In the shoulder, posterior instability requires avoiding which combination of motions?

In the shoulder, posterior instability requires avoiding which combination of motions?

• A. Internal rotation with horizontal adduction and flexion
• B. External rotation with abduction
• C. Abduction with adduction
• D. Neutral rotation with flexion

Answer: (A)

Posterior instability is most provoked when the arm is placed in a position that drives the humeral head backward against the glenoid. The specific combination that does this is when the shoulder is flexed, brought across the body (horizontal adduction), and internally rotated. In this position the humeral head translates posteriorly and presses against the posterior capsule and posterior labrum, loading those structures and reproducing instability symptoms. Movements that place the head forward or away from the posterior structures, such as external rotation with abduction, tend to tighten the front of the joint and are less provocative for posterior instability. Likewise, flexion with neutral rotation or adduction without the same internal rotation angle doesn’t load the posterior capsule as much, so they’re less likely to provoke posterior instability.

Posterior instability is most provoked when the arm is placed in a position that drives the humeral head backward against the glenoid. The specific combination that does this is when the shoulder is flexed, brought across the body (horizontal adduction), and internally rotated. In this position the humeral head translates posteriorly and presses against the posterior capsule and posterior labrum, loading those structures and reproducing instability symptoms.

Movements that place the head forward or away from the posterior structures, such as external rotation with abduction, tend to tighten the front of the joint and are less provocative for posterior instability. Likewise, flexion with neutral rotation or adduction without the same internal rotation angle doesn’t load the posterior capsule as much, so they’re less likely to provoke posterior instability.