Lateral Patellar Dislocation/Sliver Sign

Case Series!  

These findings all represent the same pathology.  

History: Generic young person takes bad step and limps into ED claiming they dislocated their knee. 

Patient #1:

Patient #2:

Patient #3:

Subtle stuff. Shall we peak at the back of the book for the answer key?

Question: What the hell is he getting at here?
Answer: Transient lateral patellar dislocation and "The Sliver Sign"

Patient #1:

Red arrow shows the bone fragments that arise from both avulsion of the medial patellofemoral ligament as well as impaction when it slams against the lateral margins of the femoral condyle.   You can appreciate the persistent subluxation too (though don't rely on this XR alignment for acute pathology - plenty of chronic lateral tilt and subluxation out there too).

Patient #2:

If you look closely over the superior margin of the infrapatellar fat pad on the lateral XR, you may see some of those small patellar avulsion/impaction slivers (red arrows) as well.  Of course, what's easier to see is the-always-abnormal joint effusion in a young person, which should make you slow down and excited for the fracture hunt.  MR arrows outline the "kissing" contusions of the patellar avulsion and femoral impaction. 

Patient #3:

Subtle!   If you do suspect a lateral patellar dislocation, either from history or typical AP/OBL/LAT 3 view, get the sunrise!  It's your highest yield view for bone abnormality.
 

Transient Lateral Patellar Dislocation has subtle findings on radiograph, but some noticeable trends surround the occasion. 

1. Joint effusion in a young person!
2. Patient often describes a "dislocation" that spontaneously reduced immediately or with personal manipulation.  Try to read between the lines when the patient (and sometimes our ED colleagues) uses vague anatomy terminology. 
3. Look for those bone slivers about the medial and inferior patella. 

Good huntin'!

Here are some good articles to review!

https://radiopaedia.org/articles/lateral-patellar-dislocation?lang=us

https://radiopaedia.org/articles/sliver-sign-patella?lang=us

Epilogue: 

Enjoy the occasional Not-transient Lateral  Patellar Dislocation too.