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Overview of the bones that define the forearm - radius and ulna.
Ever ended up with one of these? Well, I hope not, but chances are that you actually have or at least know someone who has experienced what many people might refer to as a broken wrist. But did you know that more often than not, injuries like this are actually isolated to the bones of the forearm rather than to the wrist itself? Yes, really. In fact, fractures of the distal forearm are almost three times more common in children than fractures to the actual bones of the wrist. So, if you’re interested to find more about the anatomy of these bones, then this is the tutorial for you.
Today, we’re going to be finding out all there is to know about the radius and the ulna. But before we begin looking at the bones of interest today, I want to take a moment to remind ourselves of the orientation of the bones of the upper limb.
If you can think back to your very first anatomy lesson, we always consider the body in the same position regardless of the actual position of the patient in question, and this position, of course, is known as the anatomical position. And when it comes to the forearm, it’s important to remind ourselves of a few important details. In the anatomical position, the palm of the hand is facing forwards or anteriorly with the thumb pointing outwards or laterally. When we look at the bones of the forearm and the hand, we can now see that this bone here, which is the ulna, is found along the medial or ulnar aspect of the forearm, while its partner, the radius, is over here on the lateral or the radial side. This, of course, means that we’re looking at the anterior aspect of the bones from this perspective.
Fortunately for us today, the surface anatomy of the two bones of the forearm is relatively uncomplicated and straightforward. So, rest easy. There’s nothing to fear about these bones. You’ll know them top to bottom pretty soon!
So let’s waste no time and get straight into business, beginning first with the lateral bone of the forearm – the radius.
So, the radius is the shorter of the two bones of the forearm and it is so-called due to its ability to rotate relative to its neighbor which is the ulna. As you can see in the illustration, it articulates proximally with the humerus specifically at the capitulum and it also articulates both proximally and distally with the ulna. At its distal end, it articulates with the two lateral bones of the proximal carpal row which are the scaphoid and the lunate bones. So, let’s take a closer look now at the parts, surfaces, and other points of interest of the radius.
Beginning proximally with the head of the radius which is this cylindrical structure seen here, it has a concave or cupped proximal articular surface which articulates with the capitulum of the humerus. With its outer border known as the articular circumference, articulates with the ulna at the radial notch forming the proximal radioulnar joint.
There are two important ligaments to be aware of in relation to the proximal end of the radius, and the first one of these is this one here, which works to reinforce the proximal radioulnar joint and it’s known as the annular ligament. The term annular comes from the Latin term annulus which means ring, and as you can see in the illustration, this ligament forms a ring around the articular circumference of the radial head, ensuring the head of the radius remains in position within the radial notch of the ulna.
The second ligament of interest is known as the radial collateral ligament whose fibers extend between the lateral epicondyles of the humerus and the annular ligament around the head of the radius. Just distal to the head of the radius is the neck of this bone which as you can see is a somewhat slender part of the radius. Moving slightly distal once again is this well-defined landmark which is the radial tuberosity, which is a rugged prominence located on the anteromedial aspect side of the radius. It’s also known as the bicipital tuberosity due to the fact that it serves as the distal attachment or the origin for the biceps brachii muscle of the arm.
Moving on from the proximal end of the radius, we now have the shaft or the body of the radius which runs along the length of the bone. And from this anterior perspective, we can see that it is defined by this border which runs roughly down the center of the shaft, and this is the anterior margin or the border of the radius, which delineates between the anterior surface of the radius found medial to the anterior margin and the lateral surface of the radius which is seen here. It’s also known as the oblique line and provides attachment for several muscles of the forearm.
Along the medial aspect of the radial shaft, we have another well-defined border just here which is the interosseus border or margin, and this, of course, gets its name due to the fact that the interosseus membrane located between the radius and the ulna attaches here.
