Friday, February 24, 2012

The Elbow Joint, Part 1: Anterior View, Supine Position

One of the nicest things about teaching at an art college is meeting dozens of new students every year, getting to know them, and often forging friendships that last long after graduation. Being a teacher of both anatomy and illustration means sometimes my students do unusually interesting and creative things for me, like making me little anatomical drawings (sometimes on the backs of quizzes,) alerting me to interesting anatomy-related links like this one or this one, or sending me images they think I can use for my blog. 

Last month I was fortunate to receive a nice collection of x-ray images from former student Bob Giova, who graduated from The American Academy of Art several years ago with a B.F.A. in watercolor. Bob was in one of my first anatomy classes-- quite possibly the very first semester I taught there. He was one of those students who aced every quiz and exam without breaking a sweat. We have kept in touch over the years, and recently he was kind enough to send me images of some x-rays he'd had taken of his forearm. This couldn't have come at a better time. I'm short on back muscle photos, and this gives me something else to write about while I address that. The elbow joint! We'll return to the back muscles later.

The elbow joint is one of the most complex in the human body. Most skeletal joints involve only one articulation (an articulation is a place where two bones meet) but elbow joint involves two. This is why we can produce two different arm movements there. But before we talk about that, let's look at the bones involved:

The bones involved in the elbow joint are the humerus, radius, and ulna. The radius is on the thumb side of the arm, and the ulna is on the pinky side.

The three bones involved in the elbow joint are the humerus (in the upper arm) and the radius and ulna (in the forearm.) The first thing I like to teach students about the radius and ulna is how to quickly tell them apart. The easiest way is to remember that the radius is always on the thumb side of the forearm and the ulna is always on the pinky side. This remains constant no matter the position of the hand because the hand moves with the forearm bones. More on this later. If you're looking at an image of the radius and ulna in which you can't see the wrist and hand bones, you can still tell the radius from the ulna: The radius is wider and its distal end, because it has a broad articulation with the bones of the wrist (the carpal bones.) The ulna is wider at its proximal end, where it has an intimate articulation with the humerus. That articulation will be examined more closely later.

Here is a closer look at the elbow joint itself:

The trochlea (at the distal end of the humerus) rests in the trochlear notch of the ulna. This is the articulation where are flexion and extension occurs. There is also an articulation between the proximal ends of the radius and the ulna. This is where forearm supination and pronation occurs. (Supine position is palms turned up; prone position is palms turned down.) By the way, that thin membrane between the two forearm bones is called the interosseus membrane. Inter means between and osseus refers to bones, so this name just means "membrane between bones."
The above image shows the two separate articulations at the elbow. One articulation occurs where the trochlea-- the wide, spindle shaped structure at the distal end of the humerus-- rests in the trochlear notch-- a wide depression in the proximal end of the ulna into which the trochlea fits perfectly. The proximal end of the ulna has sort of a wrench shape (the sunken area of which is the trochlear notch) and its back and forth turning motion around the humerus is what produces the flexion and extension movements (or bending and straightening) of our arm.

As you know, though, this isn't the only way our elbow joint moves. We are also able to pronate (turn our palm downward) and supinate (turn it upward.) This twisting motion of the forearm allows us to turn our hand all the way forward or backward without having to turn our body around-- or our upper arm for that matter. We're capable of independent pronation and supination of the forearm because of the proximal radioulnar articulation-- the articulation at which the head of the radius spins against the ulna and allows its twisting action. 

There will be more on pronation and supination in a future post, but for now I'd just like to take a look at the appearance of the elbow joint when the forearm is in supine position. 

Anterior view of the elbow joint with its two articulations: 1) Humero-ulnar articuation, where we flex and extend the arm; 2) Radioulnar articulation where we pronate and supinate the forearm. The medial epicondyle is also pointed out, as this is a surface landmark we might see and draw on the arm.

Bob's anterior elbow films allow us to see the relationship between the bones in the elbow joint and the soft tissue surrounding them. The image above shows us why the medial epicondyle is such a prominent surface landmark. You can see the soft tissue of the surrounding skin and how close the medial epicondyle somes to the surface.

Incidentally, the laterial epicondyle of the humerus is also a surface landmark, but it doesn't show from the anterior view. It shows from a lateral and/or posterior view, and when the arm is extended, it tends to look more like a dimple. But we'll look at that later, too.

Bob's x-ray image superimposed over my arm. Cool! This can be done pretty easily because our bone shapes are quite consistent from person to person. Notice how the medial epicondyle comes right to the surface and forms a bump on the medial elbow.

Finally, the above image shows anterior view of the arm with the forearm in supine position. The x-ray image has been superimposed over the top so we can see where the bones fall in the elbow joint. See how the medial epicondyle comes right to the surface and forms a bump on the medial elbow? This photo also shows two visible ventral wrist tendons, those of the palmaris longus muscle and the flexor carpi radialis muscle. These are covered more thoroughly in my very first blog post, The Ventral Forearm: What are those Tendons?

In an upcoming post we'll view the elbow joint from the lateral aspect, examine the landmarks that are visible laterally, and look at the mechanics of flexing and extending the elbow.

Thanks, Bob Giova, for the arm x-rays! We'll be seeing more of them in the next elbow post. To see Bob's lovely watercolor work, go here. See you next time.

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