[Displacement of the instantaneous center of rotation of the humeral head during abduction: implication for scapulohumeral muscular function].
OBJECTIVE
To date, studies of glenohumeral stress forces have been based on the notion of a unique center of rotation situated at the geometric center of the humeral head. Early work, e.g. Fisher et al in 1977, suggested that the instantaneous center of rotation can migrate during abduction producing muscle vectors with variable moments and consequently variable stress forces. We conducted a kinematic analysis of the shoulder forces during continuous movement.


METHODS
An imaging device commonly used for angiography enabled acquisition of one image per second during a continuous movement of abduction. Images were obtained for twenty healthy shoulders in ten subjects.


RESULTS
The instantaneous center of rotation was not situated at the geometric center of the humeral head. Although interindividual variability was significant, the preferential positions of the instantaneous center of rotation implied a constant reproducible succession of gliding, rolling, and translation in variable proportions during the abduction movement. When the same movement was performed with a 3500g load held in the hand, the ICR migrated towards the upper part of the humeral head implying a predominance of the rolling motion over the gliding and translation motions. Between 40 degrees and 60 degrees glenohumeral abduction, the instantaneous center of rotation exhibited a metaphyseal displacement producing a longer lever arm for the supaspinatus and an abduction function for the infraspinatus and the subscapularis, unrecognized to date. After 60 degrees, medialization of the instantaneous center of rotation lengthened the lever arm of the deltoid.


CONCLUSIONS
A valid analysis of the glenohumeral stress forces must take into consideration the localization of the instantaneous center of rotation. In the clinical setting, our findings demonstrate that the infraspinatus and the subscapularis play an important role which must be considered both in surgery and in rehabilitation.
Blaimont
P
P
Taheri
A
A
Vanderhofstadt
A
A
D000328:Adult; D001696:Biomechanical Phenomena; D005260:Female; D006801:Humans; D006811:Humerus / Q000033:anatomy & histology / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D018482:Muscle, Skeletal / Q000502:physiology*; D016059:Range of Motion, Articular; D012540:Scapula; D013997:Time Factors