The shoulder is a quite a unique joint in comparison to the rest of the joints in the human body. For example, 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 simply the joint where the head of your upper arm (the humerus
) meets your shoulder blade (the scapula
). (shoulder joint comprises of 3 joint. glenohumeral joint -Where humerus meets the glenoid cavity of the scapula, acromionclavicular joint – collar bone to acromion of scapula, sternoclavicular joint. All contributes to movements of shoulder in varying degree. The generic term shoulder joint usually refers to the glenohumeral joint
). The collar bone does not form part of the joint but instead 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 is different from the other big joints are
- The groove that the head of the humerus bone sits (glenoid cavity) in is extremely shallow (compared to say the hip joint).
- And compared to other joints, the joint is not held together by a set of ligaments found in other joint like the knee joint (anterior-cruciate and posterior-cruciate ligaments and the medial and lateral collateral ligaments). If the shoulder joint was held together by this
If the groove is very shallow and the joint is not bound by a set of strong tough ligaments, what holds up the shoulder joint that bears a lot of physical stress daily? 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 on the humerus head. The gleniod-labrum essentially deepens the sockets by acting like a 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: University of Washington, School of Medicine Why is it different?
All this leads to the question, why is this joint different from the hip or knee joint? Why do it differently in the first place? The answer is the degree of free the shoulder enjoys and the roles that freedom plays in the activities we do daily. If the glenoid cavity was deeper or the joint was held together with a set of strong ligaments, the amount of freedom of movement we now enjoyed would be severely limited.