Video: Dissection of the cubital fossa and anterior forearm
Cadaveric exploration of the cubital fossa and anterior forearm
Sometimes described as the golden age of anatomy, the 17th and 18th centuries were indeed an exciting time - a time where the study of the human body thrived and a new breed of anatomists emerged. ... Read more
Sometimes described as the golden age of anatomy, the 17th and 18th centuries were indeed an exciting time - a time where the study of the human body thrived and a new breed of anatomists emerged. The demand for cadaveric dissection was greater than ever and the anatomy theater was the place to be. Captured here in Rembrandt's 'The Anatomy Lesson of Dr. Nicolaes Tulp', public dissections were the highlight of the social calendar for the learned of Amsterdam - a gathering place of intellect and inspiration, soaring curiosity, and if my experience of studying the forearm is anything to go by, definitely confusion.
Since then, a lot of things have changed when it comes to learning anatomy. Some things, however, never changed. Doesn't matter if it's 400 years ago or today, learning about the anatomy of the forearm is a pain even for the best of anatomy students. With many muscles and complex anatomical relations to unravel, sometimes you just need to see the real thing. So, let's take the angst out of the anatomy now as we explore a dissection of the cubital fossa and anterior forearm.
We begin here with the upper limb placed in the anatomical position with the forearm supinated so the palm faces anteriorly. So taking our scalpel, we gently pinch the skin using our forceps and make a transverse incision across the bulge formed by the biceps brachii, about halfway down the arm. Remember the skin of the anterior arm is very thin; therefore, we only need to apply the gentlest of pressure with the scalpel in order to preserve the subcutaneous tissue and vessels beneath. We'll repeat this step now and make another incision in the distal region of the anterior forearm close to the wrist joint, once again ensuring to preserve the underlying subcutaneous tissue as much as possible.
Next, we make a longitudinal incision from the midpoint of our proximal incision extending distally across the forearm to meet our second incision. To create a flap and open our field of dissection, we're next going to lift a corner of the skin with our forceps where our incision lines meet. We're going to gently pull back on the skin and use the scalpel to separate it from the underlying subcutaneous tissue. Now this might be a little tedious but it's important that we do it properly and with care so that we preserve the superficial structures beneath the skin as much as possible. This actually will be one of the most time consuming parts of our dissection and it takes a degree of patience to resist the temptation to go quickly so we can get to the anatomy hidden below.
Once we have completely exposed the subcutaneous tissue, we move on to the even more tedious task of removal of this layer. As we continue with the fat removal, we'll uncover several superficial cutaneous nerves of the forearm that innervate the skin of this region as well as the superficial veins of the forearm. We'll identify the largest of these in a few moments once we've removed the bulk of the fat and connective tissue here.
Okay, now that we have removed the bulk of the subcutaneous tissue, let's identify all these neurovascular structures that we have isolated, namely the superficial veins of the forearm. We recognize veins, of course, by their collapsed lumens and dark blue color. So, first, we have these two veins here which are actually both the cephalic vein, or should I say, veins. This is an example of what is known as an inselbildung cephalic vein in which the vessel starts off as one vein, splits into two for a time, and then merges again. The larger vein which receives the cephalic veins is the median cubital vein.
The median cubital vein is a popular vein, of course, in clinical practice as it's a site for cannulation when taking blood samples, administering fluids and drugs, or when undertaking blood transfusion. Continuing medially, we see other tributaries coalescing here with the median cubital vein. Namely, there is the median antebrachial vein, the basilic vein, as well as many smaller unnamed tributaries which feed into these vessels. The large vein here in the distal part of the arm which we're now exposing is a continuation of the basilic vein. This vein, of course, will continue on towards the axilla where it will eventually become the axillary vein.
Now that we have cleared away some of the tissue around the basilic vein, we can also see a nerve here in close proximity to it which is the medial antebrachial cutaneous nerve. This is a branch of the medial cord of the brachial plexus which courses down through the arm and divides into anterior and posterior cutaneous branches. The anterior branch can be seen here crossing the coalescence of the median cubital and basilic veins and then continuing distally across the anteromedial aspect of the forearm providing sensory supply to the skin as far as the region of the wrist joint.
