Deep Layer of the Posterior Compartment of the Forearm

00:01 Now, let's carry on looking at other muscles within the posterior compartment.

00:05 And again, I appreciate there's lots of muscles here and you should look at your own respective curricular to work out how much you need to know about the origins and insertions.

00:14 But here's a comprehensive assessment of those muscles continuing into the posterior compartment.

00:22 So, let's just remind ourselves of those superficial layer muscles, brachioradialis, extensor carpi radialis longus and its sibling, brevis version.

00:31 Extensor digitorum and then, extensor digiti minimi and extensor carpi ulnaris forming the superficial layer.

00:39 You'll be glad to hear the deep layer has fewer muscles.

00:41 It has supinator which we can see here in the proximal aspect of the forearm.

00:46 Abductor pollicis longus, extensor pollicis longus, extensor pollicis brevis, it has a sibling.

00:54 And we can see extensor indicis here as well.

00:58 So, a number of muscles that form the deep layer of the posterior compartment.

01:03 So, let's talk about them and their origins and insertions.

01:06 Here, posteriorly, just distal to the elbow joint, we can see supinator.

01:11 It's origin is from the lateral epicondyle and the supinator fossa. We can see those located here.

01:17 And it's going to attach to the posterior lateral and anterior surface as of the proximal radius.

01:24 So, it really comes from a broad area from the lateral epicondyle and the supinator fossa.

01:30 And then, it's passing down onto the posterior, lateral and anterior surfaces of the proximal radius.

01:38 It's supplied again by the radial nerve, the deep radial nerve is indicated here.

01:43 The function of supinator muscle is as you'd expect, to help with supination of the forearm.

01:50 So, we can see abductor pollicis longus here.

01:53 Abductor pollicis as its name suggests is associated with abducting the wrist.

01:58 We can see that it originates form the posterior surface of the ulna which we can see here.

02:04 And also, the posterior surface of the radius, using the interosseous membrane that exists between those two bones as well.

02:13 It passes down and attaches to the metacarpal of the first digit.

02:18 So, we can see abductor pollicis longus.

02:20 It's innervated by the radial nerve, and as we've seen before, deep branches and the posterior interosseous nerve contribute here as well.

02:29 The function of abductor pollicis longus as its name suggests, helps to extend the wrist as well because of its location.

02:35 But primarily, it helps with abduction of the thumb.

02:39 And abduction of the thumb will help to move the thumb away from the midline of the hand around the axis in which the thumb locates.

02:48 We'll come back to that when we look at the thumb in more detail but controlled with - concerned with abduction of the thumb.

02:55 It also helps with extension of the thumb at the carpometacarpal joint.

03:00 If we look at extensor pollicis longus here, we can see extensor pollicis longus comes from the posterior surface of the ulnar and also, from the interosseous membrane and it passes onto the dorsal surface of the distal phalanx of the thumb, innervated again via the radial nerve and its branches, the deep branch and the posterior interosseous nerve that's coming away. The function of extensor pollicis longus, again, its name helps to give you an indication of its function.

03:29 It helps to extend the wrist as it passes across the wrist joint, but it also helps with extension of the thumb.

03:36 So, it helps with the complex movement around the axis of the thumb, which again, we'll come to later on.

03:43 Extensor pollicis brevis, we've spoken about extensor pollicis longus, this is the shorter version.

03:49 It comes from the posterior surface of the radius and also, from the interosseous membrane between the two.

03:55 And it passes through the dorsal aspect of the proximal phalanx of the first digit.

04:01 So, it's passing all the way towards the tip of the thumb.

04:06 It's innervated via the radial nerve again, the deep branch of the radial nerve and its posterior interosseous nerve.

04:14 The function of extensor pollicis brevis is to, again, help to flex the wrist as it passes over the wrist joint.

04:21 But it also goes to extend the proximal phalanx of the first digit.

04:26 So, it helps with extension of the thumb. Extensor indicis is a slender muscle.

04:33 This time, it's originating from the interosseous membrane and the posterior surface of the ulna.

04:40 And it passes all the way to the dorsal expansion of the second digit.

04:45 So, here, we can see extensor indicis passing across the wrist joint, attaching to the dorsal expansion of the second digit.

04:53 Again, it's supplied by the radial nerve, the deep radial nerve, and its posterior interosseous branch that's coming away from it.

05:01 Again, it helps to extend the wrist and extension of the second digit.

05:08 So, here, we have the muscles in the posterior compartment.

05:12 Here, we can see a whole number of these broad muscles that are filling this space.

05:17 Brachioradialis, extensor carpi radialis longus and brevis, extensor digitorum, extensor digiti minimi and extensor carpi ulnaris.

05:28 We also have abductor pollicis longus, extensor pollicis brevis and longus.

05:33 All of these muscles and the tendons that are associated with them are kept in position by the extensor retinaculum, a band of fibrous tissue, similar to the flexor retinaculum that helps to keep these tendons in position.

05:46 A lot of the tendons that we've spoken about pass to dorsal expansion of the digits.

05:51 So, these extensor expansion hoods help to form this fibrous kind of capsule around the distal tips of the fingers where these tendons pass to.

06:02 And these are really important as they pass towards this aspect of the hand.

06:07 We spoke about a number of these outcropping muscles.

06:09 What this means is these muscles are situated deep to those superficial layers and their tendons and distal muscle bellies pass out. They kind of stick out from underneath this muscle layer.

06:20 We have abductor pollicis longus and extensor pollicis brevis.

06:25 We also have extensor pollicis longus, its sibling in this space as well.

06:30 Another muscle that passes out in this direction is extensor indicis and we'll see later on that this helps to form the anatomical snuffbox which we can see located in this region here.

06:41 That's an important space we'll need to consider as it's an important space clinically when you're assessing someone for the fractured wrist.