Final answer:
For the AP projection of the shoulder in internal rotation, the humeral epicondyles should be perpendicular to the IR to accurately view the relationship between the humeral head and the glenoid cavity. The medial epicondyle faces more anteroposteriorly, and the lateral epicondyle faces laterally, assisting in diagnosing pathologies of the shoulder joint and rotator cuff.
Step-by-step explanation:
The relationship of the humeral epicondyles with the image receptor (IR) for an AP projection of the shoulder in internal rotation is that the epicondyles should be positioned perpendicular to the IR. This allows for an accurate view of the humeral head in relation to the glenoid cavity as well as a proper view of the shadow of the humeral shaft. In internal rotation, the medial epicondyle of the humerus, which is larger due to its role as an attachment point for the powerful grasping muscles of the anterior forearm, faces more anteroposteriorly and the smaller lateral epicondyle faces laterally.
This internal rotation of the shoulder joint brings the greater tubercle of the humerus out of the profile and positions the lesser tubercle in profile medially. It also allows for better evaluation of the joint space and can help identify any pathologies or abnormal positioning such as fractures, impingements, or other soft tissue disorders that might be affecting the shoulder's rotator cuff, which includes the muscles and tendons like subscapularis, supraspinatus, infraspinatus, and teres minor.