Which dislocation of the carpal bone results in the teacup effect?

• A. Scaphoid dislocation
• B. Lunate dislocation
• C. Triquetrum dislocation
• D. Capitate dislocation

Answer: (B)

The teacup effect is a radiological sign primarily associated with lunate dislocations. This term is used to describe the appearance of the lunate bone on X-ray when it is dislocated. When the lunate is dislocated, it can resemble a teacup perched on a pedestal, where the body of the lunate forms the "cup" and the distal pole makes up the "pedestal". This distinctive appearance is critical for diagnosing lunate dislocation and differentiating it from other carpal bone injuries.

In the case of lunate dislocation, the lunate typically displaces volarly (toward the palm), which alters its normal alignment with the surrounding carpal bones, creating the teacup effect in imaging. The recognition of this pattern on X-rays is vital for healthcare professionals because it indicates a significant injury that may require surgical intervention to restore normal wrist function and prevent complications such as avascular necrosis.

Other types of carpal dislocations, such as those involving the scaphoid, triquetrum, or capitate, do not exhibit this specific teacup effect and have differing visual characteristics on imaging that do not relate to the "teacup" analogy. Understanding these visual cues helps in accurate