Posterior Compartment – Anatomy of the Forearm

00:01 quite deep. So now let’s move on to the posterior compartment. And the posterior compartment is going to contain muscles that essentially extend the wrist. But first of all, we can see in this posterior compartment, brachioradialis. Brachioradialis, as I just mentioned, was that muscle that can also flex the elbow. But it’s in the posterior compartment due to its nerve supply, being supplied by the radial nerve. So here we can see brachioradialis.

00:30 We can then see two muscles. These are known as extensor carpi radialis. We have a longus version and we have a brevis version. And these are coming from the humerus and they’re coming from the lateral epicondyle as we’ll see. So we can see the brachioradialis, extensor carpi radialis longus, and extensor carpi radialis brevis. Here, we can see their attachment sites. Brachioradialis is coming from the proximal two-thirds of the lateral supracondylar ridge. Here, we’d have the lateral supracondylar ridge. That’s where brachioradialis is coming from. And it passes to the lateral surface of the distal radius. So it just crosses the elbow. It doesn’t cross the wrist. It passes to the lateral surface of the distal radius and the pre-styloid process; so just a small elevation before the styloid process.

01:31 It’s innervated via the radial nerve, and it’s a weak flexor of the elbow. It’s a strong flexor, however, when the forearm is mid-pronated. So if the elbow is fully supinated like this, it’s a very weak flexor. But in this mid-pronator position, it is a very strong flexor of the elbow joint. So, really important muscle, brachioradialis. If we then look at the two muscles slightly deeper. We have extensor carpi radialis longus. We have extensor carpi radialis brevis. We can see that these two muscles also coming from the lateral supracondylar ridge, extensor carpi radialis. And extensor carpi radialis brevis is coming from the lateral epicondyle. So we can see that they gradually work their way down the humerus. From the lateral supracondylar ridge all the way down to the lateral epicondyle. Extensor carpi radialis longus attaches to the second metacarpal. Extensor carpi radialis brevis attaches to the third metacarpal. And again, we can see that here, attaching to the second metacarpal and attaching to the third metacarpal, extensor carpi radialis longus, attaching to the second, brevis, attaching to the third. We can see these two muscles are supplied by the radial nerve; extensor carpi radialis longus by the radial nerve itself, extensor carpi radialis brevis via the deep branch of the radial nerve. And these two muscles help to extend and abduct or radially deviates the wrist, deviate the wrist to the radial aspect, abduct the wrist.

03:22 If we look at the superficial layers, if we carry on looking at the superficial layer of the posterior compartment, then we can see we have three more muscles here. We have extensor digitorum, we have extensor digiti minimi, and we have extensor carpi ulnaris.

03:42 And these muscles again are passing from this lateral epicondyle, and they’re running all the way down to the digits of the hand. So here, if we see extensor digitorum and extensor digiti minimi coming from the lateral epicondyle of the humerus, and these pass towards the digits. Now, unlike the flexor compartment, these extensor muscles don’t kind of physically attached to the distal or the middle or the proximal phalanges of the digit. But they attach to a connective tissue, fibrous band over each of the digits, known as the extensor expansion. And this is a sheath that surrounds the interphalangeal joints of the digits.

04:30 So here we can see that extensor digitorum is attaching to the extensor expansion of the medial four digits. And extensor digiti minimi is going specifically to the extensor expansion of the fifth digit. These two muscles are supplied by the posterior interosseous nerve, and this is coming from the deep radial nerve. They’re both associated with extending the wrist, they extend the medial four digits if your extensor digitorum, and the fifth digit for extensor digiti minimi. They extend the digits at the metacarpophalangeal and interphalangeal joints. And they do this by attaching to those extensor expansions.

05:16 If we look at the extensor carpi ulnaris, the extensor carpi ulnaris is running down again from the common extensor origin or at least near to the common extensor origin, specifically, the lateral epicondyle. It also is coming from the posterior surface of the ulna.

05:36 This passes to the dorsal aspect of the fifth metacarpal associated with the little finger. And again, this is supplied by the posterior interosseous nerve. This muscle is associated with extending the wrist, just like the previous muscles. It’s also associated with adducting the wrist. And extensor carpi ulnaris can work with flexor carpi ulnaris to adduct the wrist.