Shoulder instability refers to when the capsule and ligaments supporting the shoulder joint are loose, enabling the bones forming the joint to move excessively on one another.
The shoulder joint is a ball and socket joint, with the ball (= head of the humerus) being rather big for the socket (= glenoid fossa of the scapula). Therefore, ligaments, capsule and muscles are responsible for the stabilisation of the joint. Anterior glenoid deficit is the main cause for shoulder instability (Bockmann et al., 2017).
This most commonly occurs following a shoulder dislocation where the top of the arm bone is ‘popped out’ of its socket. This over stretches and injures the capsule and ligaments surrounding the shoulder joint making the joint ‘unstable’.
Similarly, the capsule and ligaments supporting the shoulder joint may be overstretched and damaged if they are repetitively stressed. This can occur, for example, during throwing which stretches out these structures. lf performed repetitively, this can make the capsule and ligaments loose and the shoulder joint ‘unstable’.
Shoulder instability may also result from ligament laxity you were born with. People with this type of laxity are often referred to as ‘double jointed’ or ‘hypermobile’ and have loose ligaments and instability at most joints in the body.
Management involves strengthening the stabilising muscles around the shoulder, modifying technical or biomechanical errors that may be causing the instability and joint proprioception (balance sense).
Bockmann B. et al., 2017, Mapping of glenoid bone loss in recurrent anterior shoulder instability: is there a particular deficit pattern, Journal of Shoulder and Elbow Surgery, pii: S1058-2746(17)30189-1. doi: 10.1016/j.jse.2017.03.022