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Lunate Dislocation and perilunate dislocation are broken down into 4 stages that relates to the progressive disruption of the carpal ligaments due to hyperextension and ulnar deviation of the wrist:
- Stage 1: Scapholunate Dislocation
- Has the characteristic sign of widening of the scapholunate joint on the PA view known as the Terry Thomas Sign as it resembles the gap between his teeth
- Gap between scaphoid and lunate should be less than 2 mm
- Stage II: Perilunate dislocation
- Best seen on lateral view of the wrist
- Associated with scaphoid fractures
- Lunate stays in its normal position with the capitate dislocation posterior when you use the distal radius as your reference point
- Stage III: Perilunate dislocation
- Also includes dislocation or fracture of the triguetrum
- Triquetrial and scaphoid malrotation
- In lateral view, all other carpal bones are dislocated posterior with respect to lunate
- Stage IV: Lunate dislocation
- On PA view you will see a triangular view of the lunate on the PA view that looks like a "piece of pie".
- On the lateral view of the wrist the lunate will look like a tea cup tipped in the volar direction AKA the "spilled teacup sign"
- Associated with a scaphoid fracture
For a good indepth review of lunate and perilunate injuries please read the article by Andy Perron with this attached link.... 
doi:10.1053/ajem.2001.21306
If you are interested in seeing some xray examples please visit LearningRadiology.com
References
Perron AD, Brady WJ, Keats TE, Hersh RE. Orthopedic Pitfalls in the ED: Lunate and Perilunate Injuries Am J Emerg Med 2001;19:157-162