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The Upper Limb
Written by Tim Sheppard MBBS BSc. Last updated 9/11/10

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What is the upper limb?

The upper limb is the part of the body which everybody knows as the arm. It is called the upper limb because it is rather unhelpfully split into the arm and the forearm - which, as I'm sure you'll agree, can become very complicated very quickly. The limb rests beside the thorax and the abdomen, and the simple truth is that the upper limb isn't simple. If you want it to be simple, accept it is the arm, and be happy. If you want to understand it further, I'm afraid you have to get into some detail. You see, it's not an issue of understanding as much as knowing, and there are lots of things to remember.

A good place to start with the upper limb is the organisation of the whole thing. We'll work with the left upper limb, although it makes absolutely no difference - your limbs are essentially symmetrical. You can see differences from the outside, but everything contained within the upper limb - in a spirit of unusual simplicity - is named the same on each.

The left upper limb, then, is on the left of the thorax and the abdomen, and hangs with the palm of the hand facing forward in the anatomical position. It is split into four major sections - the shoulder, arm, forearm, and hand. These different sections are rather helpfully divided by the joints in the upper limb, and the bones involved. The shoulder is basically made up of the scapula and clavicle, while the arm is built around the humerus, and the forearm is made up of the radius and the ulna. The hand is slightly more complicated, made up of the carpal bones, metacarpals and phalanges.

In each of these sections, the muscles attached exist to perform the wide range of operations capable by the limb. The shoulder, elbow, wrist and finger joints each have flexors and extensors, but the shoulder - capable of many other movements - has other muscles to make these possible. The elbow joint is similarly capable of more imaginative movements, and the muscles reflect this.

For humans, the wide range of movements capable of the upper limb is what makes us as developed as we are. Despite what people may argue about the level of sophistication obtained by humans in today's society, I think that what we are able to do is amazing!

Further exploration of this section describes in greater detail the muscles of the upper limb, as well as the nerves which supply it, and the arteries and veins which run through it.

What is the clavicle?

Put most simply, the clavicle is the collar bone. It is the bone which you can sometimes see coming from your shoulder down towards your chest - a stretched 'S' of bone that is often broken, and can be very painful!

The clavicle forms part of something called the pectoral girdle, a ring of bone around the top of the body which the upper limbs hang from. It is an incomplete ring, because it doesn't connect at the back, but is formed by the clavicles, scapulae and the sternum. The ring, then, is formed by the scapulae at the back (which don't join), the clavicles, and then the sternum at the front.

The clavicle joins to the manubrium sternii (that is, the head of the sternum) with a synovial joint - so there is a bag of synovial fluid between the two bones, and each of the bones has a coating of fibrocartilage around the join to try and stop damage to the bone.

It joins to the manubrium just above the place where the 1st rib (via the 1st costal cartilage) joins the manubrium, and so to secure it, the clavicle has several attachments to the rib and its costal cartilage. The most obvious is the subclavius, a muscle which attaches to the bottom of the clavicle, and joins to the first rib close to the sternum. This is called an accessory muscle of respiration, because when it contracts it can elevate the first rib, helping to increase the volume of the thorax and therefore helping to breathe in. There is also a ligament (the costoclavicular ligament) which goes from the clavicle to the costal cartilage; and an articular disc made of fibrocartilage which crosses the joint and attaches to the costal cartilage.

The image on the left looks from the back at the acromioclavicular joint, where the clavicle joins the acromion of the scapula. It is another synovial joint, so it has the synovial fluid bound in a synovial membrane (collectively the synovium) as in the sternoclavicular joint above. Again, the ends of the bones are capped in fibrocartilage, and there is a capsule, but this time there is no articular disc. To secure the joint, there is a coracoclavicular ligament, joining the clavicle to the coracoid process, but if this fails, the clavicle can slip over the top of the acromion, causing significant pain and injury.

Due to the symmetry of the clavicles, these two joints are all that are required to produce this ring. The right scapula joins to the right clavicle with the acromioclavicular joint, and the right clavicle joins to the manubrium sternii on the right side. On the left side the left left clavicle joins with another sternoclavicular joint, and then meets the left scapula at the left acromioclavicular joint. And so the ring is formed.

What is the scapula?

The scapula is the name given to your shoulder blade - the bone which forms the basis of the shoulder. It is obviously a very important bone, because it forms an important part of the frame which the upper limb hangs from. There is an incomplete ring of bone which forms around the neck, and this ring is the scafold for the bones of the arm (the humerus) and the forearm (radius and ulna).

The animation on the left shows a representation of a left scapula, rotating freely. As the scapula rotates, you can see several important features of the bone. An important thing to remember about the scapula is that its anterior surface faces into the body. Because the scapula is found at the back, its anterior surface faces into the thorax, while its posterior surface faces outwards.

The image to the right represents the anterior aspect (or surface) of the scapula. Several important features are lablelled: the scapula notch, which in humans has a ligament over the top of it, forming a hole through which things can pass; the acromion process, which forms the outermost, highest point of the scapula; the coracoid (meaning raven's claw) process, which is where several muscles and ligaments attach; the subscapular fossa, an indent in the bone which is useful in the gleno-humeral joint; and the inferior angle, which is, as it suggests, the lowest part of the bone.

The lateral aspect of the scapula (i.e. the part of the left scapula you would see if you were looking at it from the left hand side) has similar features labelled to that of the anterior aspect, but with an important addition: the glenoid fossa. I suppose I always think of this like a baseball glove - it's very shallow, but still it is the part where the scapula meets the head of the humerus. It is because of the glenoid that the joint is called the gleno-humeral joint.

