What is the shoulder?
The shoulder joint or the glenohumeral joint is the most mobile joint in the body, allowing movement in multiple planes. The shoulder joint is made up of the humerus (arm bone) and the glenoid (socket) of the shoulder blade (scapula). The shoulder joint is closely connected with other joints including the acromioclavicular (collar bone and shoulder blade), sternoclavicular (collar bone and sternum) and scapulothoracic (shoulder blade and upper back) joints. All of these joints work together to optimise the movement of the entire shoulder girdle and upper limb.
The labrum of the glenoid creates more depth to the shoulder joint, to allow for more contact area of the humerus during movement. Other passive structures including the joint capsule, ligaments, bursae and more, provide stability and allow for fluid movement of the shoulder joint.
Shoulder and surrounding musculature
Being such a mobile joint, the shoulder relies heavily on the surrounding musculature to provide stability and support. There is a small group of muscles, collectively called the rotator cuff that act to hold the shoulder joint in the optimal position and provide stability to the shoulder, especially with weight bearing and overhead movements. Other important muscles of the shoulder girdle include the biceps, pectorals, teres major, lattismus dorsi, rhomboids, trapezius and more.
So, why is the shoulder complex?
While the shoulder joint is extremely mobile, it is often considered one of the least stable joints of the body and is more prone to injuries like dislocations. The glenoid or the shoulder socket is so shallow that only a small amount of the humerus (about 1/3) sits in the socket - it has been compared to a golf ball on a tee. For this reason, the shoulder joint relies on a number of other structures to provide it with stability and control. The shoulder joint also relies on the shoulder blade (scapula) for optimal movement. Every time we elevate our arm, our shoulder blade also moves upward. Without this upward motion of the shoulder blade, the shoulder can feel restricted or uncomfortable, especially in overhead positions.
How is my shoulder related to my spine?
The shoulder works closely with all of its surrounding joints, including the neck and the upper back. If there is restriction in movement in one of these joints, it can have an impact on the other. Here is a practical example: Sitting slumped in a chair, try to lift up your arm. Now, sit up really straight and lift your arm again. Did you notice that with a longer spine you were able to lift your arm up higher? This is just one example of how the shoulder depends on the upper back for mobility.
The neck can also contribute to referred pain in the area of the shoulder. There are 8 nerves exiting from both sides of the neck, all of which innervate different muscles and supply sensation to different parts of the upper limb, otherwise known as dermatomes. If one of these nerves are irritated or inflamed, this can contribute to referred pain in the corresponding dermatome.
Take home messages
- The shoulder is extremely mobile allowing for movement in all planes of motion
- The shoulder joint works with surrounding joints including the neck and upper back
- If there is restricted movement of the spine this can have an impact on the function/ability of our shoulder
- If you have shoulder pain, consider that your neck/upper back could be involved