The humerus

Introduction

The humerus is one of the long bones in the arm, along with the ulna and radius, which provide structural support and enable mobility in the upper limb. This bone is crucial for performing various functions such as reaching, gripping, and lifting objects. The humerus is unique due to its shape and articulations, allowing a wide range of motion necessary for diverse activities in human daily life.

Anatomy and Morphology of the Humerus

Overview of the Humerus

The humerus has a slightly curved long shaft and three distinct regions: proximal, distal, and shaft (or diaphysis). It is composed of compact bone tissue that provides strength and hardness, and medullary cavity that houses marrow. The humerus consists of two articular surfaces and three distinct bony landmarks: the greater tubercle, lesser tubercle, and the capitulum.

Proximal Humerus

The proximal end of the humerus forms a ball-like surface called the head, which articulates with the glenoid cavity of the scapula (shoulder socket) to enable the wide range of motion in the shoulder joint. The head is approximately spherical and is connected proximally by the neck and tubercles.

The neck is a narrow region that connects the head to the body of the humerus. The neck can be further divided into the anatomical and surgical neck, which have clinical relevance for some procedures.

The greater tubercle is located on the lateral (outer) side of the upper part of the humerus and provides attachment for several muscles that aid in rotating and lifting the arm. The lesser tubercle is a smaller bony prominence found medial to the greater tubercle, providing additional attachment sites for muscles involved in arm movement.

Distal Humerus

The distal end of the humerus forms two articular surfaces: the trochlea and the capitulum. The trochlea is a C-shaped surface that articulates with the ulna to form the elbow joint, enabling flexion and extension motions. The capitulum is a small rounded surface that articulates with the radial head of the ulna, allowing for rotation of the forearm during supination and pronation movements.

Shaft of the Humerus

The shaft (or diaphysis) of the humerus runs from the proximal to distal end and provides the primary structural support for the arm. The shaft has an anterior (front) surface, a posterior (back) surface, medial (inner) side, and lateral (outer) side. The shaft is divided by two lines: the intertubercular sulcus (or groove) and the deltoid tuberosity ridge.

The intertubercular sulcus runs longitudinally along the humerus's anterior surface, separating the greater and lesser tubercles from each other. The deltoid tuberosity ridge is a bony prominence found on the posterior surface of the shaft, providing attachment sites for the deltoid muscle that helps in arm abduction.

Functions of the Humerus

The humerus plays several essential roles in the upper limb:

1. Structure and Support: The long, cylindrical shape of the humerus provides a stable base for the arm during various activities, ensuring that it does not collapse or buckle under weight-bearing forces.
2. Mobility: The ball-and-socket articulation between the head of the humerus and scapula allows for rotational movements and wide ranges of motion in the shoulder joint. Additionally, the elbow joint enables flexion and extension, while forearm rotation occurs at the radioulnar joint.
3. Leverage: The length and positioning of the humerus relative to the forearm and wrist provide leverage during activities such as lifting objects or performing manual tasks.
4. Protection: The humerus acts as a protective shield for internal organs in the chest cavity, reducing the risk of injury from external forces.

Clinical Relevance

Several disorders, injuries, and surgical procedures may affect the humerus. Some examples include:

1. Fractures: Humeral fractures are common due to their susceptibility to trauma, particularly during falls or accidents. Fractures can occur in various locations within the bone (e.g., proximal, distal, or shaft).
2. Dislocations: Dislocations of the shoulder joint can occur due to forced movement beyond the normal range of motion, often resulting from falls or sports injuries.
3. Arthritis: Osteoarthritis and rheumatoid arthritis may affect the articulations between the humerus and other bones, causing pain, swelling, and restricted mobility.
4. Implants: Orthopedic procedures such as joint replacements or fracture repairs may involve the use of metal or plastic implants within the humerus to restore function and mobility.