We'll start with three parts of your shoulder joint anatomy , to more easily understand the causes of shoulder pain.
First is the little joint at the outer end of your collar bone (the Acromioclavicular or ac joint.)
Next is the joint between the shoulder blade and the arm bone (the Scapulo-Humeral or Shoulder joint .)
The third part amounts to a joint, between the muscles attaching to the upper end of the arm bone, and the bony and ligamentous arch forming the point of the shoulder.
You can feel the ac joint as a little step at the end of the collar bone. Apply firm pressure as you move your fingertip side to side slightly, near the outer end of the bone. It is about 1,1/2 inches (4cm.) before the point of the shoulder.
The acromioclavicular joint is prone to osteo-arthritis and can be torn in accidents. It will now be an obvious bump, or an obvious step down.
Your shoulder joint itself is too deep to feel, covered with the large Deltoid muscle.
It is designed for a large range of movement and is prone to dislocation as a consequence.
Not being a weight bearing joint, glenohumeral osteoarthritis is uncommon.
It does easily become stiff if not put through a full range of movement regularly, as when one's arm is broken and in a sling.
Can you put your hands over the opposite shoulders, behind your neck and behind your back? If not, some stretching would probably be in order.
The third part, called the rotator cuff and coracoacromial arch, accounts for most of the trouble people have with shoulders.
In this front view of a left shoulder, CL is the collar bone or Clavicle, R is a rib, SC is the shoulder blade or Scapula and H is the arm bone or Humerus.
C and A are parts of the shoulder blade, joined by the Coracoacromial ligament shown in black. Together these form the arch mentioned above.
S is the flattened tendon of Supraspinatus, one of the muscles which start on the shoulder blade and attach to the humerus.
This and two other muscle tendons not shown, form a hood or cowl shaped structure, the rotator cuff. This passes under the arch to attach to the arm bone at the Greater Tuberosity (T) of the Humerus.)
Supraspinatus is commonly subject to wear and damage, causing pain. During a dynamic ultrasound examination, the tissues can be seen rippling against the edge of the acromion as they pass under it. This is called impingement.
The part of the tendon involved is furthest from the blood supply coming from the muscle belly at one end and the bone at the other, in the "watershed."
My knuckles are like the greater tuberosity of the humerus, moving in and out under the coracoacromial arch represented by my other hand.
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