This is a 10 year old female who fell onto her knee 
on the school playground during recess.  She jumped 
for some horizontal bars which slipped from her grasp.  
She fell onto a hard dirt surface injuring her left knee.  
She is able to walk, but this is painful. 
     Her past medical history is unremarkable.
     Exam:  VS T36.9 (oral), P1000, R18, BP 110/65.  
She is comfortable.  Her exam findings are only 
significant for the left knee.  She points to the medial 
side of her patella as the site of pain.  There is mild 
swelling noted around this region.  No abrasions, 
lacerations or bruises are noted.  The patella is tender.  
Range of motion is fair with moderately severe pain.  
There is no other deformities.  The drawer sign is 
negative and lateral stability about the knee is good.  
Her tibia, fibula, femur and hip are all non-tender.  
     Radiographs of her knee are obtained.

View knee radiographs.
View AP.


View lateral.


     Where do you suspect her to have a fracture?  Can 
you see this area adequately on an AP and lateral view 
of the knee?

     Since she fell onto her patella and the site of her 
pain and tenderness is limited to the patella, it is very 
likely that she has a patella fracture.  The patella 
cannot be viewed very well on an AP view because the 
distal femur overlaps it.  Since the distal femur is much 
thicker than the patella, it is difficult to see any bony 
detail of the patella on AP radiographs of the knee.  
The lateral view of the knee isolates the patella well, 
but this view will only detect horizontal fractures of the 
patella.  Vertical or oblique fractures will often not be 
visible on a lateral view because the width of the 
patella will obscure bony detail in the oblique and 
vertical planes.
     Another view of the knee is obtained.

View sunrise view.


     This is called the sunrise view because the patella 
appears to be rising over the horizon.  This view is 
taken with the knee flexed.  The radiograph is taken 
with the x-ray beam tangential to the patella parallel to 
the long axis of the lower extremity.  Note the small 
avulsion fracture of the medial aspect of the patella 
This fracture is in the vertical plane making it difficult to 
see on the lateral view of the patella.  The sunrise view 
helps to identify vertical fractures of the patella.  
Whenever a fracture of the patella is suspected, a 
sunrise view should be requested.  There are many 
different views of the knee and the sunrise view is not 
considered part of the standard set of views in many 
hospitals.  It must be specifically ordered.

     This second set of radiographs demonstrates the 
opposite phenomenon.  This is an older patient with 
patellar pain and tenderness.

View knee radiographs.
View AP.


View lateral.


View sunrise.


     Again, the AP view does not reveal much because 
the patella fracture is obscured by the distal femur.  
The lateral view shows the patella fracture well 
because the fracture is in the horizontal plane.  In this 
case, the sunrise view does not show anything because 
the fracture is in the horizontal plane (not the vertical 
plane).
     Despite this fracture being very large, it is not 
possible to see it if viewing it in the wrong plane.  Even 
a large vertical fracture of the patella may not be 
visible on the AP and lateral views.  A sunrise view will 
need to be ordered to visualize the fracture.  Similarly, 
the patella fracture may occasionally be oblique in 
which case all the views may fail to show it well.
     Patellar fractures are generally caused by direct 
trauma to the patella or due to an avulsion injury of the 
quadriceps or patella tendon insertions.  Comparison 
views may be useful to identify the correct position of 
the patella.  In a quadriceps or patellar tendon avulsion, 
the patella will often be out of position.  
     A "bipartite" patella is a normal variant that 
resembles a patella fracture.  A comparison view may 
be helpful to distinguish this from a fracture.  The 
lucency in a bipartite patella is usually in the superior 
lateral portion of the patella.

References:
     The Patella (Chapter 19).  In:  Simon RR, 
Koenigsknecht SJ.  Emergency Orthopedics:  The 
Extremities, thid edition.  1995, Norwalk, CT, Appleton 
& Lange, pp.  287-289.