Video: Cross sections of the upper extremity: Forearm level

Welcome back to the third part of our series on cross-sections of the upper limb. In this video tutorial, we’re going to be continuing on from the shoulder and arm regions already covered moving distally into the forearm. So, let’s waste no time and get right into it.

Before we begin, we need to first re-orientate ourselves in regards to the position of the forearm in these sections, and the forearm here in our cross-section is in a pronated position meaning that the palms of the hands have rotated from facing anteriorly to now facing posteriorly with the back of the hand now facing forwards. This, of course, is not the anatomical position for the forearm. Compared with the directions that we used for the shoulder and the arm, we can see now that directions have rotated due to this pronation of the forearm.

So looking at our cross-section of the forearm here, this is the anterior of the forearm which means that this aspect is the posterior and this is the radius over here which means that this side is lateral or radial and that makes the opposite side the medial or ulnar aspect. And do take a bit of time to have a good look at these directions now just to make sure that you’re properly orientated. Let’s continue on now and as usual, let’s start with the bones of this region.

So, as you know, the forearm extends from the elbow joint to the wrist joint and consists of two bones which are, of course, the radius and the ulna. Beginning proximally, we can see the head of the radius which articulates with the coronoid process of the ulna at the proximal radioulnar joint. As we move distally along the shaft of both bones, you can see that they’re connected by a thin, fibrous membrane seen here and this is known as the interosseus membrane, which ties bones together during pronation and supination of the arm. The interosseus membrane attaches to both the radius and the ulna at the interosseus border of each bone, and you can see that just here.

At the distal end of the forearm, the radius and the ulna will terminate as the distal end of the radius and the head of the ulna respectively. And these two parts articulate with each other at the distal radioulnar joint.

Alright now let’s explore the different muscles of the forearm of which there are many. But, of course, nothing that we can’t handle. Before we look at the muscles of the forearm, let’s first remind ourselves of the directions appropriate here, again keeping in mind that the arm in this cross-section is in a pronated position. As with the arm, the muscles of the forearm are generally grouped together into two compartments where each compartment has groups of muscles that have a similar purpose and innervation, and these two compartments are roughly divided by the radius and the ulna – and these are the anterior and posterior compartments of the forearm. And generally speaking, the flexor muscles lie anteriorly while the extensor muscles lie posteriorly with some exceptions that we’ll see shortly.

Let’s first take a look at the anterior compartment, also known as the flexor pronator compartment. The muscles of the anterior compartment are arranged into two layers which are the superficial and deep layers and we’re going to begin by focusing first on the superficial layer of muscles which all originate generally from the medial epicondyle of the humerus via a common flexor tendon. And working from radial to ulnar aspects of this group, the first muscle we’re going to encounter is the pronator teres muscle which is now highlighted in green.

So this muscle is then followed by the flexor carpi radialis muscle and usually after the flexor carpi radialis, the next muscle we would usually identify is the palmaris longus muscle. However, this particular muscle is not present in our cross-sections and the reason for this is that the muscle is probably one of the most variable muscles of the entire body and is said to be absent in about one to five people. But, on the other hand, if the muscle were to be present, you would find it just here.

Continuing on, we have the flexor digitorum superficialis muscle which when the palmaris longus muscle is present is sometimes considered as an intermediate muscle of the anterior compartment rather than a superficial muscle. And moving on, we will finally finish this layer with the flexor carpi ulnaris muscle.

Alright, now we’re going to do something which is a little bit fun. So, to help you remember these five muscles mentioned here especially when you’re doing exam, you can use this really fun mnemonic which is Pass, Fail, Pass, Fail, Fail. I’m sure that won’t be difficult for you guys to remember.

So going back to our cross-section at this level of the forearm, we can identify one of the deep muscles of the anterior compartment just here, and this is the flexor digitorum profundus muscle. And just a note as we move distally through our cross-sections along the length of the forearm, I’m just going to keep both the superficial and deep muscles highlighted as a group over here just so we can keep track of their position and sizes as we move along.

So at this level which is about the midpoint of the forearm, we can now identify the second of the deep muscles of the anterior compartment just here which is the flexor pollicis longus muscle, attached here to the anterior aspect of the radial shaft. And as we approach the distal end of the forearm, we can now see the pronator quadratus muscles, which is a rectangular muscle that runs between the anterior surface of the radius and the ulna.

And with that, we’ve covered the eight muscles of the anterior compartment of the forearm.

So let’s now turn our attention to the posterior or the extensor compartment of the forearm which contains eleven muscles. So just like the anterior compartment, we can divide the posterior compartment into a superficial layer as well as an underlying deep layer, and as before, I’m going to first begin by identifying the superficial muscles.

So working from radial to the ulnar aspects of the forearm, we firstly have the brachioradialis muscle which is neighbored by the extensor carpi radialis longus muscle and that muscle is then followed by the extensor carpi radialis brevis muscle. Next stop, we have the extensor digitorum muscle which is neighbored by the very slender extensor digiti minimi muscle, and finally, we come to the last of the superficial muscles which is the extensor carpi ulnaris muscle. And just in case you’re wondering, this muscle over here is the distal part of the anconeus muscle which we learned earlier is one of the posterior muscles of the arm.

