What is Dawbarn's sign and how is it used to diagnose subacromial bursitis?

Here's how it works:

1. Pain with adduction: When the arm is adducted (held close to the body), pressure applied to the deltoid muscle just below the acromion (the bony prominence at the top of the shoulder) will elicit pain if the subacromial bursa is inflamed.
2. Pain disappears with abduction: If the arm is then abducted (lifted away from the body), the inflamed subacromial bursa slips underneath the acromion process. When pressure is applied to the same point on the deltoid, the pain will no longer be present because the bursa is no longer directly accessible to palpation.

This disappearance of pain upon abduction is a positive Dawbarn's sign, indicating that the pain originates from the subacromial bursa.