Final answer:
The differentiation of the AP oblique projections of the elbow in medial and lateral rotation is accomplished by identifying key anatomical structures, such as the medial epicondyle of the humerus, the ulnar collateral ligament, and the olecranon process.
Step-by-step explanation:
The anatomy used to differentiate the AP oblique projections of the elbow in medial and lateral rotation includes the medial epicondyle of the humerus, the ulnar collateral ligament, and the olecranon process. The medial epicondyle is a prominent bony projection on the medial side of the distal humerus, while the ulnar collateral ligament is a strong intrinsic ligament on the medial side of the elbow joint, extending from the medial epicondyle of the humerus to the medial ulna. The olecranon process is part of the proximal ulna and serves as an attachment point for the triceps brachii muscle, which is easily identifiable in lateral rotation on the AP oblique projection.
In lateral rotation, the olecranon process of the ulna moves into the olecranon fossa of the humerus, making it more prominent in the X-ray image. Conversely, in medial rotation the coronoid process becomes more distinct. The different positions of these bony landmarks help radiologists and medical professionals to assess the elbow in various projections and rotations for diagnosis and treatment planning.