The deltoid, a forgotten muscle of the shoulder
The deltoid muscle is crucial for shoulder function, nonetheless it’s only incidentally evaluated during clinical or imaging diagnostics. It comprises three portions: a clavicular, an acromial and a spinal part – all of which insert on the deltoid tuberosity of the humerus. Innervation is provided by the axillary nerve, originating from spinal segments C5 and C6; it is vascularized by the posterior circumflex artery. Both course through the quadrilateral space between the long head of the triceps, teres minor, the humeral shaft and teres major before reaching the deltoid muscle.
Although the deltoid has traditionally been thought of as primarily an elevator of the humerus (with the exact direction depending on the parts involved in the contraction), it also stabilizes the humeral head during abduction.
Disorders of the deltoid include among others tears and nerve injuries. Tears can occur postoperatively, particularly after acromioplasty, but also in association with massive rotator cuff tears – an increasingly documented condition. Axillary nerve pathologies may have several inflammatory and mechanical causes, of which the Parsonage-Turner (brachial neuritis) and quadrilateral space syndrome (entrapment of the axillary nerve and posterior circumflex artery in the quadrilateral space) are noteworthy.
> From: Moser et al., Skeletal Radiol 42 (2014) 1361-1375. All rights reserved to ISS. Click here for the Pubmed summary.