Why is aortic knuckle absent in right upper lobe pneumonia?

why is aortic knuckle absent in right upper lobe pneumonia?

Hi, Silhouette sign is somewhat of a misnomer and in the true sense actually denotes the loss of a silhouette, thus, it is sometimes also known as loss of silhouette sign or loss of outline sign.

The differential attenuation of x-ray photons by two adjacent structures defines the silhouette, e.g. heart borders against the adjacent lung segments, and it is the pathological loss of this differentiation, that the silhouette sign refers to. In short, it denotes that a mediastinal border can only be obscured by pathology which is in direct anatomical contact.

Aortic knuckle silhouette sign is seen in left upper lobe pathology. Here, it is not properly appreciated due to inadequate exposure. Thank you.