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Overview of the structures found on different cross-section images at different levels of the wrist and hand.
Hey everyone! Welcome back to the final part of our series on cross-sections of the upper limb. So, we’ve now reached the final and most distal portion of the upper extremity which is, of course, the wrist joint and the hands. And although a relatively smaller region compared to the shoulder, arm and forearm regions, the hand packs in a huge amount of anatomical detail, so let’s waste no time and check it out.
So we’re going to begin by reminding ourselves of the anatomical directions here, remembering that the hand is in a pronated position, meaning that the palmar or anterior aspect of the hand is facing down. I’m also going to mention here that the hand in our cross-section is also slightly abducted or radially deviated and this means that the hand is not exactly perpendicular to the axial plane as you might expect. With all that in mind, let’s waste no time at all and get straight to it, starting as usual with the bones of this region.
As you probably already know, the wrist joint and hand have a complex make-up when it comes to its skeletal framework with no less than twenty seven bones in this small region. So, let’s take a moment to remind ourselves of the bones involved.
The wrist joint contains eight small carpal bones which are arranged into rows – a proximal row and a distal row. And the proximal row contains four bones which are the scaphoid, the lunate, the triquetral, and pisiform bones, and they articulate directly with the radius and indirectly with the ulna at the radiocarpal and ulnocarpal joints respectively. The distal row of carpal bones also contained four bones which are the trapezium, the trapezoid, the capitate, and hamate bones, and these articulate with the members of the proximal carpal bones at the midcarpal joint.
Distal to the carpal bones are five long bones which make up much of the hand proper or the metacarpus, and these are, of course, the metacarpal bones which are numbered one to five working from radial to ulnar aspects of the hand. And the metacarpal bones articulate with the distal carpal bones at the carpometacarpal joints and also with each other at intermetacarpal joints. Finally, at the most distal part of the hand, we have the phalanges, which make up the bony framework of the digits.
The second to fifth digits each present three phalanges – a proximal, a middle, and a distal phalanx. The thumb or the pollex, however, lacks a middle phalanx so therefore only presents a proximal and distal phalanx.
So, I’m guessing you agree with me that the bones are just a tad complicated. Let’s look at our cross-sections and see what arrangements and interrelations we can see in these images.
So, working radial to ulnar, we can first see the most distal tip of the radius which is the styloid process of the radius seen now in green. The large round bone next to this is the scaphoid bone with the lunate bone located adjacent to it, and in this section, we can see exactly why the lunate got its name with its half-moon-shaped appearance hugging the bone distal to it which is the capitate bone – the largest of the carpal bones.
Along the ulnar aspect of our cross-section, we unsurprisingly finds the head of the ulna with its styloid process along its posterior aspect. And moving slightly more distal, we can begin to see some of the distal carpal bones namely the trapezium, the trapezoid and the capitate bones. And sandwiched in between here is the distal tip of the scaphoid bone and over here, we have the triquetral bone. Also visible in this cross-section is the base of the first metacarpal bone.
Continuing on to the last section of our carpal bones, we again can see the first metacarpal here along with the radial aspect and now also the base of the second metacarpal bone articulating here with the distal parts of the trapezium and the trapezoid bones. So, in this section, we can get a better appreciation for the capitate bone which is the largest of the carpal bones. Continuing towards the ulnar aspect of the hand, we can see the hamate bone here and over here, the triquetrum bone, this time with its sesamoid neighbor which is the pisiform bone.
Moving on from the bones of the wrist joint or carpus, we can now see the five metacarpal bones together, and you can see the second through fifth metacarpal bones are relatively closely positioned relative to the first metacarpal which is much more independent and mobile.
And as we continue towards the head of the palm of the hand or the metacarpus, we can now see the expanded heads of the third, fourth and fifth metacarpals, as well as the base of the proximal phalanx of the index finger. And finally, the distal phalanx of the thumb – and that’s going to be our most distal cross-section for today. However, if we were to continue, we would see the continuation of the proximal, middle, and distal phalanges of the fingers.
Let’s continue now on to the muscles of the wrist joint and the hand.
And just like the bones, this region packs in a large amount of anatomical detail. The main reason for this complexity here is that the majority of the muscles of the forearm or rather their long tendons, continue distally to insert into the carpals, the metacarpals and the phalanges. The hand also contains several groups of small muscles, all of which support the intricate and dexterous movements that the parts of the hand is capable of.
