Humerus: Function, Anatomy, and Clinical Significance at the Elbow

What is the function of the humerus of the elbow?

The humerus, the sole bone of the upper arm, serves as the critical proximal component of the elbow joint, providing the articulating surfaces and muscle attachment sites necessary for the complex movements of the forearm and hand.

The Humerus: An Overview

The humerus is a long bone extending from the shoulder to the elbow. While its proximal end forms the glenohumeral (shoulder) joint, its distal end is intricately designed to articulate with the bones of the forearm – the ulna and radius – to form the elbow joint. This joint is not a simple hinge; it's a compound joint allowing for both flexion/extension and rotational movements.

Anatomy of the Distal Humerus at the Elbow

The distal end of the humerus is uniquely shaped to facilitate its functions at the elbow. Key anatomical features include:

• Trochlea: A spool-shaped medial condyle that articulates with the trochlear notch of the ulna. This primary articulation forms the hinge joint responsible for elbow flexion and extension. Its specific shape guides the ulna's movement, ensuring stability.
• Capitulum: A rounded lateral condyle that articulates with the concave head of the radius. This articulation allows the radius to rotate during pronation and supination of the forearm, while also contributing to elbow flexion and extension.
• Medial Epicondyle: A prominent projection on the medial side, superior to the trochlea. It serves as a crucial attachment site for the common flexor tendon of the forearm muscles and the ulnar collateral ligament (UCL), which provides significant medial stability to the elbow.
• Lateral Epicondyle: A smaller projection on the lateral side, superior to the capitulum. It provides attachment for the common extensor tendon of the forearm muscles and the radial collateral ligament (RCL), contributing to lateral stability.
• Fossae: Three depressions on the distal humerus accommodate the forearm bones during full flexion and extension:
  • Coronoid Fossa: Anterior depression superior to the trochlea, accommodating the coronoid process of the ulna during full elbow flexion.
  • Radial Fossa: Anterior depression superior to the capitulum, accommodating the head of the radius during full elbow flexion.
  • Olecranon Fossa: A large posterior depression superior to the trochlea, accommodating the olecranon process of the ulna during full elbow extension, preventing hyperextension.

Articulation with Forearm Bones

The humerus forms two distinct articulations at the elbow:

• Humeroulnar Joint: This is the primary hinge component of the elbow, formed by the trochlea of the humerus and the trochlear notch of the ulna. Its primary function is to facilitate flexion and extension of the forearm. The tight fit and strong collateral ligaments provide significant stability.
• Humeroradial Joint: Formed by the capitulum of the humerus and the head of the radius. While it contributes to flexion and extension, its more specialized role is to allow the radius to pivot around the ulna, enabling pronation (palm down) and supination (palm up) of the forearm.

Primary Functions of the Humerus at the Elbow

The humerus plays several indispensable roles at the elbow joint:

• Structural Foundation: It forms the essential proximal bony component of the elbow, providing the robust framework upon which the forearm bones articulate. Without the humerus, the elbow joint would not exist.
• Movement Facilitation: Through its specialized articular surfaces (trochlea and capitulum), the humerus directly enables the two primary movements of the elbow:
  • Flexion: Bending the elbow, bringing the forearm closer to the upper arm.
  • Extension: Straightening the elbow, moving the forearm away from the upper arm.
  • It also indirectly contributes to pronation and supination by providing the surface against which the radial head rotates.
• Muscle and Ligament Attachment Site: The prominent epicondyles and surrounding areas of the distal humerus serve as crucial origins for numerous muscles responsible for movements of the elbow, wrist, and fingers. They also provide strong anchoring points for the medial and lateral collateral ligaments, which are vital for maintaining joint stability against varus and valgus stresses.

Key Muscles Acting on the Elbow

Many muscles that move the elbow originate from or insert onto the humerus, highlighting its role as a leverage point:

• Elbow Flexors:
  • Brachialis: Originates from the anterior surface of the humerus and inserts onto the ulna. It is the primary pure elbow flexor.
  • Biceps Brachii: While its long head originates from the scapula, its short head originates from the coracoid process of the scapula, and its distal tendon inserts onto the radius. It acts as a powerful elbow flexor and supinator.
  • Brachioradialis: Originates from the lateral supracondylar ridge of the humerus and inserts onto the radius. It primarily flexes the elbow, especially in a neutral forearm position.
• Elbow Extensors:
  • Triceps Brachii: Its medial head originates from the posterior surface of the humerus, while the long and lateral heads originate from the scapula and posterior humerus, respectively. All heads insert into the olecranon of the ulna, making it the primary elbow extensor.
• Forearm Muscles: The medial and lateral epicondyles of the humerus are the common origins for the superficial flexor and extensor muscles of the wrist and fingers, respectively. These muscles, while acting primarily on the wrist and hand, cross the elbow joint and can contribute to its stability or movement.

Clinical Significance

The humerus's critical role at the elbow makes it susceptible to various injuries and conditions, including:

• Epicondylitis: Commonly known as "golfer's elbow" (medial epicondylitis) and "tennis elbow" (lateral epicondylitis), these are inflammatory conditions affecting the common flexor and extensor tendons originating from the epicondyles, respectively.
• Fractures: Distal humerus fractures can significantly impair elbow function and are often complex due to the intricate anatomy of the joint.
• Ligamentous Injuries: Damage to the ulnar collateral ligament (UCL) or radial collateral ligament (RCL), which attach to the humerus, can lead to elbow instability.

Conclusion

In summary, the humerus is far more than just a connecting bone at the elbow. Its distal anatomy, with its specialized trochlea and capitulum, intricate fossae, and robust epicondyles, establishes it as the foundational component of the elbow joint. It dictates the range and type of motion (flexion, extension, pronation, supination), provides crucial attachment sites for powerful muscles and stabilizing ligaments, and ultimately enables the complex and nuanced movements of the forearm and hand essential for daily activities and athletic performance. Understanding its precise function is paramount for anyone studying or working in the fields of exercise science, kinesiology, and rehabilitation.