Shoulder instability develops in two different
ways: traumatic (injury related) onset or atraumatic onset.
Understanding the differences is essential in choosing the best course
of treatment. Generally speaking, traumatic onset instability
begins when an injury causes a shoulder to develop recurrent (repeated)
dislocations. The patient with atraumatic instability has
general laxity (looseness) in the joint that eventually
causes the shoulder to become unstable.
Traumatic shoulder instability is most common in young, athletic people.
The younger and more active the patient is when the first dislocation
occurs, the more likely it is that recurrent instability will develop.
For example, if the first dislocation occurs during the teenage years,
there is a 70% chance that recurrent instability will develop. However,
people over 40 with a first dislocation have less than a 10% risk
of developing chronic instability. Treatment strategies should
be designed to suit each patients age and lifestyle.
The shoulder is the most mobile joint in the human body, with a complex
arrangement of structures working together to provide the movement necessary
for daily life. Unfortunately, this great mobility comes at the expense
of stability. Four bones and a network of soft tissue structures
(ligaments, tendons, and muscles), work together to produce shoulder movement.
They interact to keep the joint in place while it moves through extreme
ranges of motion. Each of these structures makes an important contribution
to shoulder movement and stability. Certain work or sports activities
can put great demands upon the shoulder, and injury can occur when the
limits of movement are exceeded and/or the individual structures are overloaded.
What is traumatic shoulder instability?
Traumatic shoulder instability begins with a first dislocation that injures
the supporting ligaments of the shoulder. The glenoid (the
socket of the shoulder) is a relatively flat surface that is deepened
slightly by the labrum, a cartilage cup that surrounds part
of the head of the humerus. The labrum acts as a bumper
to keep the humeral head firmly in place in the glenoid. More importantly,
the labrum is the attachment point for ligaments stabilizing the shoulder.
When the labrum is torn from the glenoid, the support of these ligaments
is lost. The development of recurrent instability depends upon the type
and amount of damage that is done to the labrum and the supporting ligaments.
The most common dislocation that leads to traumatic instability is in
the anterior (forward) and inferior (downward)
direction. A fall on an outstretched arm that is forced overhead, a direct
blow on the shoulder, or a forced external rotation of the arm are frequent
causes of this type of dislocation. Much less common is a posterior
(backward) dislocation, which is usually related to a seizure disorder
or electrocution, events in which the muscular forces of the shoulder
cause the dislocation.