Final answer:
The presence of a fat pad sign on an elbow x-ray indicates an intra-articular fracture that causes displacement of the normal fat pads due to accumulation of blood or fluid within the joint. It emphasizes the possibility of a subtle fracture that might not be visible but suggests injury to the articular components of the joint.
Step-by-step explanation:
A fat pad sign on a lateral x-ray of the elbow is a significant finding that suggests an intra-articular fracture, where a part of the bone within the joint has been broken. It is detected as a displacement of the normal fat pads around the joint. This displacement occurs because when there is a fracture, blood or fluid may accumulate in the joint (a hemarthrosis), displacing the fat pads. In a healthy elbow, these fat pads are typically not visible on an x-ray, but if they are elevated, it indicates pathology.
The articular capsule of the elbow, which is thin on its anterior and posterior aspects but thickened at the margins, is associated with stabilizing ligaments like the ulnar collateral ligament on the medial side and radial collateral ligament on the lateral side. The lateral epicondyle of the humerus is found on the lateral side of the elbow, playing a role in muscle attachment, susceptible to injury from repetitive movements such as throwing. Meanwhile, the proximal radioulnar joint supported by the annular ligament allows for rotation which is crucial for forearm movements.