The shoulder with a complex arrangement of structures working together is the most mobile joint in the human body. But this great mobility comes at the expense of stability. Atraumatic instability is a condition in which the shoulder starts to slip part way out of joint without having had a significant injury. Atraumatic instability may arise from a variety of causes. Certain shoulders may be more susceptible to atraumatic instability. Atraumatic shoulder instability is generally treated with rehabilitation using stabilizing exercises and surgery is only very rarely required.
The other type of instability is Traumatic instability of the shoulder and this is a common condition, which, especially in young patients, is associated with high recurrence rates. In this type of instability the anterior dislocation due to trauma is the most common type, corresponding to more than 90% of the cases. Posterior shoulder instability occurs less frequently, only accounting for less than 5% of traumatic shoulder dislocations. Atraumatic instability commonly results from repetitive overhead movements or congenital joint features. Atraumatic shoulder instability is not related to injury or trauma to the shoulder. It is the result of stretching of the shoulder capsule (a ligament that protects the muscles and ligaments of the shoulder) over a period of time.
Atraumatic instability could be either structural or positional instability which is muscle patterning usually seen in younger patients. Atraumatic instability is an acquired instability that could be brought on by repetitive microtrauma, which places undue stress upon the soft tissues; or rapid, forceful movements that contribute to the overall laxity of the joint. This is a common problem in athletes, particularly throwers, weight trainers, swimmers, golfers, tennis players and gymnasts, where they develop symptoms of instability due to overload and fatigue in the stabilizing muscles of the shoulder.
The two main types of atraumatic instabilities are Congenital Instability which is due to laxity of structures in the shoulder which may be present since birth and Chronic Recurrent Instability caused by repetitive extreme external rotation with the humerus abducted and extended. Congenital instability may be the result of bone anomalies, increased amount and composition of collagen and elastin, hypoplastic glenoid or increased retroversion of glenoid. The top symptoms of atraumatic shoulder instability can be feelings of looseness or instability of the shoulder or recurring (repeated) subluxation or partial dislocation. Shoulder instability can be tested with the help of an anterior apprehension test, or crank test.
Treatment strategies for atraumatic shoulder instability are devised to suit each patient’s age and lifestyle. The goal of the rehabilitation program may vary greatly based on the onset and mechanism of injury. If physical therapy does not improve shoulder stability, the doctor may recommend surgery. The goal of surgery for atraumatic shoulder instability is to tighten the ligaments that have been overstretched in order to reduce the risk of recurrent dislocations. Sometimes this type of surgery can be done arthroscopically and it is called a capsular shift. The recurrence rates of shoulder instabilities vary based on age, activity level and arm dominance and in collision sport athletes the rate of recurrence is pegged between 86-94%.