As we move on to the distal end of the radius, we can see that it has a somewhat expanded appearance. On its medial aspect, we have this feature here which is the ulnar notch which, true to its name, accommodates the distal end of the ulna forming the distal radioulnar joint. On the lateral aspect, we can see this sharp projection here which is known as the styloid process of the radius. And the border leading into the styloid process is known as the suprastyloid crest and it provides attachment for the brachioradialis muscle.
So, let’s flip over for a moment now to the posterior aspect of the radius. And once again, we see the circular form of the articular circumference of the radial head. The posterior aspect of the radial shaft is largely by this border here which is known as the posterior margin of the radius and which separates the posterior surface of the radius from the lateral surface. If we follow the posterior margin distally, we will find a large and well-defined tuberosity here and this is known as the dorsal tubercle or Lister’s tubercle.
And with that, we’ve identified all of the major parts and landmarks of the radius bone.
So we’re halfway there, so let’s not lose any momentum. Let’s head straight into the brother of the radius which is, of course, the ulna.
So as I mentioned earlier, the ulna is the longer of the two bones of the forearm. It articulates proximally with the trochlea of the humerus as well as with the head of the radius. Distally, it again articulates with the radius, however, it does not articulate directly with any of the carpal bones of the wrist joint. Instead, it’s separated from them by a small articular disc which allows for a greater degree of rotation of the forearm as well as ulnar deviation of the hand.
So, let’s take a closer look now at the prominences, the borders and the projections which define the surface of this bone. So beginning once again on the proximal anterior aspect, we’re going to start with this prominence just here, which is known as the coronoid process. And the term coronoid is derived from the Latin word for “crown”, and as you can see in the illustration as the name suggests, this bony landmark has a crown-like or mitered appearance.
Immediately distal to the coronoid process is a roughened depression which is known as the ulnar tuberosity and this is also known as the brachial tuberosity as it gives attachment to the brachialis muscle. And just lateral to this, we have the radial notch of the ulna which accommodates the articular circumference of the radial head. Supporting the proximal end of the ulna with the humerus is this ligament here which is known as the ulnar collateral ligament which extends from the medial epicondyle of the humerus and the medial aspect of the proximal ulna.
Moving distally now, let’s take a few moments to examine the shaft or the body of the ulna. And the ulnar shaft has three surfaces which are the anterior surface, the medial surface, and the posterior surface. So from an anterior perspective, the anterior surface is limited laterally by the interosseus border or the crest, which as you guessed, provides attachment for the interosseus membrane. Medially, the anterior surface of the ulna is bounded by the anterior margin of the ulna or the border which separates it from the medial surface of the ulna. Continuing to the distal end of the bone where we can see that the ulna has a somewhat rounded form, this is known as the head of the ulna.
So, let’s turn our attention to the posterior aspect of the ulna now beginning once again at the proximal end which as you can see is dominated by this large rounded projection of bone just here and this is known as the olecranon and it forms the posterior part of the elbow joint. To get a better idea of the shape of this process, let’s briefly look at its profile in cross-section.
And as you can see, as a whole, the olecranon and the coronoid process form a cup-shaped fossa which accommodates the trochlea of the humerus. And this all together forms the elbow joint. And this depression is known as the trochlear or semilunar notch. It’s otherwise known as the greater sigmoid cavity of the ulna.
Moving onto the posterior ulnar shaft, we can see that it is largely defined by this border running along its length which is the posterior border or the margin of the ulna, and this marks the division between the posterior and medial surfaces of the ulna.
If we follow the posterior border down to the distal end, we will find it terminates at this somewhat pointed process here which is known as the styloid process of the ulna.
And with that, we’ve covered the ins and outs of the ulna.
So let’s finish up now by quickly putting our knowledge together into a clinical perspective. So as we mentioned at the beginning of our tutorial, injuries which are often referred to as wrist fractures most often actually involved the radius and the ulna. So let’s take a look and see what type of fractures are characteristic of this type of injury.
