What is a common feature of avascular necrosis visible on X-ray in its late stages?

In the late stages of avascular necrosis, subchondral collapse is a notable feature visible on X-ray. This occurs as the blood supply to the bone is compromised, leading to the weakening of the bone structure beneath the cartilage, specifically in the subchondral region. As the bone loses its integrity and support, it can no longer withstand normal physiological loads, resulting in microfractures and eventual collapse.

The presence of subchondral collapse indicates significant deterioration of the bone, which can lead to joint deformities and pain. It is often a critical point in the disease process that signals the transition from potential to irreversible damage, making it an important feature for diagnostic purposes.

In contrast, other features mentioned in the question, such as radioopacity, increased vascularity, and joint effusion, do not typically represent the characteristic changes seen in the later stages of avascular necrosis. Radioopacity may suggest bone sclerosis, but this does not directly indicate avascular necrosis. Increased vascularity is contrary to the condition, as avascular necrosis involves a lack of blood supply. Joint effusion might be present due to secondary phenomena but isn't a defining feature of the necrosis itself.