Joint Knowledge: The Shoulder

Of all the joints in the body, the shoulder joint is capable of having the most range and flexibility.

In many ways, the shoulder joint is a mechanical marvel because it can move in directions no other joint can with minimal architecture.

Despite its “minimalist” parts, the shoulder is very complex when it comes to function.

In this article, we’ve included the proper scientific names of structures of the shoulder, but we’ll use the common names–like “shoulder blade” rather than “scapula”–throughout just to make things simple! We’ve also tried to use bullet points to make skimming easier if you’re just wanting a quick overview!

Without further ado, let’s learn about the shoulder.

 

Source: National Institute Of Arthritis And Musculoskeletal And Skin Diseases (NIAMS); via Wikimedia Commons.

The 3 Bones of the Shoulder

The classic description of the shoulder joint is its ball-and-socket construction. (The other is the hip!)

It consists of two of bones:
1. The humerus (the “ball”): the long bone of the upper arm
2. The shoulder blade (the “socket”): scapula is its anatomical name

It’s this setup that gives the joint the ability to move in many different directions. Imagine throwing a ball. You reach back behind you then pull your arm forward to create force before releasing the ball. You can do that because the “ball” of the upper arm bone is able to roll in the socket.

Don’t forget about the collarbone!

When you think “shoulder,” you probably don’t think of your collarbone (or clavicle), but it’s what connects your shoulder to your axial skeleton (think spine or trunk).

In other words, without it, your shoulder and the rest of your arm would just flop down to your side.  That’s why you have to wear a sling if you break your collarbone.

When we bring the collarbone into the discussion, we should technically say “shoulder complex” because each connection between it and the shoulder blade are separate joints.

Yep, your “shoulder” is actually a combination of multiple joints all working to together.

The Joints of the Shoulder Complex

Here are the main joints of the shoulder complex and the bones involved*:
• GH joint – the ball-and-socket joint (the humerus and shoulder blade)
• AC joint – located at the top, front “corner” of your shoulder (the shoulder blade and collarbone)
• SC joint – where your collarbone connects to your sternum (just under your neck)

There are two other joints–or articulations–that aren’t like the classic joint we typically think of*:
• ST joint – the gliding surfaces between the shoulder blade with the ribcage
• SA (or SH) space – a little space between the humerus and shoulder blade that tendons pass through very near the AC joint

Connecting it all together…

You know the bones and joints of the shoulder, but without the following structures, the shoulder just wouldn’t work correctly:
• Labrum (cartilage)
• Bursa (fluid-filled, sac-like cushion)
• Ligaments (connect bone to bone)
• Tendons (attach muscle to bone)

…and, of course, the muscles!

When it comes to muscles of the shoulder complex, we have some major movers (i.e., the ones that generate the most strength, power, and movement):
• Deltoids (the “delts”)
• Pectoralis major and minor (the “pecs”)
• Latissimus dorsi (the “lats)
• Teres major

While those are undoubtedly important for a fully functioning shoulder, it’s the small, stabilizing group of muscles called rotator cuff that is just as critical but tends to get neglected and injured.

The Rotator Cuff

This group of muscles (which consists of the supraspinatus, infraspinatus, teres minor, and subscapularis) holds the “ball” in its “socket.” It’s what keeps your arm in its socket when you move it, pick things up, hold something overhead, or throw a ball.  When weak and used repetitively, the stability decreases and the risk of injury increases.

 

Take a look at our article on the top shoulder injuries, here.

 

* Here are the full names of the joints in the order they are introduced: glenohumeral joint, acromioclavicular joint, sternoclavicular, scapulothoracic joint, and subacromial (or suprahumeral) space