Posterior region of the arm

External landmarks and boundaries

The posterior region of the arm can be identified by a number of distinct external landmarks. These include the contours of the deltoid muscle (m. deltoideus), the pronounced bulge of the triceps brachii muscle (m. triceps brachii), the insertion site of the latissimus dorsi muscle (m. latissimus dorsi), as well as the medial and lateral epicondyles of the humerus (epicondylus medialis et lateralis).

The boundaries of the region are defined as follows:

• Superior boundary: an oblique line running along the posterior margin of the deltoid muscle down to the latissimus dorsi muscle.
• Inferior boundary: a conventional circular line drawn 4 cm above the epicondyles of the humerus.
• Lateral boundaries: vertical lines extending upward from the corresponding epicondyles.

Layer-by-Layer Structure and Fascial Compartment

The tissues of the posterior region of the arm are arranged in distinct layers, each with its own characteristics:

1. Skin. Significantly thicker than on the anterior surface of the arm and characterized by low mobility.
2. Subcutaneous adipose tissue. As a rule, well developed. Numerous cutaneous nerves pass through it to reach the skin. This abundance of nerve branches explains why intramuscular injections into the triceps brachii are often accompanied by significant pain.
3. Fascia. The deep fascia (fascia propria), together with the lateral and medial intermuscular septa, forms the posterior fascial compartment (compartimentum brachii posterius). Anteriorly, it is bounded by the humerus and the septa, and posteriorly by the deep fascia.

Contents of the posterior fascial compartment:

• Triceps brachii muscle. Immediately beneath the fascia laterally lie the long head (medially) and the lateral head, while the medial head is located deeper.
• Radial nerve and profunda brachii artery.
• In the lower third of the arm, the ulnar nerve (n. ulnaris) and the superior ulnar collateral artery pass in transit, heading toward the posterior cubital region.

Humeromuscular Canal and Radial Nerve

The radial nerve (n. radialis) is a key structure of the posterior region of the arm. It enters this region from the anterior compartment by passing through the gap between the long and lateral heads of the m. triceps brachii.

Next, the nerve descends into the humeromuscular canal (canalis humeromuscularis). In the middle third of the arm, it spirals around the humerus. The walls of this canal are formed by the humerus (humerus) and the lateral head of the triceps.

The projection of the radial nerve corresponds to a spiral line: it begins at the lower edge of the latissimus dorsi muscle and ends at a point on the border between the middle and lower thirds of the lateral boundary of the region.

Clinical significance: in the middle third of the arm, the radial nerve lies directly against the bone. This makes it extremely vulnerable. Prolonged application of a tourniquet to the middle of the arm or a fracture of the humeral shaft can lead to paresis or paralysis (so-called "tourniquet syndrome" or "Saturday night palsy").

Vessels and Cutaneous Innervation

The main arterial trunk of the region is the profunda brachii artery (a. profunda brachii), which accompanies the radial nerve. It gives off several important branches:

• Deltoid branch (ramus deltoideus): originates at the very beginning and plays an important role in collateral circulation between the shoulder girdle and the arm (anastomosing with the deltoid branch of the thoracoacromial artery and the circumflex humeral arteries).
• Radial collateral artery (a. collateralis radialis): in the lower part, it pierces the lateral intermuscular septum, returns to the anterior compartment, and forms an anastomosis with the a. recurrens radialis.
• Middle collateral artery (a. collateralis media): forms an anastomosis with the recurrent interosseous artery (a. interossea recurrens).

Several nerves are responsible for the sensation of the posterior region of the arm:

• Superior lateral cutaneous nerve of the arm (a branch of the axillary nerve).
• Inferior lateral and posterior cutaneous nerves of the arm (branches of the radial nerve).
• Posterior cutaneous nerve of the forearm (also a branch of the radial nerve), which pierces the deep fascia at the border with the posterior cubital region.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Topography of collateral circulation of the shoulder girdle and arm (anastomoses of the deltoid branch)
• Skin innervation: distribution zones of the branches of the radial and axillary nerves
• Connection of the posterior compartment of the arm with the anterior compartment and cubital region
• Clinical presentation of radial nerve injury in humeral shaft fractures

Other topics in the "Upper Extremity" section

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