The shoulder is a ball & socket type of a joint. The top of the arm bone, the Humeral head ,contributes the ball & socket is formed by the shoulder blade .(the scapular glenoid).The importance of the shoulder joint is that it takes our hand to different position & to reach objects in space . Without a mobile shoulder the hand would be severely dysfunctional.
The shoulder is the most mobile joint in the body, but mobility come at the cost of stability.
Its function & stability is mainly dependent on the following factors:
- Various muscles that other attached to the scapula (shoulder blade) or the ball humeral head. These muscle group act like the suspensory ropes of a hanging bridge. The tone & tension in these muscles in responsible for the joint stability and efficient functioning.
- The Second most important structure in the stability is the labrum. Unlike the hip joint where the socket is deep & ball is conforming, here the socket or the glenoid is shallow, it can be compared to a saucer. The ball , humeral head is almost 3 times the size of this glenoid ‘ saucer’. Hence, to make the saucer deeper, mother nature has built a structure around the saucer like a Dynamic compound wall called as labrum. The labrum deepens the socket & also give a negative suction effect to the ball & socket joint.
*The labrum is a soft tissue structure similar to the knee meniscus.
- The third factor or structure is the layer surrounding the ball & socket joint it is called the capsule. It is like a coat covering the joint. This layer is strengthened at places by special ligaments.
These fortification of the capsule are mainly present in the lower (inferior) part of the socket, where the shoulder joint is most prone for dislocation.
The first fortifications is the IGHL or the inferior glenhumeral ligament with its two arms (anterior & posterior) supports the glenoid & labrum from below & prevents the humeral head from dislocation.
Is can be compared to the two arm of Atlas supporting the Earth.
The IGHl is one of most crucial structure in the stability of the shoulder joint.
The capsule in the front part is strengthened by the MGHL or the middle glenhumeral ligament.
In the event of the shoulder joint dislocating, i.e. the ball coming out of the socket there is a 95% possibility that the ball will come out either from the front (anterior) or the lower (interior) part of the socket.
Hence to prevent this, nature has given these anterior (MGHL) & lower-inferior(IGHL) ligaments as check reins !!