Monday, June 27, 2011

The Dorsal Hand: The Dorsal Foot's Better Looking Sibling

Writing about the dorsal foot last time got me thinking about the dorsal hand, which despite its completely different outward appearance, is a pretty close relative structurally. For starters, the bones of the hand sit in a curved arrangement so, like the foot, it's convex on its dorsal side and concave on its palmer side. (The palmar side of the hand is also considered ventral, just like the plantar side of the foot.) Because of this arrangement, most of the soft muscle tissue in the hand is either on the palmar side or tucked between the metacarpals. Most of what we can feel on the dorsal side of the hand is bone, and most of what we can see are tendons (along with a few superficial veins.) Like the dorsal foot, the dorsal hand has seven visible tendons (although we don't always see them all at the same time) and only one clearly visible muscle. And finally, the visible tendons in the dorsal hand, like those in the dorsal foot, are named for the structures onto which they insert and for the movements they facilitate. 

The easiest tendons to identify in the dorsal hand are those of the extensor digitorum muscle. Its name means extensor of the digits, which is why there were also extensor digitorum tendons in the feet. (Toes are digits, too.) But notice the absence of "longus" or "brevis" here; unlike the toes, the fingers have only one extensor muscle, so it needs no further qualifier in its name. The extensor digitorum muscle, which lies in the dorsal forearm, splits off into four individual tendons that insert onto digits II through V (the index finger through the small finger.) The thumb has its own muscles, some of which we'll look at later in this post. It's easy to identify the four extensor digitorum tendons on the dorsal side of the hand because they are clearly heading in the direction of digits II through V. These tendons show most of the time, but more so when the fingers are extended (which means straight as opposed to curled inward) or abducted (which means spread apart.)

The thumb extensors are also pretty easy to spot as well, especially when the thumb is... well... extended. There are two thumb extensors, extensor pollucis longus and extensor pollucis brevis. Pollucis comes from the Greek word pollux. These two tendons are most visible at the base of the hand on the radial (thumb) side. At this point, they rest about one centimeter apart. Of the two, extensor pollucis longus is the more dorsal and, as its name implies, the longer. It inserts all the way on the first distal phalanx. Extensor pollicis brevis is more ventral, and it inserts on the first proximal phalanx. Another tendon, that of abductor pollucis longus, runs right along side the extensor pollucis brevis tendon at this point, but it is difficult to differentiate one from the other. The bodies of all three of these muscles are on the distal end of the forearm, radial side, and you can see them moving around if you extend and abduct your thumb.

Things look a little different on the dorsal hand when we adduct the thumb. Adduction of the thumb is bringing it inward so it tucks up right along the index finger. When we do this, the extensor digitorum tendons are still somewhat visible, but the extensor pollicis longus and brevis tendons disappear almost entirely. And in this position, a whole new structure pops out-- the first dorsal interosseous muscle. Interosseous means "between bones" and these muscles were given that name because they run in between the metacarpals, the bones in the body of the hand. There is a dorsal set of interosseous muscles and a palmar set (which are also between the metacarpals but on the palmar side.) Of all these, the only one that's ever really visible on the surface is the first dorsal interosseous. That means the dorsal interosseous muscle between the first and second metacarpals. When the thumb is adducted, the first dorsal interosseous muscle plumps up like a little pillow just next to the thumb. The more tightly adducted the thumb, the more this muscle shows. At the base of this muscle lies the trapezoid bone, one of the eight small carpal bones at the base of the hand. The trapezoid bone is not always easy to see, but it can be palpated easily.

There are two more elusive tendons on the dorsal hand that can be seen in certain situations. While the extensor digitorum longus muscle extends digits II through V, two fingers on the hand have extensor muscles exclusively their own. The second digit (or digiti indices) has its own private extensor called extensor indices. As you can probably guess, this tendon is most visible when the index finger is extended separately from the others.

The other elusive dorsal hand tendon is that of extensor digiti minimi. This name means "extensor of the small digit" and, you guessed it, it can be seen most clearly when the fifth digit (digiti minimi) is extended separately from the rest. It runs right along side the extensor digitorum tendon heading toward the fifth digit, but the extensor digiti minimi tendon is closer to the ulnar side of the arm.

All this dorsal hand talk has me amped up to write about the dorsal forearm soon. That's the mother of all complex muscular areas, but it's very cool. I've already chosen my forearm models, which was no easy task! I look forward to that and, as always, I'm open to suggestions for other areas to cover.

Tuesday, June 14, 2011

The Dorsal Foot: How Do I Love Thee? Let Me Count Your Tendons

A few weeks ago I had the pleasure of spending the day with 20 of my students at Body Worlds exhibit at the Museum of Science and Industry here in Chicago. It was a great day spent in the company of many wonderful students and friends, and it was unique opportunity to use real human specimens to discuss many of the structures we've covered in anatomy class.

