Volume 6, Case 8
Loren G. Yamamoto, MD, MPH
Kapiolani Medical Center For Women And Children
University of Hawaii John A. Burns School of Medicine
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.