Final answer:
To distinguish between DISI and VISI deformities on lateral wrist films, assess the alignment of the lunate bone with the capitate. A DISI deformity shows lunate tilting dorsally, while a VISI deformity shows it tilting volarly.
Step-by-step explanation:
To determine whether you are looking at a Dorsal Intercalated Segment Instability (DISI) or a Volar Intercalated Segment Instability (VISI) deformity on lateral wrist films, you need to evaluate the alignment of the carpal bones. A DISI deformity is characterized by the lunate bone tilting dorsally, causing the scaphoid to flex and creating a greater than 10-degree difference between the lunate and capitate line. Conversely, a VISI deformity occurs when the lunate tilts volarly (towards the palm), causing the triquetrum to flex and showing a greater than 10-degree difference between the lunate and the capitate line in the opposite direction. These conditions are associated with the instability of the wrist joint and are important to identify as they can affect the function of the wrist and require different management approaches.