The shoulder joint has the greatest freedom of movement of all the joints in the body. It has to allow us the range we need to reach up, down, behind our back, brush our hair etc., but also be strong enough to hold our arm in position while we lift, garden, drive, play sports etc. This balance is achieved by the shape of the joint – a ball and socket, versus strong stabilising muscles, ligaments, cartilages and tendons.
• The shape of the joint
• The glenoid labrum – deepens the socket and forms a seal.
• The ligaments – act as check rains preventing and stopping the joint surfaces moving to a point where muscle damage occurs.
• The rotator cuff muscles, as their name suggests, rotate the humerus within the glenohumeral joint, but primarily act as compressors and stabilizers pulling the humerus into a stable position while the power muscles, responsible for movement like deltoid, pectoralis major, latissimus dorsi work to move and articulate our arms.
Damage to any, or a mixture of the supporting structures in the arm will result in an instability. This can be as a result of an injury or gradual wear of the tissues or development of muscular imbalance, as is often the case in younger people who have not had any trauma.
MULTI DIRECTIONAL INSTABILITY
This is defined as abnormal amount of excursion of the humeral head in the glenoid in more than one direction. This can be a congenital or an acquired enlargement of the gleno-humeral joint volume and a very redundant capsule anteriorly, inferiorly and/or posteroinferiorly.
If the instability is congenital there is usually generalised joint laxity and defects of the posterior humeral head and anterior labrum are often associated with it.
There is a much lower incidence of trauma with MDI. If it is acquired it is usually through repetitive micro-trauma from sports which leads to selective stretching out of the shoulder in comparison to other joints. Pain is generalised and often reported while carrying a weight or overhead activity.