What is shoulder instability?
Shoulder instability is when the ball of the shoulder joint repeatedly slips out of the socket or dislocates.
Shoulder instability can either be due to an acute episode or a long-standing issue. Anterior and posterior instability commonly occur as a result of trauma. This can be an acute episode or repeated trauma over time. Multidirectional instability is often an atraumatic injury and can be associated with general hypermobility.
Anterior instability: When the shoulder is unstable or dislocates out the front of the joint.
Posterior Instability: When the shoulder dislocates out the back of the joint.
Multidirectional instability: When the shoulder is unstable in various directions.
How does it present and why does is happen?
A patient usually describes the feeling of the shoulder ‘popping out’ when describing a dislocation or instability.
When a shoulder is unstable, it can fully dislocate or it can sublux. A full dislocation means that the ball is fully out of the socket. This may spontaneously relocate or require the intervention of a medical professional to relocate the shoulder in the joint. A subluxation of the joint occurs when the shoulder partially comes out of the joint and then relocates back into position.
Acute, first time dislocation usually results as a result of a traumatic incident. An anterior dislocation most often occurs in a ‘stop’ position. The arm is out to the side an external force pushing the arm back can cause the shoulder to pop out of the front. More subtle anterior instability can be due to repeated trauma. Often, this can involve repetitive movements such as throwing.
A posterior dislocation, when the ball slips out of the back of the socket, can commonly be caused by a fall onto the arm. Instability can be due to ligamentous laxity and repetitive movements involved in overhead throwing sports, swimming and weightlifting.
Multidirectional instability presents as the feeling of the shoulder being unstable in a variety of directions. This can often be associated with general joint laxity and/or repeated trauma such as participation in sports that require large amplitudes of movement.
Anatomy of shoulder instability.
The shoulder joint is made up of the glenoid fossa (the end of the shoulder blade) and the head of the humerus (the top of the arm bone). Although the shoulder is called a ‘ball and socket joint’, the socket covers less than one third of the head of the humerus. This is great for function and allows us to move the shoulder in a wide variety of directions; however, due to the lack of coverage, the shoulder joint is highly dependent on ligaments and muscular control to maintain stability. Therefore, if there is injury to the ligaments of the shoulder, this can easily lead to laxity of the joint. Similarly, if there is dysfunction and a lack of control of the muscles of the shoulder joint, this can mean that instability is much more likely to occur.
How do we differentiate from other diagnoses?
A clinical examination should be undertaken to fully assess the range of motion and function of the shoulder. Strength testing will determine whether there is any weakness in the muscles of the shoulder. Special tests are included in the physical examination to test for instability and laxity of the shoulder joint.
Medical imaging options
After an acute episode, the shoulder is x-rayed to assess whether there is any bony damage to the ball or the socket and to out rule any major fractures. It is not uncommon for there to be some minor bony damage after an acute dislocation. A small compression fracture to the head of the humerus is called a ‘Hills Sachs Lesion’. Sometimes there can also be a small fracture of the socket or the shoulder called a ‘Bony Bankart Lesion’.
An MRI may also be included in the medical imaging to assess whether there is any damage to the ligaments, musculature and the cartilage or labrum of the shoulder.
How is it treated?
Treatment options may include surgical interventions and/or rehabilitation of the shoulder. Treatment options are very much dependent on the type of instability and the age of the patient.
An acute, first time, anterior dislocation in people younger 30 years of age can often lead to a high rate of recurrent dislocations. For this reason, a surgical opinion is usually sought. Rehabilitation for shoulder strength and function should also be undertaken. In those older than 30years of age, the recurrent dislocations tend to be less, and the choice of whether to have surgery or not is decided on an individual basis.
More subtle subluxations or mild instability is often managed conservatively. This involves strengthening of the shoulder musculature as well as sport or job specific retraining to return to full function.
The risk of recurrence after an acute, first time, dislocation of the shoulder is much lower than that of an anterior dislocation. Again, the risk is increased in the younger population. For this reason, conservative rehabilitation is often trialled first. If this fails, then a surgical opinion is sought.
Multidirectional instability if often treated with conservative management. This involves retraining of the rotator cuff and other muscles of the shoulder, as well as retraining of the movement patterns of the scapula to improve shoulder strength and biomechanics.
Brukner, P., & Khan, K. (2012). Clinical Sports Medicine (4th Ed.). North Ryde, NSW: McGraw Hill
Old, M., Ellis, R., Donaldson, K., Parmar, P. & Kersten, P. (2015), Risk factors which predispose first-time traumatic anterior shoulder dislocations to recurrent instability in adults: a systematic review and meta-analysis. British Journal of Sports Medicine, 49, 92-922
Robinson, C. M., Seah, M, Akhtar, M. A, (2011). The epidemiology, risk of recurrence and functional outcome after an acute posterior dislocation of the shoulder, The Journal of Bone and Joint Surgery, 93, 1605-3
Tannenbaum, E., & Sekiya, J. K. (2011). Evaluation and Management of Posterior Shoulder Instability, Sports Health, 3(3), 253-263