Abstract
Summary Statement
We measured scapulothoracic motions during humeral abduction with different humeral rotations in healthy subjects and whole cadaver models and clarified that humeral rotation significantly influenced scapular kinematics.
Introduction
Scapular dyskinesis has been observed in various shoulder disorders such as impingement syndrome or rotator cuff tears. However, the relationship between scapular kinematics and humeral positions remains unclear. We hypothesised that humeral rotation would influence scapular motions during humeral abduction and measured scapular motion relative to the thorax in the healthy subjects and whole cadavers.
Methods
Healthy Subjects: Twenty-four shoulders of twelve healthy subjects without shoulder disorders were enrolled. Three electromagnetic sensors were attached on the skin over the sternum, scapula and humerus. Scapular motions during scapular plane abduction (abduction) were measured. The measurements were performed with four hand positions, palm up, thumb up, palm down and thumb down. The elbow was kept extended in all measurements. Each measurement took 5 seconds and repeated three times.
Cadavers: Twelve shoulders from 6 fresh whole cadavers were used. A cadaver was set in sitting position on a wooden chair without interrupting scapular motions. Electromagnetic sensors were attached on the thorax, scapula and humerus rigidly with transcortical pins. The elbow was kept in extended position by holding the forearm and the arm was moved passively. The measurements were performed during scapular plane abduction and scapular kinematics were measured in four hand positions, 1: thumb up, 2; palm up, 3; palm down, 4; thumb down as well as the healthy subjects. Each measurement took 5 seconds and repeated three times.
Data Analysis: The coordinate system and rotation angles of the thorax, scapula and humerus were decided following ISB recommendation. A one-way analysis of variance was used to test the differences in 4 arm positions. Dunnet's multiple post hoc tests were used to identify the difference between thumb up model (neutral rotation) and other three arm positions.
Results
Scapular posterior tilt increased during palm up abduction (healthy subjects −2.0° to 0.1°, cadaver −3.2° to −1.4° at 120° of abduction). During thumb-down abduction, scapular posterior tilt decreased (healthy subjects −4.1° to −8.0° at 110° of abduction, cadaver −3.2° to −8.6° at 120° of abduction) and scapular upward rotation increased (healthy subjects 21.0° to 26.1° at 110° of abduction, cadaver 25.3° to 31.1° at 120° of abduction). Thumb down abduction demonstrated no significant difference from thumb up position.
Discussion
Scapular motions measured in healthy subjects and cadaver models showed similar patterns indicating that surface markers on the healthy subjects could track scapular motions successfully as bone markers in cadaver models. Humeral external rotation increased scapular posterior tilt and humeral internal rotation increased scapular anterior tilt and upward rotation. This suggests that position of the greater and lesser tuberosity and tension of the joint capsule caused scapular tilt and scapular upward rotation. Kinematic changes caused by humeral rotations were observed in earlier phase of abduction in healthy subjects than in cadaver models. This suggests that healthy subjects set scapular position beforehand not to increase subacromial pressure.
Conclusion
Humeral rotation significantly influenced scapular kinematics. Assessment for these patterns is important for evaluation of shoulder pathology associated with abnormal scapular kinematics.
