Shoulder Joint: A Deeper Look Into It

The shoulder is quite a unique joint in comparison to the rest of the joints in the human body. But what holds it up? Let’s take a deeper look. Deep beneath the skin and muscles, things turn out to be quite surprising.

The Shoulder Joint

The shoulder joint is the joint where the head of your upper arm (the humerus) meets your shoulder blade (the scapula). It comprises 3 joints:

1. Glenohumeral joint -where the humerus meets the glenoid cavity of the scapula
2. Acromioclavicular joint – collar bone to the acromion of the scapula
3. Sternoclavicular joint

All of them contribute to movements of the shoulder to varying degrees. The generic term shoulder joint usually refers to the glenohumeral joint). The collar bone does not form part of the joint. Instead, it joins the shoulder blade and the sternum, the large hard bony portion in the centre of your chest.

Two things about the shoulder joint that are different from the other big joints are

1. The groove that the head of the humerus bone sits in is an extremely shallow groove (glenoid cavity) (as compared to say the hip joint).
2. The joint is not held together by a set of ligaments found in other joints, such as the knee joint (anterior-cruciate and posterior-cruciate ligaments and the medial and lateral collateral ligaments).

How Is The Shoulder Joint Held?

If the groove is very shallow, and the shoulder joint is not bound by a set of strong ligaments, what holds it?

The answer is suction power.

The glenoid-labrum (a fibrous cartilage-like material) and joint fluid work together to create a suction-cup holding effect. This effect is on the humerus head. The glenoid labrum essentially deepens the sockets by acting by skirting around the shallower glenoid cavity. The joint fluid through adhesive-and-cohesive forces holds the joint together even under great stress. This is akin to ‘wetting’ the suction-cup hook to stick better to a wall. (Watch the video).

Source: the University of Washington, School of Medicine

Why Is It Different?

All this leads to the question, why is the shoulder joint different from the hip or knee joint? Why do it differently in the first place?

The answer lies in the degree of freedom the shoulder enjoys, and the roles that freedom plays in the activities we do daily. If the glenoid cavity is deeper, or if the joint was held together with a set of strong ligaments, the amount of freedom of movement we now enjoy will be severely limited.

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