My awesome students: Kris and her sister, Nick, Amanda, Lucas, Bryan, Paul A., Paul H., Matt, Vanessa, Leslie, Hali, Kristen, Jake, Sarah (not a student, but a fellow Anatomy instructor), Nate, Kirsten, Dustin, Aaron  

I have a real soft spot for the human foot-- particularly the dorsal side, where several superficial tendons can be seen just beneath the skin, sweeping gracefully across the convex arrangement of metatarsal and phalanges. So I experienced a particularly intense nerd buzz at Body Worlds when given the opportunity to explain these relationships using an actual human foot. We weren't allowed to touch the foot, but frantically stabbing my finger on the glass case in which it rested seemed to suffice. Talking about these structures while students listened (or pretended to listen) was really a lovely moment for me, so today's post is going to center on the remnants of that discussion.

So... first things first. There just aren't many visible muscles in the foot. There are several on the plantar surface (the bottom--the part that touches the ground) but they are not particularly defined or visible externally. The bones of the foot, especially the metatarsals, lie in a curved arrangement, so the basic structure of the foot is concave on the plantar side, convex on the dorsal (top) side. A few layers of muscles are nestled within the concavity on the plantar side, but they are obscured superficially by the plantar aponeurosis, a thin, flat tendinous covering that helps protect the underside of the foot. We can feel the soft tissue of the muscles through this aponeurosis (which is why the bottom of the foot is not bony) but we can really see much of their definition.

There are muscles on the dorsal side of the foot as well, but only a small portion of one of them shows clearly on the surface. More on that later. Most of what we can feel on the dorsal side of the foot is bone, and most of what we can see are tendons. The number of tendons we see depends on the position of the foot, but it ranges from one to seven. And there are a few other nice ones on the laterial and posterior sides of the ankle as well. How are there so many visible tendons on the foot ankle if we see so few muscles? Because the tendons we can see there come from muscles that are up higher, usually somewhere on the lower leg.

One of the easiest sets of muscle tendons to identify on the dorsal foot are those of extensor digitorum longus. That name means "long extensor of the digits." This muscle has four tendons, one each that inserts onto toes 2 through 5 and extends them. (Extension of the toes is kind of like curling them upward.) These tendons stand out clearly, sometimes even when the toes aren't extended. Sometimes we can also see the single extensor digitorum longus tendon up higher on the ankle-- before it has split into four separate tendons.

The presence of the word longus in a muscle name implies that there's a brevis, or shorter counterpart. (Brevis is Latin for "short" and it's the root of the English word "brief." We'll look at the extensor digitorum brevis muscle a little later.

(By the way, there is also a finger extensor in the forearm called extensor digitorum-- digitorum can be fingers or toes-- but there is no need for a longus or brevis qualifier in this name because there is only one extensor digitorum muscle up there.)

Another tendon that is easy to locate is that of the extensor hallucis longus muscle. Digiti hallicus is the Latin name for toe number 1 (or the big toe) so muscles that move it around often have the word hallucis in their names. The extensor hallicus longus tendon is also easy to find, as it insert on the dorsal side of the big toe and extends it upward. Like the other foot tendons, the muscle from which the extensor hallucis longus tendon originates is up on the lower leg. But the tendon can be seen surfacing anywhere from the anterior ankle all the way down to just before the big toe. The point at which it appears depends on the position of the big toe; the more it's extended, the more of the extensor hallucis longus muscle is visible.

Another visible tendon on the dorsal foot is that of tibialis anterior. It's easy to confuse the tibialis anterior tendon with the extensor hallucis longus tendon because they run side by side on the anterior ankle. What tends to confuse things more is the fact that the tibialis anterior tendon sometimes becomes less visible on the surface right at the point at which the extensor hallucis longus tendon begins to emerge! So we tend to see the end of one tendon and the beginning of another as just one long tendon. This is one of the most common mistakes made when drawing the feet.

In the image on the left, we can see both the tibialis anterior tendon and the extensor hallucis longus tendon. If they're mistaken for a single tendon, it's often because often tibialis anterior is disappearing just as extensor hallucis longus is emerging.

Below is a photo in which we can see almost all the tendons mentioned so far. In this case we can only see three of the four extensor digitorum longus tendons. The tendon that goes to the second digit tends to show less often than the other three (although I've included a leader line showing where it would be.) Notice also that the diameter of the tibialis anterior tendon much greater than that of the extensor hallucis longus tendon, and that, as mentioned above, the tibialis anterior tendon submerges and disappears just as the extensor hallucis longus tendon surfaces. We can also just barely see the great saphenous vein, which runs very superficially, just anterior to the medial malleolus of the tibia (which is the bony lump on the medial side of the ankle.)

Despite the great number of tendons dominating the landscape of the dorsal foot, one small muscle manages to peek through them. The extensor digitorum brevis muscle, as its name implies, is the shorter extensor muscle of the toes. It's so short, in fact, that the whole structure fits on top of the foot. It lies just deep to the tendons of extensor digitorum longus, and only the lateral most part of it shows on the surface.  

The extensor digitorum brevis muscle appears as a little lump just distal to the lateral malleolus of the fibula. In fact, it sometimes looks like another malleolus. But one gentle touch will tell you it's soft muscle tissue, not bone. I'm not sure its lovely shape would quite have inspired Elizabeth Barrett Browning, but it adds, in its own odd way, a depth and breadth and height to the poetry of the human foot.

Thank you to my foot models. If you have requests for upcoming posts, I'm always happy to take suggestions!