Effects of shoulder external rotation on dynamic changes in the glenohumeral joint in baseball players

The purpose of this study was to determine the effect of glenohumeral joint dynamic changes on the proximity between the humeral head and posterior glenoid rim using magnetic resonance imaging (MRI).

The participants included 25 university and three adult baseball players who were part of a competitive-level team. MRI was performed using an AIRIS Vento 0.3-Tesla whole-body MRI under T2* imaging with gradient echo. The imaging positions were 90° abduction at 90°, 100°, and 110° of shoulder ER. The HHT measures the displacement rate of the humeral head relative to the scapular glenoid. The posterior glenohumeral distance (PGHD, mm) was measured by determining the distance between the posterior glenoid rim and the humeral head. Statistical analysis was performed using the Tukey or Steel-Dwass multiple comparison tests for comparisons of HHT for 90°, 100°, and 110° ER position of the throwing shoulder joint. A simple regression analysis was performed with PGHD as the dependent variable and HHT as the independent variable. Statistical analysis was using R4.4.1, with a significance level of 5%.

The participants’ average age (20.1±1.4 years), height (173.4±3.4 cm), weight (71.2±5.7 kg), body mass index (23.6±1.9 kg/m²), years of baseball experience (12.2±2.2 years), and practice frequency of the participants (5.3±1.5 times/week) were collected. 

In a comparison of the HHT with the ER angle of the throwing shoulders, the HHT at the 110° ER position was significantly lower than that at the 90° ER position (4.5 vs. –2.5°, p 0.01). Simple linear regression analysis showed that the HHT was associated with the PGHD at 90° (β coefficient = –0.649, p 0.001) and 100° (β coefficient = –0.556, p 0.001) ER positions.

The humeral head was displaced posteriorly relative to the scapular glenoid as the ER angle increased. The humeral head translation was associated with the posterior glenohumeral distance. These findings suggest that an increased ER angle of the glenohumeral joint contributes to contact between the humeral head and posterior glenoid rim of the scapula.

These findings provide evidence for understanding the dynamic changes in the glenohumeral joint during the late cocking phase for the assessment and approach to throwing shoulder injury.