard of MRI.

Clinicians who wish to evaluate PCT in baseball pitchers may use this validated technique to quantify PCT and evaluate for potential contributors to glenohumeral internal rotation deficit.