Clinical CT – August 2016

Clinical CT for August 2016
by Adam Blumenberg MD

A 75 year-old woman with a history of left femur ORIF and hypertension was “clipped” on the left knee by a turning car as she was crossing the street. She is dressed in religious garb with a head scarf and long sleeves. She has been unable to bear weight since, and has a slowly expanding knee effusion. Distal pulses are intact. A CT of her knee just superior to the patella is shown.

1. Please describe this image.
   This is a transverse CT slice through the distal femur. There is a circular radio-dense structure in the marrow space, likely an intramedullary nail. There is also an effusion in the suprapatellar bursa (continuous with joint space) with a fluid-fluid level.
   
2. What is the significance of the radiographic findings? How does the patient’s history influence your interpretation of this image?
   The patient is an elderly woman with a history of femur fracture and her clothing lowers her exposure to sunlight thereby increasing her risk of hypovitaminosis D and subsequent risk of osteoporosis. The patient’s risk factors for fracture include female sex, elderly, low exposure to vitamin D, prior fracture with an implanted device, and mechanism (struck by a vehicle). She has a traumatic knee effusion and is unable to bear weight. The layered joint effusion may represent either a hemarthrosis with the cellular components settling away from the serum, much like hematocrit effect. The fluid-fluid level may also indicate a lipohemarthrosis. The presence of fat in the joint space would imply disruption of bony cortex because the bone marrow would be the source of fat.
   
3. What similar radiographic finding is highly specific for fracture?
   A double fluid-fluid level would imply presence of fat, serum, and cellular layers and is highly specific for fracture.
   
4. What is the next step in management?
   Assess the density of each fluid layer in Hounsfield units. Then, closely scrutinize the entirety of the CT image for a fracture.
   
5. What is the patient’s disposition?
   Further review of the images reveals a non-displaced tibial plateau fracture. Place the patient in knee immobilization with non-weight bearing status and have the patient follow up closely with orthopedics as an outpatient.