The glenoid (or glenoid cavity) has a rim around the edge called the glenoid labrum, which is made out of fibrocartilage. Because the glenoid is such a shallow indent, it doesn't hold the humerus very well (in the same way that a shallow bowl won't hold a football very well). The glenoid labrum, although hardly that deep itself, just gives the gleno-humeral joint that extra bit of stability - which, combined with the other structures in place and the rotator cuff mechanism, keeps the shoulder joint intact.

The posterior aspect of the scapula has a few more features to note - again we have the acromion and scapula notch which we have met before. But this time we have the spine, which is named because it looks the spine of the bone, and which forms a helpful landmark which we can name everything else by. The infraspinous fossa is clearly below the spine, and the supraspinous fossa is obviously above it. These two become important when we deal with the shoulder joint.

The remainder of the labels are simply explaining which border is which - and they're all relatively obvious if you know which way round the scapula is.

The important thing to remember about the scapula is its orientation, and this becomes easy if you remember that the coracoid process, the acromion and the glenoid all have to face outwards, as they all relate to the gleno-humeral joint.

What is the gleno-humeral joint?

The gleno-humeral joint is just a posh name for the shoulder joint, and it involves the interaction between the humerus and the scapula. Because the part of the scapula central to the joint is called the glenoid, it makes logical sense to call it the gleno-humeral joint, even if it makes it more difficult to remember!

The image on the right builds up the muscles of the left shoulder. Level 1 shows the bones and their orientation, showing where anterior, posterior, superior, inferior, medial and lateral are.

Level 2 labels two very important parts of the scapula - the coracoid process (so named because it is supposed to look like a crow's claw) anteriorly and the spine posteriorly.

More parts of the scapula are named on Level 3 - the indents or fossae found on the anterior and posterior surfaces. The subscapula fossa is found on the front. The supraspinous and infraspinous fossae are found above and below the spine of the scapula respectively - hence their names.

From level 6 we start to build up the muscles. Starting with teres (meaning plump) major, which causes the arm to rotate medially, we move on to teres minor (7), supraspinatus (9) and infraspinatus (8), and subscapularis (10). These last four are all part of the rotator cuff mechanism. While teres minor is named due to its plump appearance, the other three are all named according to the fossae they arise from.

Deltoid, the muscles which gives the shoulder its rounded shape, can be found on level 11, with 12 there just to see what everything looks like when together. The reason deltoid is named as such is that is is, in a sense, triangular shaped, like the great letter delta (Δ). It has two attachments to the scapula, and then joins the humerus at a special bump called the deltoid tuberosity.

If we look at the joint itself, there are some important - but complicated - features. The gleno-humeral joint is a synovial joint, which means the two bones have a bag of fluid called synovial fluid packaged between them. This stops the bones rubbing against each other, thereby preventing damage to them. This particular joint has other features to perform the same purpose - there is a bursa, and the bones have a hyaline cartilage coat. This will be more obvious in a moment.

The image on the left shows the shoulder joint, but as though we've cut through part of it. A transverse section of the acromion (part of the scapula) appears completely independent of the scapula, even though in real life it is attached. Imagine we're taken a slice of the shoulder, and are looking at it here.

Level 2 orientates you so that you can tell we are looking at the left shoulder again, while level 3 confirms the bones involves - the scapula, the acromion and the humerus. Level 4 points out some important features of the humerus, and then we look at the muscles on level 5. The new muscle we meet here is biceps brachii, which is called biceps because it has two heads, and brachii because it is in the arm. The image focusses on the long head, because it inserts into the joint, as we'll see in a minute.

Beneath the acromion is the sub-acromial bursa (6), which is like another bag of fluid to cushion the joint and prevent rubbing of the bones.

Then we look at the joint itself. Level 7 labels the capsule which surrounds the synovial joint. Then we go further down to look at the synovial fluid, encapsulated in a synovial membrane (8). You can't actually see the membrane, but basically the fluid has two coats - the synovial membrane, then the capsule on the outside. Level 9 shows that the long head of biceps brachii actually passes right over to the glenoid fossa, but it isn't actually submerged in synovial fluid - the synovial membrane forms a kind of sheath or tube around the muscle, allowing it to pass through the joint without actually touching any of the fluid.

Finally level 10 shows the glenoid fossa and the head of the humerus, each coated in hyaline cartilage (represented in blue) to try and prevent damage to the bone.

What are the rotator cuff muscles?

The rotator cuff mechanism may sound like some complicated or abstract anatomical name for something which you have no hope of understanding. In actual fact they are essentially very simple - and it's all explained in the name. They form a cuff, and they rotate. Well, that's almost true anyway.

The image on the right shows the rotator cuff muscles around the head of the humerus. We're looking at the right humerus, and we are on the lateral side, facing medially. The humerus is of course made of solid bone, but for the purposes of understanding the image, it has been made to appear slightly transparent, and you can just about make out the glenoid fossa on the other side, with the long head of biceps brachii attaching, as seen in the section on the gleno-humeral joint above.

Then we have four muscles surrounding the head - supraspinatus, infraspinatus, teres minor, and subscapularis. The latter three can all rotate the humerus when they rotate, and collectively they form a kind of cuff around the humerus, helping it to stay in place. As previously discussed, the glenoid fossa is quite shallow, and to keep the joint in place, there are several added features - notably the glenoid labrum and this mechanism.

Unfortunately the shaft of the humerus, extending below the head, prevents muscles attaching all the way around the bone. This leads to a weakness in the rotator cuff mechanism, and it means that if ever there is a dislocation of this particular joint, it usually happens with the humerus slipping downwards, where there are no muscles to stop it.

Further Reading