What is the Difference Between Radius and Ulna

The main difference between radius and ulna is that the radius is the long bone that extends from the lateral side of the elbow to the thumb side of the wrist. Meanwhile, the ulna is the long bone that extends from the elbow to the smallest finger in the medial side of the forearm. Furthermore, the ulna is the larger and longer of these two bones. 

In brief, the radius and ulna are the two long bones which extend from the elbow to the wrist of the forearm. AlsoGenerally, they are attached to around to 20 muscles of the forearm, enabling its unique movements. 

Key Areas Covered 

1. What is Radius
     – Definition, Anatomy, Function
2. What is Ulna
     – Definition, Anatomy, Function
3. What are the Similarities Between Radius and Ulna
     – Outline of Common Features
4. What is the Difference Between Radius and Ulna
     – Comparison of Key Differences

Key Terms 

Forearm, Long Bones, Radius, Ulna

What is Radius 

Radius is one of the two long bones that run parallel to each other in the forearm. However, it is shorter and smaller than the ulna. Also, it extends from the elbow to the thumb in the lateral side of the body. In overall, the radius is a prism-shaped bone, slightly curved longitudinally. Furthermore, long bones including the radius compose of three parts. They are the shaft, upper extremity, and the lower extremity. Here, the upper extremity of the radius occurs at the elbow while the lower extremity of the radius occurs at the wrist.

Figure 1: Bones of Left Forearm. Anterior Aspect

Moreover, the radius is thickest along the interosseous border or the internal border. However, it is thinnest at the extremities with the cup-shaped articular surface (fovea) on the head. Furthermore, the head of the upper extremity of the radius consists of a cylindrical head, articulating with the humerus and ulna, a neck, and a radial tuberosity. In comparison, the lower extremity of the radius is roughly quadrilateral in shape, and it consists of articular surfaces for the ulna, scaphoid and lunate bones.  

What is Ulna 

The ulna is the second long bone which extends from the elbow to the smallest finger of the hand in the medial side of the forearm. It is longer and larger than the radius. Typically, the ulna is broader close to the elbow and narrows as it approaches the wrist. Since ulna is a long bone, it also has three parts: the shaft, upper extremity, and a lower extremity. Here, the central part of the shaft is straight while the lower part is smooth, rounded and bent a little lateralward.

Figure 2: Bones of the Left Forearm, Posterior Surface

The Upper Extremity of the Ulna

Besides, the upper extremity of the ulna has two curved processes: they are the olecranon and the coronoid process. Generally, the olecranon process is a curved eminence and is large and thick. Also, it occurs at the upper back part of the ulna. The coronoid process is a triangular eminence which projects forward from the upper and front part of the ulna. In addition to these, the upper extremity of the ulna consists of two concave articular cavities known as the semilunar and radial notches. Here, the semilunar notch is a large depression formed by the olecranon and the coronoid process. And, it articulates with the trochlea of the humerus. Meanwhile, the radial notch is a narrow articular depression which occurs on the lateral side of the coronoid process.  

The Lower Extremity of the Ulna

The articular surface of the lower extremity of the head of the ulna contains two portions. One portion is oval or semilunar form, which is directed downward and articulates with the upper surface of the triangular articular disk. Meanwhile, the second portion is narrow, convex, and directed lateralward. Typically, it receives into the ulnar notch of the radius. Furthermore, this lower extremity contains a styloid process, projecting from the medial and back part of the bone. Also, its end is rounded and affords attachment to the ulnar collateral ligament of the wrist joint. 

Similarities Between Radius and Ulna  

• Radius and ulna are the two long bones of the forearm.  
• They run parallel to each other.  
• Also, both have a body, an upper, and lower extremity.  
• Their upper extremity connects to the humerus bone of the upper arm at the elbow joint, while their lower extremity connects to the wrist.  
• Besides, a  series of muscles and ligaments act as a hinge, enabling the movements of the upper limb, including supination and pronation. 

Difference Between Radius and Ulna 

Definition 

Radius refers to the bone that is thicker and shorter of the two bones in the forearm of humans. While, the ulna refers to the bone that is thinner and longer of the two bones in the human forearm. 

Location 

Moreover, radius extends from the lateral side of the elbow to the thumb side of the wrist. Meanwhile, ulna extends from the elbow to the smallest finger in the medial side of the forearm. 

Size 

Besides, radius is the shorter and smaller bone of the forearm while ulna is the longer and the larger bone of the forearm. 

Anatomy at the Elbow 

Also, thr radius contains a cylindrical head, round and smooth neck, and an eminence called the radial tuberosity at the elbow. Meanwhile, the ulna contains two curved processes, the olecranon, and the coronoid process; and two concave, articular cavities, the semilunar and radial notches at the elbow. 

Articulation at the Elbow 

Furthermore, radius joins with the capitulum of the humerus and with the ulna at the radial notch in two separate regions at the elbow. Meanwhile, the ulna fits into the olecranon fossa of the humerus through the olecranon process and the radial notch of the ulna fits with the head of the radius. 

Anatomy of the Body 

In addition to these, the body of the radius is prismoid in form, narrower above than below, and slightly curved, while the body of the ulna is prismatic in the form at its upper part; its central part is straight; its lower part is rounded, smooth, and bent a little lateralward.  

Anatomy at the Wrist 

Moreover, the radius is large and of quadrilateral form at the wrist while one portion of the articular surface of the ulna is oval or semilunar form and directed downward and the other portion is narrow, convex, and directed lateralward. 

Articulation at the Wrist 

Radius contains two articular surfaces, one for the carpus and the other for the ulna while a portion of the articular surface of the ulna articulates with the upper surface of the triangular articular disc and the second portion receives into the ulnar notch of the radius. 

During Pronation 

In addition, radius rolls around the ulna at both elbow and the wrist during pronation while ulna remains in a crossed position. 

Corresponding Bone in the Lower Leg 

While radius resembles the fibula in the lower leg, ulna resembles the tibia. 

Conclusion 

Radius is one of the two long bones of the forearm. It is shorter and smaller and extends from the elbow to the thumb at the lateral side. During pronation, it rolls around the ulna at both elbow and the wrist.  The ulna is the other long bone which occurs parallel to the radius. Therefore, it extends from the elbow to the smallest finger of the hand at the medial side. However, it is longer and larger. Therefore, the main difference between radius and ulna is their anatomy and location. 

References:

1. Bengochea, Kim. “Radius and Ulna.” Kenhub, Kenhub, 22 Nov. 2019, Available Here.

Image Courtesy:

1. “Gray213” By Henry Vandyke Carter – Henry Gray (1918) Anatomy of the Human Body (See “Book” section below)Bartleby.com: Gray’s Anatomy, Plate 213 (Public Domain) via Commons Wikimedia    
2. “Gray214” By Henry Vandyke Carter – Henry Gray (1918) Anatomy of the Human Body (See “Book” section below)Bartleby.com: Gray’s Anatomy, Plate 214 (Public Domain) via Commons Wikimedia