As the radius is the primary load-bearing bone of the forearm, it’s therefore no surprise that the radius is more commonly fractured than its neighbor, the ulna. And most cases of distal radial fracture tend to involve falling onto an outstretched hand as we all tend to brace ourselves with our hands when we slip or trip ourselves. In the case of young adults, their bones generally tend to have a strong structural integrity, meaning that significant force is required for such injuries. In the elderly, however, in particular, in all the women, the long bones tend to present significantly reduced bone density and are therefore much more susceptible to facture.
So, there are several classifications of distal radial fractures based on the fracture type, the location, the displacement of bone fragments, and the joint involved. So, again, I’ll take a couple of minutes to talk through some of these.
So, one of the most well-known classes of distal radial fracture is called Colles’ fracture, and in this situation, a transverse fracture of the distal metaphyseal region of the radius is observed with displacement of the bone fragment posteriorly and broken into pieces. In the case of a Colles’ fracture, no damage is incurred to the articular plate of the distal end of the radius but the ulnar styloid process will often also be avulsed or broken off in this instance as well. And the Colles’ fracture presents normally with dorsal angulation of the wrist joint meaning that the joint appears displaced in the posterior direction.
Another type of distal radial fracture that I wanted to talk about today is known as a Smith fracture, which is in essence, the reverse of a Colles’ fracture in that broken bone fragments are displaced anteriorly causing palmar angulation of the wrist joint instead. Other types of distal radial fracture include Barton’s facture and chauffeur fracture.
As the radius and the ulna are tightly bound together by the interosseus membrane, it’s also important to look for secondary fracture or damage to one of the radioulnar joints when a primary fracture to the radius or ulna is observed. And the radial fracture combined with the dislocation of the proximal radioulnar joint is known as the Monteggia fracture dislocation while a radial fracture combined with dislocation of the distal radioulnar joint is called the Galeazzi fracture dislocation.
And that’s where we’re going to leave it for today. I hope you’ve managed to take in all the points mentioned in this tutorial. To help reinforce what we’ve learned today, let’s quickly summarize the surface of the radius and ulna before we wrap it up.
So we began at the proximal end of the radius where we looked at the radial head which presented a depressed articular facet on its proximal surface and an articular circumference around its border. We then learned that the radial head is held in place by the radial collateral and annular ligaments.
Distal to the head, we saw the neck of the radius and distal again to that we found the radial tuberosity. On the anterior shaft of the radius, we identified the anterior margin of the radius which separated the anterior and lateral surfaces of the radial shaft. You may also remember that we referred to the medial border of the bone as the interosseus border due to the fact that it provides attachment for the interosseus membrane of the forearm. At the distal end, we described the styloid process of the radius as this point of protuberance right here.
On the posterior aspect of the radius, we saw the posterior margin of the radius which separates the posterior and lateral surfaces, and at the distal end of the radius, we finally identified the dorsal tubercle right here.
Moving onto the ulna, we first identified the coronoid process which forms the anterior lip of the trochlear notch and articulates with the trochlea of the humerus. We also talked about the ulnar collateral ligament which is located between the coronoid process and the medial epicondyle of the humerus.
Just distal to that, we found the ulnar tuberosity which provides attachment for the brachialis muscle as well as the radial notch which accommodates the head of the radius. The anterior ulnar shaft is limited medially by its anterior margin and laterally by its interosseus border. And we identified the distal end of the ulna as the ulnar head as seen here on the illustration.
Turning to the posterior aspect, we began with the large proximal process that is the olecranon which forms the posterior lip of the trochlear notch and the posterior shaft is largely defined by the posterior border of the ulna which separates the medial and the posterior surfaces.
Finally, at the distal end, we discussed the sharp prominence down here known as the styloid process of the ulna.
And that brings us to the end of this tutorial. If you want to test your knowledge of the radius and ulna, please be sure to give our quiz on these bones a try, and don’t forget to check out our articles and atlas images on the radius and ulna also.
Until next time, happy studying!