Now once we remove the continuation of the cephalic vein into the arm, we can see the lateral antebrachial cutaneous nerve, which is the terminal branch of the musculocutaneous nerve that supplies the anterior muscles of the arm. This nerve often courses in close proximity to the cephalic vein as it goes distally through the forearm. It gives off a posterior branch that courses laterally to the posterior aspect of the forearm. The main trunk of the lateral antebrachial cutaneous nerve, however, continues as the anterior branch which divides further as it courses distally through the forearm.
Let's prepare to continue a little deeper now with our dissection. First, we'll cut the cephalic vein like this with our scissors. Now when we reflect this vessel, we can see another pair of veins draining into the cephalic vein which are known as the perforating veins. These veins connect the superficial and deep venous drainage of the forearm. Now, once we've exposed the muscle belly of the biceps brachii, we carefully remove the fascia from the distal tendon of this muscle. Next, we'll further dissect the lateral antebrachial cutaneous nerve from the surrounding tissues and track its course proximally to where it emerges from beneath the biceps brachii.
Next, we're going to cut and reflect the basilic vein in the arm to fully expose the biceps brachii, and actually, while we're at it, let's cut these perforating veins so that we can completely remove all the superficial veins. With that cleared, we can proceed with further dissection and exposure of the bicipital aponeurosis and distal attachment of the biceps brachii.
Here we see the distal attachment of the biceps brachii muscle which dives deep towards the radial tuberosity. We can also see the fascial attachment of the biceps brachii extending from its tendon which we know is called the bicipital aponeurosis.
After removing the brachial fascia, let's move on to the antebrachial fascia, or the deep fascia of the forearm. We'll start by dissecting the lateral or radial part of the fascia. This way, we've uncovered the anterior surface of the brachioradialis muscle. Next, let's separate the bicipital aponeurosis from the antebrachial fascia and get our first glimpse of what lies beneath.
When we pull the bicipital aponeurosis away, we see the trunk of the brachial artery and we'll just take a few moments to clear away the connective tissue around it a bit more so we can get a better appreciation of what it looks like. Now if you look closely at the brachial artery, you will see two veins running alongside it which are its venae comitantes, otherwise known as the brachial veins. Let's remove one of them as well as its tributaries - the radial veins - in order to get a clearer view of the brachial artery.
With that complete, we now clearly see the brachial artery dividing into its two major branches that supply the forearm - the radial and ulnar arteries.
Now let's retract the biceps brachii muscle to identify the structures lying deep to it. Here we see the brachial artery coursing through the arm close to the brachialis muscle. Medial to it, we find the median nerve which dives deep to the pronator teres muscle as it enters the cubital fossa. Having cleared up the field around the median nerve, we can now get a better view of these arteries again. The proximal one is the inferior ulnar collateral artery - which we can see giving off a branch here - will anastomose with the anterior branch of the ulnar recurrent artery while the inferior ulnar collateral artery itself will continue on to anastomose with the middle collateral and superior ulnar collateral arteries.
The vessel distal to the inferior ulnar collateral artery is a muscular branch to the brachialis muscle. And if we continue distally along the length of the brachial artery, we can see another muscular branch here once again. Holding the artery in this position, we can also have a better look at the radial artery. We see the main trunk of the artery continuing down the forearm as well as its branch - the radial recurrent artery.
Let's now remove the antebrachial fascia from the distal forearm. We'll pull out the retraction needle and proceed with the in situ identification of the muscles that we see.
Right here is the medial margin of the brachioradialis muscle. Notice how the radial artery courses just deep to its margin, and this here is the pronator teres muscle, and this muscle over here is the palmaris longus muscle with its characteristic tendon that starts midway down the forearm towards the hand. The next muscle we can see here is the flexor carpi radialis muscle, and finally, lateral and deep to the palmaris longus is the flexor digitorum superficialis muscle.
