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Deltoid muscle

Synonym: deltoideus
German: Musculus deltoideus

1 Definition

The deltoid muscle forms part of the dorsal group of the shoulder muscles. It is a strong muscle, triangular when spread-out, and it wraps around the proximal part of the humerus from the cranial, ventral, dorsal and lateral side.

2 Devolution

2.1 Origin

The deltoid muscle can be divided into three parts that all have different areas of origin:

  • Lateral deltoid (clavicular part): It has its origin at the lateral third of the clavicle. This part has parallel fibers.
  • Medial deltoid (acromial part): This part has its origin at the acromion. This muscle part is predominantly pinnate.
  • Posterior deltoid (spinal part): This has its origin at the spine of the scapula.

2.2 Insertion

The deltoid muscle inserts at the deltoid tuberosity of the (humerus).

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3 Innervation

The innervation of the deltoid muscle is provided by the axillary nerve from the posterior cord of the brachial plexus (segments: C5-C6). An accessory innervation is provided by the ventral branches of the thoracic nerves.

4 Function

The functions of the deltoid muscle are manifold and complex. Depending on which parts of the muscle are contracted there are different forms of movement resulting from the contraction.

The medial part performs an abduction of the arm. In an abduction of over 60°, the other two parts support the movement.

The medial part and the lateral part together perform an anteversion of the arm. Whereas, when the medial part contracts together with the posterior part, this leads to a retroversion of the arm.

When the arm is hanging down, the lateral part supports the internal rotation, whereas the spinal part supports external rotation.

When the arm is adducted again starting from the abducting position, the medial and posterior parts support the adduction when going below the 60∞-mark (abduction axis). Moreover, the posterior part prevents additional internal rotation during adduction.

5 Clinical presentation

The deltoid muscle is often used for intramuscular injections of vaccines or drugs, since the risk of causing nerve damage is low here.

In case of paralysis of the muscle, problems get manifold. Abductions is as good as impossible, and it cannot be compensated by the supraspinatus muscle and the biceps either. Moreover, the muscular joint protection by the deltoid muscle drops out. The arm is in an adducted and internally rotated position.

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Specialties: Anatomy

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