Review Question - QID 219500

The deltoid inserts on the lateral humeral shaft and contributes to the abduction of the proximal fragment in this humeral shaft fracture. The deltoid is innervated by the axillary nerve, which arises from the posterior cord of the brachial plexus (Answer 3).

In humeral shaft fractures below the deltoid tuberosity, the proximal fragment tends to abduct. This force is often counteracted by the adduction force of the pectoralis major, inserting at approximately the same level. The deltoid is innervated by the axillary nerve, a terminal branch of the posterior cord of the brachial plexus (Illustration A). The axillary nerve carries fibers from C5 and C6 and arises from the posterior cord to supply muscle innervation to the deltoid (via anterior and posterior branches) and the teres minor (via posterior branch). Sensation of the lateral shoulder and the glenohumeral joint capsule is also supplied by sensory branches of the axillary nerve. The brachial plexus is divided into roots, trunks, divisions, cords, and branches. The cords, named by their relationship to the axillary nerve, are the posterior, lateral, and medial cords.

Moatshe et al. performed a cadaveric study on the proximal humerus muscular attachments and the axillary nerve. After dissecting and studying ten cadaveric shoulders, the deltoid was found to insert on the deltoid tuberosity about 101 mm distal from the greater tuberosity. The axillary nerve was on average 50.3 mm distal to the greater tuberosity. The authors concluded that an understanding of the proximal humerus anatomy is critical to avoid nerve damage during surgical procedures.

Schumaier et al. published a review on proximal humerus fractures in elderly patients. When discussing the anatomy of the proximal humerus, the authors note the deltoid insertion on the lateral humerus results in a laterally deforming force. The authors concluded by recommending conservative treatment for minimally displaced proximal humerus fractures.

Figure A is a radiograph of a right proximal humeral shaft fracture with lateral displacement of the proximal segment. A functional brace is seen on the radiographs on the right humerus.
Illustration A is a diagram of the brachial plexus.

Incorrect Answers:
Answers 1 & 5: The three cords of the brachial plexus are the lateral, medial, and posterior cords.
Answer 2: The axillary nerve is an extension of the posterior cord. The lateral cord gives rise to the lateral pectoral nerve, fibers to the median nerve, and the musculocutaneous nerve.
Answer 3: The axillary nerve is an extension of the posterior cord. The medial cord gives rise to the medial pectoral nerve, the medial antebrachial cutaneous nerve, the medial brachial cutaneous nerve, the ulnar nerve, and fibers to the median nerve.