Topic Overview

Posterior Compartment of Arm 

Overview

• The posterior compartment is also called the extensor compartment of the arm.

• It is larger than the anterior compartment.

• It contains:

  • Muscles: Triceps brachii and Anconeus.

  • Nerve: Radial nerve.

  • Artery: Profunda brachii artery and its branches.

• Main action: Extension of elbow joint.

Triceps Brachii Muscle 

Origin

• Long head → Infraglenoid tubercle of scapula.

• Lateral head → Posterior surface of humerus, above the radial (spiral) groove.

• Medial head → Posterior surface of humerus, below the radial groove.

Insertion

• Upper surface of the olecranon process of ulna and posterior capsule of elbow joint (via triceps tendon).

Nerve Supply

• Radial nerve (C6, C7, C8).

  • Each head receives a separate branch.

  • Nerve to long head arises in axilla.

  • Nerve to medial head also supplies anconeus.

Actions

• Chief extensor of elbow joint.

• Long head also extends and adducts arm at the shoulder.

• Assists in stabilizing the head of humerus during adduction.

Relations

• Radial nerve and profunda brachii artery run in the spiral groove between the long and medial heads.

• Medial intermuscular septum separates triceps from the flexor group anteriorly.

Dissection of Posterior Compartment 

Steps

1. Place cadaver in prone position and make a midline incision along the posterior arm.

2. Reflect skin and superficial fascia to expose the deep fascia.

3. Identify the triceps brachii muscle:

   • Long head → from infraglenoid tubercle.

   • Lateral head → upper posterior humerus (above radial groove).

   • Medial head → deep, below radial groove.

4. Observe the triceps tendon inserting into olecranon process.

5. Clean the radial nerve and profunda brachii artery running obliquely in the spiral groove between the long and lateral heads.

6. Follow radial nerve to its point of piercing the lateral intermuscular septum to enter the anterior compartment.

7. Trace branches to triceps and anconeus.

8. Examine anconeus muscle near the posterior elbow joint (small, triangular).

Clinical Anatomy of Triceps and Posterior Compartment 

1. Triceps Reflex

• Tapping over triceps tendon → causes elbow extension.

• Tests C7–C8 segments of the spinal cord via radial nerve.

2. Radial Nerve Injury

• Commonly injured in:

  • Fracture of mid-shaft of humerus (spiral groove).

  • Improper use of crutches (crutch palsy).

• Effects:

  • Paralysis of triceps (if injury high in axilla).

  • Wrist and finger drop due to paralysis of extensors.

  • Sensory loss over posterior arm, forearm, and dorsum of hand.

3. Isolated Long Head Paralysis

• Loss of shoulder extension and adduction, but elbow extension usually intact.

4. Triceps Tendon Rupture

• Rare; causes inability to extend forearm.

• May follow sudden resisted flexion (e.g., heavy lifting).

5. Injection Landmark

• Posterior aspect of arm near deltoid–triceps junction → used for radial nerve block.

6. Surgical Note

• The radial nerve and profunda brachii artery must be protected during posterior approaches to humeral shaft fractures.