Association Between Shoulder Morphological Changes and Glenohumeral Maximum External Rotation During Late Cocking Phase.

Background: Excessive external rotation of the glenohumeral joint during the late cocking phase of throwing is a factor in internal impingement; however, the relationship between maximum external rotation (MER) of the glenohumeral joint and morphological changes in the shoulder joint is unclear.

Purpose: To clarify whether glenohumeral MER is associated with quantified assessment of morphological changes in the throwing shoulder joint.

Study design: Descriptive laboratory study.

Methods: This study included 15 male university and adult baseball players from a competitive team. The posterior glenohumeral distance (mm) and area of impingement (mm²), reflecting morphological changes in the shoulder joint, were measured using open magnetic resonance imaging. The percent posterior glenohumeral distance (%PGHD) and percent area of impingement (%AOI) were calculated as these values of the throwing shoulder divided by those of the nonthrowing shoulder. With a 3-dimensional motion analysis system, bar markers were affixed to the acromion and humerus, and the glenohumeral MER angle was measured.

Results: Simple linear regression analysis revealed that the glenohumeral MER angle was associated with the %PGHD (β coefficient = 0.685; P = .005) and %AOI (β coefficient = 0.754; P = .001).

Conclusion: The glenohumeral MER angle was associated with the %PGHD and %AOI, which reflects morphological shoulder-joint changes.

Clinical relevance: Assessment of excessive external rotation of the glenohumeral joint during the late cocking phase contributes to the understanding of morphological changes in the shoulder joint and related throwing injuries.