What type of injury would cause pain with external rotation?

Ligaments & capsule. Pain with external rotation of a joint can be casued by a sprain of the normal structures that control or limit the specific external rotational movement. In the shoulder the static stabilizers that most limit exteranl rotation are the capsule and anteroinferior and middle glenohumeral ligaments. Injured joint surface (cartilage) can also cause pain as the injured portion rotates across the joint.