So let’s take a look at our cross-sections and see how this translates.
At this level of the proximal wrist, we can see that the tendons of several muscles of the forearm present here, and from the anterior compartment, we can identify the tendons of the flexor carpi radialis, the flexor pollicis longus, the flexor digitorum superficialis, the flexor digitorum profundus, and finally, the flexor carpi ulnaris.
From the posterior compartment of the forearm, we can see the tendons of the abductor pollicis longus, the extensor pollicis brevis, the extensor pollicis longus, the extensor carpi radialis longus, the extensor carpi radialis brevis, the extensor indicis, the extensor digitorum, the extensor digiti minimi, and finally, the extensor carpi ulnaris. And that’s quite a lot to take in but it’s important for us to identify these tendons as they all act on the joints of the wrist and the hand.
And as we continue onto a more distal cross-section, we can now see the bellies of the three muscles which belong to the thenar eminence, which is also known as the ball of the thumb. The muscles present here are the abductor pollicis brevis, which is the most superficial of the thenar muscles. Next, we have the opponens pollicis muscle which is flanked either side by the superficial and deep heads of the flexor pollicis brevis muscle, all of which are found anterior to the first metacarpal bone.
Continuing distally through our sections now at the level of the head of the first metacarpal bone, we can again see the abductor pollicis brevis and the flexor pollicis brevis muscles. And deep to the thenar muscles, we can now identify both heads of the adductor pollicis muscle specifically its oblique head which we can see over here and its transverse head which we can see over here.
Crossing over to the ulnar aspect of the hand, let’s take a look at these muscles over here which are the muscles of the hypothenar eminence. And these muscles include the palmaris brevis muscle which is the most superficial of the hypothenar muscles, the flexor digiti minimi brevis, the opponens digiti minimi, and finally, the abductor digiti minimi.
So, let’s turn our attention now to three final groups of muscles, all of which are essential for the fine dexterous motor movements of the hand.
So, the first group are the dorsal interossei muscles of which there are four. Each interposed between adjacent metacarpal bones – and you can see this being highlighted now – and these are bipennate muscles, meaning that each one presents two heads which originate from the dorsal aspect of adjacent metacarpal bones. Anterior to the dorsal interossei muscles are the palmar interossei muscles which consists of three unipennate muscles. The first palmar interosseus is located along the ulnar aspect of the second metacarpal bone with the second and third located along the radial aspect of the fourth and fifth metacarpal bones respectively.
Finally, located on the anterior or palmar aspect of the hand, we have the lumbrical muscles of the hand of which there are four examples. The first and second lumbrical muscles are unipennate arising from the lateral two tendons of the flexor digitorum profundus, and the third and fourth lumbricals are bipennate, arising from the three medial tendons of the flexor digitorum profundus.
And that brings us to the end of the muscles of the hand.
Just one topic left to quickly look at before we finish, and that’s the neurovasculature of the hand.
As we mentioned when looking at the vessels of the forearm, the more distal we travel along the upper limb, the more difficult it becomes to identify blood vessels and nerves in cross-section. That being said, we can still usually manage to identify some of the major vessels and nerves of this region. Focusing first on the blood vessels, the main arteries of this level that we can see are the ulnar artery seen here along the ulnar aspect of the flexor digitorum tendons as well as the radial artery, which is not as easy to spot here, and we would expect it to be somewhere around here close to the tendons of the abductor pollicis longus and extensor pollicis brevis muscles.
The venous structures of the hand are even harder to identify, however, we would expect to see the ulnar vein as well as the radial veins accompanying their respective arteries just here.
Moving on to the nerves, and the main structures in question here are, of course, the median and the ulnar nerves. The median nerve can be seen here between the tendons of the flexor carpi radialis and flexor digitorum superficialis while the ulnar nerve, as you would expect, can be relatively easily located here adjacent to the ulnar artery. The superficial branch of the radial nerve is not visible at this level of cross-section but, however, we would expect for it to be located somewhere around here.
So, that’s it. We’ve now covered the entire upper limb in cross-section, and with that, we’ve reached the end of this challenging and information-packed video tutorial series. I hope you found it interesting.
Don’t forget, you can explore each cross-section of the upper limb in more detail by checking out our individual study units on kenhub.com where you’ll be able to find great articles, videos, and challenging quizzes on all of the structures we’ve encountered today.
So until next time, happy studying, and thanks for watching!