Posterior Compartment of Arm

Introduction

Introduction to the Posterior Compartment of the Arm

The posterior compartment of the arm (also known as the extensor compartment) lies behind the humerus, separated from the anterior compartment by the medial and lateral intermuscular septa. It contains only one muscle – the triceps brachii – which is the primary extensor of the elbow. The compartment is innervated by the radial nerve (C5-C8) and receives its blood supply mainly from the profunda brachii artery (deep brachial artery), a branch of the brachial artery. The radial nerve and profunda brachii travel together in the radial groove of the humerus, making them vulnerable to mid-shaft humeral fractures.

Study

Triceps Brachii: The Only Muscle of the Posterior Compartment

Triceps brachii has three heads: 1. Long head: Originates from infraglenoid tubercle of scapula. Passes between teres minor (superiorly) and teres major (inferiorly) – forms the medial border of the quadrangular space. 2. Lateral head: Originates from posterior surface of humerus, superior to radial groove. 3. Medial head: Originates from posterior surface of humerus, inferior to radial groove (deep to lateral head). All three heads converge to insert on the olecranon of the ulna. Actions: All heads extend the elbow; the long head also extends and adducts the shoulder (glenohumeral joint). Innervation: Radial nerve (C6-C8, via branches to each head).

Neurovascular Structures in the Posterior Compartment

Radial nerve (C5-C8, T1): Arises from posterior cord of brachial plexus. In the arm, it passes through the triangular interval (with profunda brachii artery) to enter the radial groove on the posterior humerus. It then runs laterally between lateral and medial heads of triceps, supplying all three heads. Distally, it pierces the lateral intermuscular septum to enter the anterior compartment of the arm (cubital fossa), where it divides into superficial (sensory) and deep (motor) branches. Profunda brachii artery (deep brachial artery): Branch of brachial artery (distal to teres major). Runs with radial nerve in radial groove, giving off nutrient artery to humerus and middle collateral artery (supplies medial head, anastomoses with interosseous recurrent) and radial collateral artery (anastomoses with radial recurrent). Posterior cutaneous nerve of the arm: Branch of radial nerve, supplies skin over posterior arm.

Clinical Relevance

Radial nerve injury in the arm (e.g., mid-shaft humeral fracture, 'Saturday night palsy' from compression) causes wrist drop (loss of wrist extensors), loss of elbow extension (triceps weakness – only if lesion is proximal to branches to triceps), and sensory loss over posterior arm, forearm, and dorsal hand. The radial nerve is vulnerable in the radial groove. Triceps weakness is more common with high radial nerve lesions (e.g., axilla). Triceps tendon reflex (C7) tests radial nerve integrity. Olecranon bursitis (student's elbow) is not a muscle problem but affects the triceps insertion area.

Summary

Key Takeaways – Posterior Compartment of Arm

Single muscle: Triceps brachii (long, lateral, medial heads). Action: Elbow extension (all heads); long head also extends/adducts shoulder. Innervation: Radial nerve (C6-C8). Blood supply: Profunda brachii artery (from brachial artery). Radial groove: Contains radial nerve and profunda brachii – common injury site. Clinical: Radial nerve injury → wrist drop; high lesion → triceps weakness; reflex loss (C7).