Anatomy and Innervation
The deltoid muscle is a large and powerful muscle of the shoulder joint. It distinctively shapes the surface anatomy of the shoulder and this muscle is supplied by the axillary nerve, a branch of the brachial plexus.
The deltoid has three parts, each with their own origin:
- clavicular (or anterior) part: lateral third of the clavicle
- acromial (or middle) part: acromion
- spinal (or posterior) part: scapular spine
Together they all insert laterally at the humeral shaft into the deltoid tuberosity. In its course, the muscle lies in close relation to the cephalic vein which runs in the deltopectoral groove between the deltoid and pectoralis major muscle becoming the axillary vein.
The deltoid moves and stabilizes the shoulder joint. The movements of the different deltoid parts can interact both synergistically and antagonistically depending on the specific part and the position of the humerus. The deltoid is the most important abductor of the shoulder joint.
The abduction is mainly initiated and held by the acromial part. Both the clavicular and spinal parts function as adductors up to 60° and abductors over 60°. Furthermore the clavicular part causes an inward rotation and anteversion, the spinal part an outward rotation and retroversion.
An axillary nerve injury may lead to paralysis of the deltoid muscle. The affected patients typically show a clinical picture of an atrophic shoulder in comparison to the healthy side.
Another classic sign is the acromion standing out prominently (“scaphoid sign”). Even though the limitations of movement are partly compensated by the other shoulder muscles especially the abduction and outward rotation become tremendously difficult. Isolated lesions of the axillary nerve occur rarely. More common are injuries due to a dislocated shoulder or proximal humerus fracture.