THE SHOULDER
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The best way for us to learn about the upper extremity is to begin at the very
beginning, right up here. We'll start by looking at the bones of the shoulder region:
the clavicle, the scapula and the humerus. Then we'll look at the joints that let them
move, and the muscles, which make them move. Lastly we'll look at the principal
blood vessels and nerves in the region. First, the bones.
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BONES, JOINTS AND LIGAMENTS
The bones that connect the upper extremity to the trunk are the clavicle, or collar
bone, and the scapula, or shoulder blade. The parts of them that we can feel
beneath beneath the skin can be seen in this dissection: here's the spine of the
scapula, here's the clavicle. In the dry skeleton, here's the clavicle, here's the
scapula.
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The proximal long bone of the upper extremity, the humerus articulates with the
scapula at the shoulder joint. The scapula and clavicle articulate with the bones of
the thorax at one point only, here, at the sternoclavicular joint.
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The lateral end of the clavicle articulates with this projection on the scapula, the
acromion, forming the acromio-clavicular joint. Apart from this one very movable
bony linkage, the scapula is held onto the body entirely by muscles. It's thus
capable of a wide range of movement, upward and downward, and also forward and
backward around the chest wall.
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Looking at the clavicle from above we can see that it's slightly S-shaped, with a
forward curve to its medial half. At its medial end this large joint surface articulates
with the sternum. At the lateral end this smaller surface articulates with the scapula.
On the underside, massive ligaments are attached, here laterally and here medially.
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The scapula is a much more complicated bone. The flat part, or blade, is roughly
triangular with an upper border, a lateral border, and a medial border. The blade
isn't really flat, it's a little curved to fit the curve of the chest wall.
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This smooth concave surface is the glenoid fossa. It's the articular surface for the
shoulder joint. Above and below the glenoid fossa are the supraglenoid tubercle, and
the infraglenoid tubercle, where two tendons are attached as we'll see.
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A prominent bony ridge, the spine of the scapula, arises from the dorsal surface, and
divides it into the supraspinous fossa, and the infraspinous fossa. At its lateral end
the spine gives rise to this flat, angulated projection, the acromion, which stands
completely clear of the bone. The clavicle articulates with the scapula here, at the
tip of the acromion. This other projection, looking like a bent finger, is the coracoid
process.
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Here's how the clavicle and the scapula look in the living body. Round the edge of
the shallow glenoid fossa, a rim of fibrocartiilage, the glenoid labrum, makes the
socket of the shoulder joint both wider and deeper. This flat ligament, the coracoacromial
ligament, joins the coracoid process to the acromion. Here's the acromioACLAND'S
DVD ATLAS OF HUMAN ANATOMY VOL 1 PART 1 2
clavicular joint. Two strong ligaments, the trapezoid in front and the conoid behind,
fix the underside of the clavicle to the coracoid process. There's very little movement
at the acromio-clavicular joint.
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As we've seen, the medial end of the clavicle articulates with the sternum at the
sterno-clavicular joint. Strong ligaments between the clavicle and the sternum and
between the clavicle and the underlying first rib, keep the two bones together but
permit an impresssive range of motion: up and down, and backward and forward.
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Now let's see how the clavicle and the scapula move, relative to the trunk. Upward
movement of the scapula is called elevation; downward movement is called
depression. Forward movement around the trunk is called protraction; the opposite
movement is retraction. This movement is called upward rotation. The opposite
movement is downward rotation. In real life these movements of the scapula are
often combined.
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The range of motion of the scapula provides fully one third of the total range of
motion of the humerus, relative to the body, sometimes more. Without this
movement of the scapula, we'd only be able to abduct our arm to here. That's as far
as the shoulder joint goes, before bone hits bone. It's scapular movement that lets
us get all the way to here.
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Now let's look at the shoulder joint. To understand the shoulder joint, let's get
acquainted with the upper half of the humerus.
This is the head of the humerus. The articular surface is half of a sphere. On the
anterior aspect is a well marked groove known as the bicipital groove, because the
tendon of the long head of the biceps runs in it. At the proximal end of the groove
are the lesser tubercle, and the greater tubercle. Because it's between two
tubercles, the bicipital groove is also known as the inter-tubercular groove. Down
here on the lateral aspect of the humerus, almost halfway down the bone, is a rough
spot, the deltoid tubersosity.
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Here's the shoulder joint, also known as the gleno-humeral joint. This loose sleeve
of tissue which encloses the joint is the joint capsule. The capsule doesn't hold the
bones together, it's quite a weak structure. What it does is to permit movement.
The structures which hold the two bones together are muscles, as we'll see. Here's
the tendon of one of those muscles.
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Let's look at the movements that can occur at the shoulder joint. Movement forward
and upward is called flexion. Movement downward and backward is called extension.
Movement away from the side of the body is ab-duction. The opposite movement is
ad-duction. Rotation which moves the front of the arm towards the body, is internal
rotation. Rotation the other way is external rotation.
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Now that we've taken a look at the bones, joints and ligaments, let's spend about a
minute reviewing what we've seen so far.
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REVIEW
Here's the clavicle, for an easy start. On the scapula here's the blade, the glenoid
fossa, the supraglenoid, and infraglenoid tubercles, the spine of the scapula, the
supraspinous and infraspinous fossa the acromion, and the coracoid process.
ACLAND'S DVD ATLAS OF HUMAN ANATOMY VOL 1 PART 1 3
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Here's the proximal humerus, with the head, the greater tubercle and lesser
tubercle, the bicipital groove, and the deltoid tuberosity.
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Here's the sterno-clavicular joint, and here's the acromio-clavicular joint, with the
conoid ligament and the trapezoid ligament.
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On the scapula, here's the glenoid labrum, and the coraco-acromial ligament. Lastly,
here's the capsule of the shoulder joint
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MUSCLES
Now let's move on to look at the muscles. We'll build our understanding pretty much
from the inside to the outside. First we'll look at the deepest muscles, the ones that
go from the scapula to the humerus. Then we'll look at the ones that go from the
trunk to the scapula, and lastly we'll look at the big three muscles on the outside,
which cover up almost all the others.