Biomechanics of the deltoideus.

The objective of this study was to determine the direction of the migration engendered by the middle deltoideus on the upper end of the humerus. Eleven patients suffering from shoulder pathology underwent an MRI examination (3 mm thick slices). From these MRI slices, 3D reconstructions were obtained for each patient by using a manual data capture system (SliceOmatic((R))). From this geometry, a mechanical model of the deltoideus was produced, taking into account the contacts between the latter and the following anatomical parts: supraspinatus, infraspinatus and humeral head. For the 11 shoulders, we have obtained a deltoideus showing a global resultant oriented upwards. There was, however, a component oriented downwards (at the level of the humeral head), its intensity being 40-80% less than the component oriented upwards (at the level of the deltoideus V). It is important to note that this study is valid only in the initial degrees of lateral elevation. The deltoideus is an elevator muscle of the humeral head in the glenoid, presenting nevertheless a component oriented downwards. The deltoideus would, therefore, intervene to recenter the shoulder during an abduction movement.