So, like their anterior compartment, the superficial extensors of the forearm all originate from the humerus, specifically the lateral supracondylar ridge and the lateral epicondyle.

Alright, let’s move on now to the muscles of the deep layer of the posterior extensor compartment whose origins are located along the length of the proximal two-thirds of the forearm. In this section, we can see the most proximally located muscle of the deep posterior muscles and that’s the supinator muscle. And the humeral head of this muscle originates from the lateral epicondyle of the humerus and therefore works on the elbow joint. And over here, we can see it wrapping around the radius extending over to the posterior aspect of the ulna.

If we continue to the next cross-section, we can see the supinator has decreased in size and is now neighbored by the next deep muscle of the posterior compartment which is the abductor pollicis longus muscle which arises along the posterior aspect of the ulnar shaft. And as you can see in the diagram, the abductor pollicis longus muscle crosses from its origin across to the radial aspect of the forearm as it descends through towards the thumb. And let’s have a look at a cross-section taken just distal to the midpoint of the forearm where we can see the abductor pollicis muscle now along the posterior radius as expected.

Okay, so at this level, we can also now identify the last of the deep muscles of the posterior compartment, and the first of these is the extensor pollicis brevis muscle which originates from the posterior radial shaft and interosseus membrane and it lies sandwiched between the abductor pollicis, and our next muscle, which is the extensor pollicis longus muscle. And finally we have the extensor indicis muscle originating here on the posterior ulnar shaft around the same level as the extensor pollicis brevis muscle, and this muscle, as the name suggests, works to extend the index finger.

And with that, we’ve identified all the muscles of the posterior compartment of the forearm. And, personally, I think it’s quite interesting to see at this stage how the posterior muscles of the forearm even though greater in number are smaller as a group compared to those of the anterior compartment.

We’re now going to continue through with our cross-sections at the wrist joint following the posterior muscles of the forearm as we go along and as before, I’m going to highlight the superficial muscles again in blue and the deep muscles in green to help you keep track. At the distal end of the forearm, we can see that most of the muscular bellies of the posterior muscles have tapered off into their respective tendons which will continue on to their insertion points in the hand.

So, that’s it, we’ve now looked at all of the muscles of the forearm.

Our final set of structures which we need to look at quickly before we finish is, of course, the neurovasculature, and let’s begin with the major blood vessels of the forearm. So to learn about these, we’re going to have a look at the cross-section of the proximal forearm as the neurovasculature is much smaller and more difficult to identify at its distal end. And there are two major arteries for us to identify in the forearm both of which are branches of the brachial artery which we saw in the arm.

And the first of these is the radial artery, which as you would expect in our cross-section is located along the radial aspect of the forearm. The second is the ulnar artery which at this level is located close to the median nerve, however, it runs closer to the ulna in the distal forearm. And both of these arteries have vene comitantes, which means that their equivalent veins run along beside them. And here we can see the radial veins and here we can see the ulnar veins.

Hopefully, you’ll still remember the two other major veins which we identified in the arm and if you don’t, it doesn’t matter, I’m going to remind you anyway because these are the basilic vein located on the medial or the ulnar aspect of the arm and the cephalic vein here on our lateral or radial aspect.

And with that, we’ve identified all the major vessels of the forearm.

The last thing we want to talk about are the nerves of the forearm which like the arteries are basically continuations of those which we encountered in the arm. So those of you who studied hard learning the muscles of the forearm will already know that most of the muscles of the anterior compartment of the forearm are in general supplied by branches of the median nerve which as the name suggests runs through the center or the middle of the forearm along the deep aspect of the flexor digitorum superficialis muscle.

The one exception to this rule – because there’s always exceptions to rules in medicine – are the flexor carpi ulnaris muscle and the medial part of the flexor digitorum profundus muscle which are both innervated by the ulnar nerve, which as you would expect, runs in the line with the ulna between the two muscles that it innervates.

The muscles of the posterior compartment of the forearm are innervated by branches of the radial nerve without exception which makes sense given that they mainly lie towards the posterolateral or radial aspect of the forearm. And the radial nerve branches around the level of the lateral epicondyle of the humerus into the superficial and deep branches of the radial nerve. The superficial branch of the radial nerve usually runs within the anterolateral aspect of the forearm close to the radial artery and deep to the brachioradialis muscle which is right here.

The deep branch of the radial nerve usually runs posterior to the supinator muscle between the superficial and deep layers of the posterior compartment which is around about here.

And with that, we are three-quarters of our way through the cross-sections of the upper limb. Remember this video tutorial is the third in a four-part series on the cross-sections of the upper limb. And in our next and final installment, we’ll be continuing distally into the region of the wrist joint and hand once again identifying the bones, muscles, and neurovasculature present there.

I hope you’ve enjoyed this tutorial so far, and hope to see you in part four of this series.