The Shoulder: Stability vs. Mobility

This is a guest post from Samuel Christian. Sam graduated from Curtin University with a Bachelor of Science (Physiotherapy) degree with Distinction, and is currently working full time as a mobile physiotherapist providing in-home services to those who are unable to attend a clinic. He treats a variety of conditions ranging from neck and back pain, rehabilitation following surgery, and assisting the elderly in maintaining or improving their balance, mobility and function. His sporting interests include basketball and mixed martial arts (UFC), and he is a die-hard Dallas Mavericks and Dirk Nowitzki fan.

Let us talk a little bit about one of the most complex joints in the human body – the shoulder!

The shoulder girdle consists of a number of joints that work together in synergy. These are the:

  • Acromio-Clavicular (AC) joint
  • Sterno-Clavicular (SC) joint
  • Scapulo-Thoracic joint (where your shoulder blade attaches to the rib cage)
  • Glenohumeral joint (what most people refer to as the actual “shoulder joint”)

Therefore, it is important that we consider all these different joints when we are talking about, or treating the shoulder.

Today will be an introduction to the anatomy of the glenohumeral joint – the articulation between the humerus and the scapula (shoulder blade).

If you think about the normal activities you carry out each day – such as putting on a shirt, brushing your hair or reaching up to grab something from the cupboard – you will notice that they require a fair amount of mobility in your shoulder.

The shoulder is the most mobile joint in our body and allows movement in almost every single direction. Unfortunately, like with most things in life, we can’t always have the best of everything! In this case, the stability of the shoulder is sacrificed to provide a greater mobility and this is why it is the most commonly dislocated joint in the body!

But WHY is the shoulder unstable you ask? Let’s have a look…

Our joints get their stability from a number of different factors. One of them is the congruency or the “fit” of the combining surfaces. Take a look at the hip for example; it is a ball-and-socket joint just like the shoulder. The acetabulum of the pelvis (socket) wraps nicely around the head of the femur (ball) and provides a tight “fit”. This makes the hip a stable joint but in turn reduces its mobility – unless you are a gymnast or dancer which means this doesn’t apply to you!

Now let’s take a look at the shoulder… The glenoid fossa (or cavity) of the scapula is like a small and shallow socket, and provides little support for the relatively large head of humerus – some people compare this to a golf ball sitting on top of a tee. This difference in size means that the combining surfaces do not “fit” well, making the shoulder a relatively unstable joint. However, this is what gives the shoulder its mobility! With that being said, there are other structures around the shoulder that help to make it a little bit more stable.

A piece of cartilage called the glenoid labrum lies around the margin of the glenoid fossa, making the socket slightly deeper and providing a better “fit” for the head of humerus – increasing the joint’s stability. Fun fact – the depth of the socket is greatest at the top and bottom of the glenoid fossa and most shallow at the front and back, which is why we see more shoulders dislocate in a forward or backward direction!

Secondly, the capsule that surrounds the glenohumeral joint is reinforced by ligaments at the front, which are connective tissue that attach bone together with bone and limit excessive movement of a joint. These ligaments get tight when you move your arm in certain directions, such as when you try to throw a ball from behind your head. The tightening of these ligaments protects your shoulder from “popping out” by compressing the head of humerus into the glenoid fossa and restricting excessive movement, further stabilising the joint.

Last but not least, a group of four small muscles famously known as the rotator cuff provide added stability to our shoulders. In brief summary, their job is to keep the position of the head of humerus within the glenoid fossa during movements of the arm. The rotator cuff will be discussed in more detail in a later post about the shoulder!

Tune in next time – same Bat-time, same Bat-channel!

By Samuel Christian

 

References

  • Brukner P, Khan K (2009) Clinical Sports Medicine (revised 3rd edn). NSW: McGraw-Hill Australia
  • Kalogrianitis S, McBride T (2011) Dislocations of the shoulder joint. Trauma 14: 47-56
  • Norris CM (2011) The Shoulder. In Norris CM (Ed) Managing Sports Injuries. Churchill Livingstone, pp. 334-374
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