Now, we will proceed with separating the muscles one from another. We'll start with the palmaris longus and its tendon. Moving over to the radial aspect of the arm, we're going to repeat this process by separating the fascia joining the brachioradialis and flexor carpi radialis muscles. When we lift the medial border of the brachioradialis muscle, we'll uncover the continuation of the radial artery coursing in an almost straight line through the forearm. As you can see in the distal half of the forearm, it is simply covered by the antebrachial fascia which we are cutting away now and is here, of course, between the tendons of the brachioradialis and flexor carpi radialis muscles that we find our radial pulse in clinical practice.
Let's now detach and reflect the biceps brachii muscle. Pulling the brachial artery away, we see the anterior surface of the brachialis muscle and its tendon that attaches to the proximal ulna. Bringing the biceps back for a moment, we can now nicely see most of the borders of the cubital fossa. The lateral margin of the pronator teres represents the medial border of the fossa, the medial margin of the brachioradialis comprises the lateral border, while the brachialis muscle comprises the proximal part of the floor of the cubital fossa. The superior border is an imaginary line extending between the epicondyles of the humerus which would be here. And if we think back to earlier in our dissection, we remember that the roof of the cubital fossa was reinforced by the bicipital aponeurosis.
All the contents of the cubital fossa except the radial nerve which lies a little deeper are visible here. They are the median nerve, the brachial artery, and the tendon of the biceps brachii.
Now to quickly memorize the contents of the cubital fossa, try using the following mnemonic - My Blood Turns Red. Here, the first letter of each word stands for the first letter of each of the structures you need to remember. So My Blood Turns Red stands for the Median nerve, the Brachial artery, the Tendon of the biceps brachii, and of course, R is for the radial nerve.
With the brachioradialis muscle now retracted, let's turn our attention to the radial nerve which has been hiding in a deep groove here between the brachialis muscle medially and the brachioradialis and extensor carpi radialis longus laterally. Let's dissect the radial nerve going proximal to distal so we can track its course. And if we follow this nerve down roughly to the level of the lateral epicondyle, we can see it dividing into its superficial and deep branches. The deep branch passes under the humeral head of the supinator muscle which it supplies with motor innervation. The superficial branch of the radial nerve, however, continues along the length of the anterior forearm lateral to the radial artery and deep to the brachioradialis muscle when it's not retracted.
As it approaches the distal end of the forearm, the superficial branch of the radial nerve passes under the tendon of the brachioradialis muscle and continues around the lateral side of the radius where it will innervate the dorsal aspect of the hand and fingers.
If we move back to the proximal forearm, we can also take a look at some of the muscular branches of the radial nerve here such as this one to the extensor carpi radialis longus and this one slightly more distal which is for the extensor carpi radialis brevis. Here you can also see another branch to the extensor carpi radialis brevis arising from where the radial nerve divides into its superficial and deep branches. If you look closely, we can see these branches are accompanied by several arterial branches arising from the radial recurrent artery which also supply the extensor carpi radialis brevis.
If we place the biceps brachii muscle back into its original position, we can once again get a better look at the radial recurrent artery here curling around the radial attachment of this muscle where it will continue to eventually anastomose with the radial collateral artery of the arm, and again here, we have its muscular branches to the extensor carpi radialis brevis and longus.
Now, we're going to cut some of these muscular branches to help us get a better visualization of the deep branch of the radial nerve, which we can now clearly see passing deep to the supinator muscle. Now, let's go back over to the ulnar aspect of the distal arm where we're going to slightly retract the pronator teres muscle to reveal some of the muscular branches of the median nerve which supply this muscle.
Our next step is to quickly remove the fascial covering of the flexor carpi radialis muscle, which is this muscle here. We do this by making the finest of incisions along the length of the muscle belly and then using two sets of forceps to lift and bluntly remove the fascia. We can then use our scissors to cut away the loose fascia on either side. Next, we can bluntly separate the muscle from its neighbors and do the same for the palmaris longus muscle.
