The Shoulder

The shoulder is a region and the shoulder is a joint. As a joint it is made up of more than one bone. There is the wing bone ( alias = scapula) and the humerus.

The shoulder blade (or wing bone) is technically called the scapula. It rests against the chest wall which is rounded. From that, (see left image) it does not sit with the shoulder joint pointed laterally (lateral = toward the side) but rather it points a bit forward as seen on the left. An x-ray taken front to back of the body (anterior to posterior, or 'AP') finds the scapula obliquely forward rather than flat on.

On the right we see the view perpendicular to the scapula itself. This right shoulder is viewed at an angle to the front of the body. This view shows the joint. The scapula side of the joint is called the glenoid .

Sitting in the glenoid is the rounded head of the humerus.

Arching over top of the humeral head are two projections of the scapula. The one closest to us in these views is the coracoid process. The other one (more lateral and coming from behind) is the acromion (Latin for crow's beak).

With the humerus removed we see the acromion better. Acromion starts with the letter 'A' while clavicle starts with the letter 'C'. Those are the letter names for the AC Joint - the joint between the clavicle and the acromion. The AC joint isn't much of an articulation - noting fits into something else. It is an articulation held by ligaments. This is the joint most commonly "sprung" with a blow to the side of the shoulder area (like trying to bust a door down Hollywood style - Does anybody really do that?). AC sprains are a dime a dozen in hockey from ramming the side walls.

Looking from the side, we see the glenoid joint surface better. It isn't much of a socket. It is deepened by a ring of tissue at the edges ( a labrum - like lips). Even so, a cuff of muscles and balanced forces are key in keeping the humeral head in this shallow dish.

We see the acromion extending up like a hood from the rear ( at 11 o'clock) and the coracoid protruding forward (at 1 o'clock) from the top of the glenoid area. The coracoid is the anchor for a biceps accompanying muscle (coraco-brachial) and the lesser pectoral (minor) and one part of the biceps. The space between the coracoid and acromion is spanned by a tough ligament (nearly like rope) called the coracoid acromial ligament. That ligament and any protrusions of bone at either insertion may dig (impinge) into the cuff of muscle covering the top of the humerus. It pinches when the arm is raised to the side - the impingement syndrome. If it does that enough it can wear right through the muscular cuff called the rotator cuff producing one kind of a rotator cuff rupture.

From the back of this right scapula we see that the acromion is an extension of a horizontal-ish ridge. Muscle sits above and below this ridge applied to the flat of the scapula. That muscle converges on the humeral head to form the rotator cuff.

At the lower edge of the glenoid, from the back, the triceps tendon attaches.

Overlying the scapula and attaching to that prominent ridge (called the scapula spine), the trapezius muscles suspend the scapula along with muscles which attach all along the medial (to our left) edge. Those muscles run from the vertebrae downward and laterally (outward) like this \\\\.

From the medial edge we better appreciate the curved deep surface designed to ride on the rib cage. The glenoid is pointing away from us.

The acromion is to the far right of the image. The coracoid points to the left. A very violent pull on the pectoralis minor with help from a muscle that often gets lumped in with the biceps, though it is it's own muscle, can pull off (avulse) the coracoid. Ouch.

ABDUCTION, OUTWARD ROTATION

Here the arm is in the stick'em up  posture
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The big bump on top of the humerus which just barely passes below the acromion is the greater tuberosity (another way of saying big bump). To that, attaches the rotator cuff and forward it, the biceps tendon sits in a groove. This is a tight fit and is the place and posture for impingement when it occurs. Putting a hat on can hurt.

ADDUCTION, INWARD ROTATION

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With the hand on the belly button, the arm is rotated inward and the greater tuberosity is well away from the arch of the coracoid, acromion and the spanning coraco-acromial ligament. The smaller bump is called the 'smaller bump' - but in different lingo: the lesser tuberosity.