Innervation of the deltoid is supplied by the axillary nerve (C5 and C6) branched from the posterior cord of the brachial plexus. The axillary nerve passes through the quadrilateral space from anterior to posterior direction and splits into two branches (the anterior and the posterior branches) within the quadrilateral space. Anterior branch travels around the surgical neck of the humerus and supplies the middle and anterior portions of the deltoid (Fig. 26.3). Moreover, sub-branches from the anterior branch supplies the posterior portion [18, 35]. The posterior branch runs posteriorly and supplies the posterior portion of the deltoid. The posterior branch also sends a motor branch to the teres minor muscle and a sensory branch (superior lateral brachial cutaneous nerve) to the superolateral area of the shoulder called “regimental badge.” The posterior portion of the deltoid has double-supply from the anterior and posterior branches of the axillary nerve in 89.1 % of the cadaveric specimens [18]. In their series, the posterior portion is supplied by the anterior branch alone in 2.3 % and by the posterior branch alone in 8.5 % [18]. The mean diameter of the anterior and posterior branches are 4.0 and 3.3 mm, respectively [29]. The mean distance between the acromial edge and the axillary nerve varies among the reporters [20] reported that the distance from the humeral head and the axillary nerve ranged from 4.0 to 6.7 cm, however, the axillary nerve moves superiorly during abduction. The distance between the acromion and the axillary nerve ranged from 66.6 to 72.6 mm in the hanging arm position and from 53.9 to 61.6 mm in 60° of abduction [7].

Electromyographic assessment suggests that the deltoid consists of at least seven segments coordinated by the central nervous system [3, 4, 34]. However, these functional seven segments may differ from the anatomical seven segments divided by the intramuscular tendons. In other words, the relation between the innervated segments and the anatomical segments is still unclear and needs further investigation.

The vascular supply of the deltoid is largely derived from the posterior circumflex humeral artery, which travels with the axillary nerve through the quadrilateral space (Fig. 26.4). The posterior circumflex humeral artery supplies the middle and the posterior portions of the deltoid. The thoracoacromial artery also provides a supply to the deltoid muscle. The thoracoacromial artery is branched from the axillary artery and separates into two branches: the deltoid artery and the acromial artery. The deltoid artery runs near the deltopectoral groove and supplies the anterior portion of the deltoid [5]. The acromial artery travels in a deep layer of the deltoid near the clavicle and acromion and gives some branches to the middle portion. The anterior circumflex humeral artery sends a branch to the anterior portion in 63 % of cadaveric specimens [11].

The venous branches accompany the arterial branches, except the cephalic vein , which runs in the deltopectoral groove.

The motion of the shoulder is a complex of actions in many directions such as flexion/extension, abduction/adduction, internal rotation/external rotation. In addition, many muscles co-work in any single motion. Therefore, it is quite difficult to assess the participation of each muscle to the shoulder movement. The muscle function is evaluated by various methods, such as the physiological cross-sectional area (PCSA), the moment arm, the potential moment, and the electromyographic (EMG) activity [8, 15, 25, 33]. The deltoid has a largest PCSA in the shoulder girdle and is a key muscle in abduction of the shoulder. Moreover, the deltoid co-works with the rotator cuff muscles and constructs a force couple to move the shoulder joint smoothly.

The anterior portion of the deltoid elevates the arm forward with some contribution by the clavicular portion of the pectorals major and the biceps brachii. The middle portion elevates the arm laterally. The posterior portion works with the teres minor and the latissimus dorsi and elevates the arm backward [22]. In a moment arm study, the posterior portion was estimated to provide 14 % of the shoulder extension torque in addition to 20 % of the shoulder extension torque [8].

The deltoid used to be thought to push the humeral head superiorly during arm elevation. However, recent biomechanical studies have demonstrated that the deltoid also stabilizes the humeral head against the glenoid fossa during arm elevation [2, 23, 31] (Fig. 26.5). This function is very important in a rotator cuff deficient shoulder.

Biomechanical studies during the throwing motion have demonstrated that the deltoid acts as an anterior stabilizer along with the rotator cuff muscles at 90° of abduction and 90° of external rotation. Although this anterior stabilizing effect is stronger in 90° of elevation in the scapular plane than in the coronal plane, it is still weaker than the stability provided by the rotator cuff [13, 17].

Axillary nerve palsy is not uncommon, but the other conditions of the deltoid are rare.