We're then going to repeat this process of fascial removal with the muscle deep to the flexor carpi radialis and palmaris longus which is, of course, the flexor digitorum superficialis muscle. As we remove the fascia, you can see that it is continuous with the muscle adjacent to the flexor digitorum superficialis which is the flexor carpi ulnaris, so we'll use a scalpel to remove this fascia and separate these two muscles from one another. And when they are fully separated, we will find two very important structures lying between which are distal parts of the ulnar artery and nerve.
So first let's look at the ulnar nerve which is the relatively thick white ropey structure here. We can see it too gives off muscular branches like this one which supplies the medial half of the flexor digitorum profundus. We'll uncover this muscle in a short while. It also gives off muscular branches to the flexor carpi ulnaris. Lateral to the ulnar nerve is the ulnar artery which is easy enough to tell apart from its neighbor.
Okay, so it's time for us now to begin delving a little deeper into the anterior forearm and we're going to begin by opening up the region around the cubital fossa, and to do this, we will first cut the muscular belly of the humeral head of the pronator teres muscle. While we're looking at the pronator teres, we can also take the opportunity to look at a distal attachment or insertion of the pronator teres muscle along the lateral border of the radius. It is located distal to the supinator muscle and deep to the brachioradialis and extensor carpi radialis longus muscles.
So let's take a look at our widened dissection field and see what structures we can now identify. Let's begin with these small muscular branches of the median nerve to the pronator teres which arise a couple of centimeters proximal to the level of the medial epicondyle. A little more distal along the median nerve is another larger muscular branch which usually innervates the flexor carpi radialis, but we'll take a closer look at these hopefully when we go deeper into our dissection.
As for the median nerve, it passes deep between the humeral and ulnar heads of the pronator teres as you can see here. This is also around the point where the median nerve passes from the medial to the lateral side of the ulnar artery.
Let's continue with the removal of our superficial flexor muscles now using our forceps to lift the palmaris longus. We'll next take our scalpel to gently separate the muscle from the bulk of the common flexor tendon before using our large scissors to cut the muscle belly. Now we're going to take the flexor carpi radialis with the forceps and again use our scalpel to carefully begin separating it from the underlying flexor digitorum superficialis before taking our scissors to further separate them before cutting through the muscle belly of the flexor carpi radialis,
We can now also get a better appreciation of the muscular branch of the median nerve to the flexor carpi radialis which we identified earlier. Let's cut that nerve now so that we can reflect the muscle back and fully expose the underlying flexor digitorum superficialis.
So looking at the distal part of the flexor digitorum superficialis, we can see the two tendons of the superficial stratum or layer of this muscle which will continue onto the middle and ring fingers. And while we're here at the distal end of the flexor digitorum superficialis, we should also take note of the median nerve which passes along the lateral border of the flexor digitorum superficialis after having passed deep to the muscle between its humeroulnar and radial heads in the proximal forearm.
Since we have removed the pronator teres and flexor carpi radialis muscles, we can now identify yet another group of fairly large muscular branches of the median nerve which this time provide motor supply to the flexor digitorum superficialis. We can also see the anterior interosseous branch of the median nerve here which we will speak about later on.
So out of the five superficial flexor muscles of the forearm, four of them are supplied by the median nerve. The only one not to be innervated by the median nerve is the flexor carpi ulnaris which is, unsurprisingly, supplied by the ulnar nerve which we will see later on. If you find tricky to remember the five superficial flexors of the forearm in your anatomy exam, try using a very apt mnemonic which is Pass Fail Pass Fail Fail. The first letter of each of these words stands for one of the superficial flexors. So, Pass Fail Pass Fail Fail stands for the Pronator teres, Flexor carpi radialis, Palmaris longus, Flexor digitorum superficialis, and Flexor carpi ulnaris.
