Posterior Region of the Arm.

Posterior Region of the Arm

The posterior region of the arm is located on the back portion of the upper limb between the shoulder and the elbow. It contains muscles primarily involved in the extension of the elbow joint, as well as important neurovascular structures such as the radial nerve and the profunda brachii artery. The posterior arm muscles are responsible for movements such as elbow extension and forearm supination. This region also plays a key role in maintaining upper limb function and strength.

Anatomy of the Posterior Region of the Arm

Boundaries:

• Anterior boundary: The brachialis and biceps brachii muscles in the anterior compartment.

• Posterior boundary: The triceps brachii and anconeus muscles.

• Medial boundary: The medial intermuscular septum and the medial side of the arm.

• Lateral boundary: The lateral intermuscular septum and the lateral side of the arm.

Muscles of the Posterior Region of the Arm:

1. Triceps Brachii:

   • Origin:

     • Long Head: Infraglenoid tubercle of the scapula.

     • Lateral Head: Posterior surface of the humerus, above the radial groove.

     • Medial Head: Posterior surface of the humerus, below the radial groove.

   • Insertion: The olecranon process of the ulna.

   • Innervation: The radial nerve (C6, C7, C8, T1).

   • Function: The primary function of the triceps brachii is elbow extension, but it also assists with shoulder extension (long head) and adduction (long head).

   • Clinical Significance: Injury to the radial nerve can result in triceps weakness and loss of elbow extension. This is clinically relevant in humeral fractures that can damage the radial nerve or its branches.

2. Anconeus:

   • Origin: The lateral epicondyle of the humerus.

   • Insertion: The olecranon process and the posterior surface of the ulna.

   • Innervation: The radial nerve (C7, C8).

   • Function: The anconeus assists in elbow extension and stabilizes the elbow joint.

   • Clinical Significance: The anconeus is a small muscle that is often involved in cases of elbow joint instability due to its stabilizing role.

Neurovascular Structures in the Posterior Region of the Arm

Nerves:

1. Radial Nerve (C5, C6, C7, C8, T1):

   • Pathway: The radial nerve enters the posterior compartment of the arm by traveling around the radial groove of the humerus (along with the profunda brachii artery).

   • Motor Functions: The radial nerve innervates the triceps brachii and the anconeus muscles, as well as the extensor muscles of the forearm and hand (through its branches that pass into the forearm).

   • Sensory Functions: It provides sensory innervation to the posterior arm, posterior forearm, and dorsal surface of the hand.

   • Clinical Significance: Injury to the radial nerve in the arm (often from humeral fractures or compression at the radial groove) can lead to wrist drop, weakness in the triceps and extensor muscles of the forearm, and sensory loss in the affected area.

Arteries:

1. Profunda Brachii Artery (Deep Brachial Artery):

   • Origin: The profunda brachii artery is a branch of the brachial artery, which supplies the posterior compartment of the arm.

   • Pathway: It runs along with the radial nerve through the radial groove of the humerus, supplying blood to the triceps brachii and the anconeus muscles.

   • Clinical Significance: The profunda brachii artery is crucial for the blood supply of the muscles in the posterior arm. It is at risk of injury in fractures of the humerus, particularly in the radial groove area, potentially leading to muscle ischemia and impaired function.

2. Brachial Artery:

   • The brachial artery is the main artery supplying blood to the upper limb. It continues down the arm and supplies branches to the anterior and posterior compartments of the arm. The profunda brachii is one of its main branches.

Blood Supply and Anastomoses in the Posterior Region of the Arm

The blood supply to the posterior arm is primarily from the brachial artery and its branch, the profunda brachii artery. The profunda brachii artery is the main artery that supplies blood to the posterior muscles, particularly the triceps brachii.

Anastomoses:

1. Profunda Brachii Artery Anastomosis:

   • The profunda brachii artery can form anastomoses with the ulnar artery in the region of the elbow, providing collateral circulation in cases of obstruction or injury to the main arterial supply. This is important for maintaining blood flow to the forearm and hand if there is damage to the main arteries.

2. Radial Artery Anastomosis:

   • The radial artery, after emerging from the brachial artery and running distally, communicates with the ulnar artery through the palmar arches, providing collateral circulation in the forearm and hand.

Clinical Significance of the Posterior Region of the Arm

1. Radial Nerve Injury:

   • One of the most common injuries in the posterior arm is damage to the radial nerve due to humeral fractures, especially fractures involving the radial groove. This injury can result in wrist drop (inability to extend the wrist and fingers), weakness in elbow extension, and loss of sensation on the posterior forearm and dorsum of the hand.

2. Triceps Brachii Injury:

   • Triceps brachii weakness can result from radial nerve injury, leading to a loss of elbow extension strength, which is critical for tasks like pushing or straightening the arm. An isolated triceps injury can result from direct trauma or chronic overuse.

3. Profunda Brachii Artery Damage:

   • Damage to the profunda brachii artery (e.g., due to a fracture or contusion) can compromise the blood supply to the posterior arm muscles, especially the triceps brachii. This can lead to muscle ischemia, delayed healing, and muscle dysfunction.

4. Elbow Joint Pathology:

   • The posterior arm muscles, particularly the triceps brachii and anconeus, are involved in stabilizing and extending the elbow joint. Elbow joint instability or ligamentous injuries (e.g., in throwing athletes) can affect these muscles' function, leading to weakness in elbow extension and increased risk of joint dislocation or instability.

5. Brachial Plexus Injuries:

   • In severe cases, brachial plexus injuries can affect the posterior region of the arm, particularly the radial nerve, leading to deficits in the extension of the elbow, wrist, and fingers, and affecting the sensation of the arm and hand.

Summary

The posterior region of the arm is primarily concerned with the extension of the elbow and involves key muscles such as the triceps brachii and anconeus, both of which are innervated by the radial nerve. The profunda brachii artery provides the blood supply to these muscles, running along with the radial nerve in the radial groove of the humerus. Clinical issues such as radial nerve injury and triceps weakness can occur due to trauma or fractures in the humerus, leading to impaired function in elbow extension. Anastomoses between the profunda brachii and ulnar arteries help maintain circulation to the forearm and hand in case of vascular compromise. Understanding the anatomy and clinical significance of the posterior arm is essential for diagnosing and managing upper limb injuries and dysfunctions.