Scapular positioning assessment: is side-to-side comparison clinically acceptable?

Clinicians routinely assess scapular position and motion of the symptomatic shoulder taking as reference for the contralateral asymptomatic side. A different positioning between sides (scapular asymmetry) is often assumed as pathological, however, the symmetry of scapular kinematics in healthy individuals is yet to be demonstrated. This study tested the hypothesis of scapular symmetry during arm elevation. The 3-dimensional scapular positioning of the dominant and non-dominant shoulders of fourteen healthy young adults was simultaneously measured by a 6 degrees of freedom electromagnetic tracking device at three positions of arm elevation: rest, hands on hips, and 90° of shoulder abduction with internal rotation. The scapula on the dominant shoulder showed greater retraction (P < 0.001; η(2)(p) = 0.68) and upward rotation (P < 0.001; η(2)(p) = 0.70) at all positions of arm elevation. From rest to 90° of shoulder abduction, the mean (±SD) amount of scapular angular displacement was, respectively for dominant and non-dominant shoulders, 7.2° (±7.8°) and 7.2° (±4.4°) for retraction, 17.4° (±5.1°) and 17.8° (±6.4°) for upward rotation, and 3.8° (±3.6°) and 0.9° (±3.6°) for posterior tilting. These findings suggest that scapular positioning on the thorax are not the same despite the observation of an identical kinematic pattern during arm elevation. This should be taken into consideration when comparing scapular position and motion of symptomatic and contralateral shoulders.