So, as I mentioned before, we can see the median nerve passing deep to the flexor digitorum superficialis right here, and what we're going to do is now slip the hemostat right under the radial head of the flexor digitorum superficialis which attaches to the anterior border of the radius and cut through it like this with our scalpel. This will allow us to reflect the superficial stratum or layer away so we can now see the full course of the median nerve through the forearm. So let's pin back the reflected head of the flexor digitorum superficialis like so and let's actually do the same with the median nerve to make our dissection feel a little bit more open. So we'll add a pin here and maybe one more a little further distally right here.
Now after doing this, we have what seems like another muscle right here, but it's actually another part of the flexor digitorum superficialis, specifically, it's deep stratum or layer. This part of the muscle will taper off into two tendons just like the superficial part; however, this time, the tendons will proceed to the index and little fingers instead.
We're back in the proximal forearm now where I'm going to also retract and pin back the humeroulnar head of the flexor digitorum superficialis in addition to the median nerve.
Now that we have further open up our dissection field, I want us to quickly take another look at the arterial supply here once again. So, of course, we begin with the brachial artery right here descending through the arm lateral to the median nerve. We then have the radial recurrent artery here which is the branch of the radial artery which we see continuing distally here along the lateral side of the forearm. We then have the ulnar artery right here which is of a larger caliber or a size than the radial artery. We see it passing deep to a slip of muscle here which we'll identify in a moment before continuing distally along the forearm across the belly of the flexor digitorum profundus muscle.
Now that it's fully exposed, we can see many muscular branches which supply the flexor digitorum superficialis which overly it. Now if we move the ulnar artery aside, we'll find its nervous counterpart - the ulnar nerve - traveling right beside it, and if I quickly pin back the ulnar artery, we will now have three major nerves of the anterior forearm exposed together. So going ulnar to radial or medial to lateral, we have the ulnar nerve, the median nerve, and the superficial branch of the radial nerve.
Now so here once again we have the flexor digitorum superficialis muscle in its totality. We have the two tendons of the superficial stratum or layer here, and deep to those are two more tendons coming from the deep stratum or layer of the flexor digitorum superficialis.
So let's retract and pin this muscle back out of the way before we look at this muscle here which is the flexor pollicis longus. Now this is quite an interesting example of the flexor pollicis longus because if you're familiar with your muscle attachments, you might know that this muscle is usually described as originating from the anterior surface of the radius and interosseous membrane only. But if I lift the muscle here, we can also see an additional slip of muscle going in the direction of the coronoid process of the ulna. This here is a well-known anatomical variation of the flexor pollicis longus and this accessory head is known as the Gantzer muscle and is said to be present in around seventy percent of the population.
Now as we lift up the flexor pollicis longus, we are further exposing the muscular belly of the flexor digitorum profundus and we can now nicely see all four tendons of this muscle. Notice especially how the most lateral of them - the tendon for the index finger - is quite distinct and somewhat independent of the others which appear connected with loose connective tissue in the distal forearm.
So now we have both the flexor pollicis longus and the flexor digitorum profundus retracted and pinned back to reveal what is basically the anterior surface of the interosseous membrane as well as the pronator quadratus muscle here at the distal end of the forearm.
To further open up the dissection field, we are going to take the accessory head of the flexor pollicis longus which we discussed a moment ago and cut through this slip of muscle so we can reflect it back like this. With that moved away, we can now identify this branch of the median nerve which is the anterior interosseous nerve, and if we follow along its length, we can see many muscular branches to both the flexor pollicis longus and flexor digitorum profundus.
So now that we have located and isolated the anterior interosseous nerve, let's take a look at its arterial counterpart. So if we backtrack up to the ulnar artery, we can see it gives off a large branch here which arises just distal to the attachment of the biceps brachii at the radial tuberosity. This is the common interosseous artery. We can see that this artery also immediately bifurcates into two branches which are the posterior interosseous artery that we can just about see here and passes along the proximal border of the interosseous membrane to the posterior aspect of the forearm. The other branch which we see here is the anterior interosseous artery coursing distally between the flexor pollicis longus and flexor digitorum profundus to which it gives off several muscular branches.
And that is it for our exploration of the forearm today. I hope you enjoyed it and we'll see you next time.