Shoulder Instability

Shoulder Instability Injury

There are 2 general types of glenohumeral instability described as it relates to the direction in which the shoulder is unstable.

Multidirectional and Unidirectional Shoulder Instability

Multidirectional Instability (MDI) is named based on the instability pattern being in multiple directions- anterior, inferior, and posterior. This type of instability pattern is often the result of global ligament laxity and is often congenital as opposed to being the result of a single traumatic event. This pattern of instability can be seen in individuals who are ligamentously lax – ‘double jointed’ or who have subject their shoulder to high frequency stress over time and have as a result stretched out the ligaments in the shoulder causing them to be lax.

Unidirectional Instability. This name is based on the instability pattern being overwhelmingly in a single direction usually anterior or posterior instability. This type of instability pattern is often related to a focal failure of an anatomic feature important for shoulder stability and is often related to a traumatic event. For instance as the result of a fall the shoulder might dislocate. This often happens because the fall generates enough tension in the ligaments as a result of force to tear the ligament or labrum from the skeleton. After this happens instability in the direction of where the failure occurred is likely.

A common pattern of failure is at the anterior labrum where the anterior inferior glenohumeral ligament attaches. This pattern of injury is called a Bankart Lesion. If the injury is on the posterior glenoid it is called a Posterior Bankart Lesion or Reverse Bankart Lesion. If the failure occurs at the glenoid labrum and in addition a piece of bone from the glenoid accompanies the labrum and ligament as part of the injury pattern this is called a Bony Bankart Lesion.

Physiology

The glenohumeral joint is stabilized in 2 ways- passive and dynamic stabilizers. The passive stabilizers are those structures that are static anatomic features with no capacity to generate force and include the glenoid labrum, the inferior glenohumeral ligament complex both anterior and posterior, the middle and superior glenohumeral ligament.

The dynamic stabilizers are those structures that have the ability to generate force and include the rotator cuff muscles as well as the long head biceps tendon.

The congruity of the glenohumeral articulation along with the labrum, ligaments, biceps and rotator cuff functions to maintain the center of rotation of the humeral head on the glenoid whereby it maintains stability. The rotator cuff muscles surround the glenohumeral joint and improve stability by generating force through muscle contraction and it does this by drawing the humeral head into the glenoid cavity creating what is called concavity-compression.

The biceps tendon is situated at the superior pole of the glenohumeral articulation and functions to depress the humeral head resisting the natural tendency of the humeral head to want to ride high on the glenoid.

This natural tendency for the humeral head to ride superiorly is based on the force vector that the deltoid muscle exerts with the deltoid muscle being the largest and strongest of shoulder muscles and thereby being the primary force generator during shoulder activity.

Shoulder Instability Treatment

To learn more about the treatment of shoulder instability, visit our shoulder instability treatment page.