The fibrous frame of the deltoid muscle. Its functional and surgical relevance.

Black lines seen on magnetic resonance imaging in the middle part of the deltoid suggest the presence of fibrous bands. Anatomic study of 30 deltoid muscles was done. Eight half-cone shaped distal fibrous structures merged together into the distal tendon of the deltoid muscle. The middle part of the deltoid muscle contains four deep fibrous bands that glide inside the distal half-cones. The anterior and posterior parts of the deltoid muscle lacked such bands. Histologic study confirmed the presence of the bands and cones. In the middle part of the deltoid, muscle fibers are oblique between the bands or between the bands and the half-cones. This multipennate structure favors strength instead of range of excursion of the muscle. Because of its significant change of direction around the humeral head, at the onset of elevation of the arm, the deltoid muscle sustains forces that press the muscle against the head which then leads to deformation of the muscle. The fibrous bands make the muscle strong enough to support these forces. The middle part of the muscle is of greatest importance in comparison with the other parts. This should be considered during shoulder rehabilitation. The presence of the fibrous band originating from the anterior corner of the acromion may help create a strong repair after splitting the deltoid.